{ "name": "ES6", "order": 2, "time": "5 hours", "helpRoom": "Help", "challenges": [ { "id": "587d7b86367417b2b2512b3e", "title": "Introduction to the ES6 Challenges", "description": [ [ "", "", "ECMAScript is a standardized version of JavaScript with the goal of unifying the language's specifications and features. As all major browsers and JavaScript-runtimes follow this specification, the term ECMAScript is interchangeable with the term JavaScript.

Most of the challenges on freeCodeCamp use the ECMAScript 5 (ES5) specification of the language, finalized in 2009. But JavaScript is an evolving programming language. As features are added and revisions are made, new versions of the language are released for use by developers.

The most recent standardized version is called ECMAScript 6 (ES6), released in 2015. This new version of the language adds some powerful features that will be covered in this section of challenges, including:



Note
Not all browsers support ES6 features. If you use ES6 in your own projects, you may need to use a program (transpiler) to convert your ES6 code into ES5 until browsers support ES6.", "" ] ], "releasedOn": "Feb 17, 2017", "challengeSeed": [], "tests": [], "type": "waypoint", "challengeType": 7, "isRequired": false, "translations": {} }, { "id": "587d7b87367417b2b2512b3f", "title": "Explore Problems with the var Keyword", "description": [ "One of the biggest problems with declaring variables with the var keyword is that you can overwrite variable declarations without an error.", "
var camper = 'James';
var camper = 'David';
console.log(camper);
// logs 'David'
", "In a small application, you might not run into this type of problem, but when your code becomes larger, you might accidently overwrite a variable that you did not intend to overwrite. Because this behaviour does not throw an error, searching and fixing bugs becomes more difficult.", "Another problem with the var keyword is that it is hoisted to the top of your code when it compiles. This means that you can use a variable before you declare it.", "
console.log(camper);
var camper = 'David';
// logs undefined
", "The code runs in the following order:", "
  1. The variable camper is declared as undefined.
  2. The value of camper is logged.
  3. David is assigned to camper.
", "This code will run without an error.", "A new keyword called let was introduced in ES6 to solve the problems with the var keyword. With the let keyword, all the examples we just saw will cause an error to appear. We can no longer overwrite variables or use a variable before we declare it. Some modern browsers require you to add \"use strict\"; to the top of your code before you can use the new features of ES6.", "Let's try using the let keyword.", "
", "Fix the code so that it only uses the let keyword and makes the errors go away.", "Note
Remember to add \"use strict\"; to the top of your code." ], "challengeSeed": [ "var favorite = redNosedReindeer + \" is Santa's favorite reindeer.\";", "var redNosedReindeer = \"Rudolph\";", "var redNosedReindeer = \"Comet\";" ], "tests": [ "assert(redNosedReindeer === \"Rudolph\", 'message: redNosedReindeer should be Rudolph.');", "assert(favorite === \"Rudolph is Santa's favorite reindeer.\", \"message: favorite should return Santa's favorite reindeer.\");" ], "type": "waypoint", "releasedOn": "Feb 17, 2017", "challengeType": 1, "translations": {} }, { "id": "587d7b87367417b2b2512b40", "title": "Compare Scopes of the var and let Keywords", "description": [ "When you declare a variable with the var keyword, it is declared globally, or locally if declared inside a function.", "The let keyword behaves similarly, but with some extra features. When you declare a variable with the let keyword inside a block, statement, or expression, its scope is limited to that block, statement, or expression.", "For example:", "
var numArray = [];
for (var i = 0; i < 3; i++) {
numArray.push(i);
}
console.log(numArray);
// returns [0, 1, 2]
console.log(i);
// returns 3
", "With the var keyword, i is declared globally. So when i++ is executed, it updates the global variable. This code is similiar to the following:", "
var numArray = [];
var i;
for (i = 0; i < 3; i++) {
numArray.push(i);
}
console.log(numArray);
// returns [0, 1, 2]
console.log(i);
// returns 3
", "This behavior will cause problems if you were to create a function and store it for later use inside a for loop that uses the i variable. This is because the stored function will always refer to the value of the updated global i variable.", "
var printNumTwo;
for (var i = 0; i < 3; i++) {
if(i === 2){
printNumTwo = function() {
return i;
};
}
}
console.log(printNumTwo());
// returns 3
", "As you can see, printNumTwo() prints 3 and not 2. This is because the value assigned to i was updated and the printNumTwo() returns the global i and not the value i had when the function was created in the for loop. The let keyword does not follow this behavior:", "
'use strict';
let printNumTwo;
for (let i = 0; i < 3; i++) {
if (i === 2) {
printNumTwo = function() {
return i;
};
}
}
console.log(printNumTwo());
// returns 2
console.log(i);
// returns \"i is not defined\"
", "i is not defined because it was not declared in the global scope. It is only declared within the for loop statement. printNumTwo() returned the correct value because three different i variables with unique values (0, 1, and 2) were created by the let keyword within the loop statement.", "
", "Fix the code so that i declared in the if statement is a separate variable than i declared in the first line of the function. Be certain not to use the var keyword anywhere in your code.", "Note
Remember to add \"use strict\"; to the top of your code.", "This exercise is designed to illustrate the difference between how var and let keywords assign scope to the declared variable. When programming a function similar to the one used in this exercise, it is often better to use different variable names to avoid confusion." ], "challengeSeed": [ "function checkScope() {", " var i = \"function scope\";", " if (true) {", " i = \"block scope\";", " console.log(\"Block scope i is: \", i);", " }", " console.log(\"Function scope i is: \", i);", " return i;", "}", "// only change the code above this line", "checkScope();" ], "tests": [ "// TEMPORARILY COMMENTED OUT: assert(!/var/g.test(code) && /let/g.test(code), 'message: The var keyword should be replaced with let. (This test is temporarily disabled)');", "assert(code.match(/(i\\s*=\\s*).*\\s*.*\\s*.*\\1('|\")block\\s*scope\\2/g), 'message: The variable i declared in the if statement should equal \"block scope\".');", "assert(checkScope() === \"function scope\", 'message: checkScope() should return \"function scope\"');" ], "type": "waypoint", "releasedOn": "Feb 17, 2017", "challengeType": 1, "translations": {} }, { "id": "587d7b87367417b2b2512b41", "title": "Declare a Read-Only Variable with the const Keyword", "description": [ "let is not the only new way to declare variables. In ES6, you can also declare variables using the const keyword.", "const has all the awesome features that let has, with the added bonus that variables declared using const are read-only. They are a constant value, which means that once a variable is assigned with const, it cannot be reassigned.", "
\"use strict\"
const FAV_PET = \"Cats\";
FAV_PET = \"Dogs\"; // returns error
", "As you can see, trying to reassign a variable declared with const will throw an error. You should always name variables you don't want to reassign using the const keyword. This helps when you accidentally attempt to reassign a variable that is meant to stay constant. A common practice is to name your constants in all upper-cases and with an underscore to separate words (e.g. EXAMPLE_VARIABLE).", "
", "Change the code so that all variables are declared using let or const. Use let when you want the variable to change, and const when you want the variable to remain constant. Also, rename variables declared with const to conform to common practices.", "Note
Don't forget to add \"use strict\"; to the top of your code." ], "challengeSeed": [ "// change 'var' to 'let' or 'const'", "// rename constant variables", "var pi = 3.14;", "var radius = 10;", "var calculateCircumference = function(r) {", " var diameter = 2 * r;", " var result = pi * diameter;", " return result;", "};", "// Test your code", "console.log(calculateCircumference(radius));" ], "tests": [ "// Test user replaced all var keyword", "// Test PI is const", "// Test calculateCircumference is const", "// Test pi and calculateCircumference has been renamed" ], "type": "waypoint", "releasedOn": "Feb 17, 2017", "challengeType": 1, "translations": {} }, { "id": "587d7b87367417b2b2512b42", "title": "Mutate an Array Declared with const", "description": [ "The const declaration has many use cases in modern JavaScript.", "Some developers prefer to assign all their variables using const by default, unless they know they will need to reassign the value. Only in that case, they use let.", "However, it is important to understand that objects (including arrays and functions) assigned to a variable using const are still mutable. Using the const declaration only prevents reassignment of the variable identifier.", "
\"use strict\";
const s = [5, 6, 7];
s = [1, 2, 3]; // throws error, trying to assign a const
s[2] = 45; // works just as it would with an array declared with var or let
console.log(s); // returns [5, 6, 45]
", "As you can see, you can mutate the object [5, 6, 7] itself and the variable s will still point to the altered array [5, 6, 45]. Like all arrays, the array elements in s are mutable, but because const was used, you cannot use the variable identifier s to point to a different array using the assignment operator.", "
", "An array is declared as const s = [5, 7, 2]. Change the array to [2, 5, 7] using various element assignment.", "Note
Don't forget to add \"use strict\"; to the top of your code." ], "challengeSeed": [ "const s = [5, 7, 2];", "// change code below this line", "", "s = [2, 5, 7];", "", "// change code above this line", "// Test your code", "console.log(s);" ], "tests": [ "assert(code.match(/const/g), 'message: Do not replace const keyword.');", "assert(code.match(/const\\s+s/g), 'message: s is declared with const.');", "assert(code.match(/const\\s+s\\s*?=\\s*?\\[\\s*?2\\s*?,\\s*?5\\s*?,\\s*?7\\s*?\\]\\s*?;/g), 'message: Do not change the original array declaration.');", "assert.deepEqual(s, [2, 5, 7], 'message: s should be equal to [2, 5, 7].');" ], "type": "waypoint", "releasedOn": "Feb 17, 2017", "challengeType": 1, "translations": {} }, { "id": "598f48a36c8c40764b4e52b3", "title": "Prevent Object Mutation", "description": [ "As seen in previous challenge, const declaration alone doesn't really protect your data from mutation. To ensure your data doesn't change, JavaScript provides a function Object.freeze to prevent data mutation.", "Once the object is freezed, you can no longer add/update/delete properties from it. Any attempt at changing the object will be rejected without any error.", "
\nlet obj = {\n name:\"FreeCodeCamp\"\n review:\"Awesome\"\n};\nObject.freeze(obj);\nobj.review = \"bad\"; //will be ignored. Mutation not allowed\nobj.newProp = \"Test\"; // will be ignored. Mutation not allowed\nconsole.log(obj); \n// { name: \"FreeCodeCamp\", review:\"Awesome\"}\n
", "
", "In this challenge you are going to use Object.freeze to prevent mathematical constants from changing. You need to freeze MATH_CONSTANTS object so that noone is able alter the value of PI or add any more properties to it." ], "challengeSeed": [ "const MATH_CONSTANTS = {", " PI: 3.14", "};", "// change code below this line", "", "", "// change code above this line", "MATH_CONSTANTS.PI = 99;", "// Test your code", "console.log(MATH_CONSTANTS.PI);// should show 3.14" ], "tests": [ "// Do not replace const keyword.", "// MATH_CONSTANTS is declared with const.", "// Do not change original MATH_CONSTANTS", "assert.deepEqual(MATH_CONSTANTS, {PI: 3.14}, 'message: MATH_CONSTANTS.PI should be equal to 3.14.');" ], "type": "waypoint", "releasedOn": "Aug 12, 2017", "challengeType": 1, "translations": {} }, { "id": "587d7b87367417b2b2512b43", "title": "Use Arrow Functions to Write Concise Anonymous Functions", "description": [ "In JavaScript, we often don't need to name our functions, especially when passing a function as an argument to another function. Instead, we create inline functions. We don't need to name these functions because we do not reuse them anywhere else.", "To achieve this, we often use the following syntax:", "
const myFunc = function() {
const myVar = \"value\";
return myVar;
}
", "ES6 provides us with the syntactic sugar to not have to write anonymous functions this way. Instead, you can use arrow function syntax:", "
const myFunc = () => {
const myVar = \"value\";
return myVar;
}
", "When there is no function body, and only a return value, arrow function syntax allows you to omit the keyword return as well as the brackets surrounding the code. This helps simplify smaller functions into one-line statements:", "
const myFunc= () => \"value\"
", "This code will still return value by default.", "
", "Rewrite the function assigned to the variable magic which returns a new Date() to use arrow function syntax. Also make sure nothing is defined using the keyword var.", "Note", "Don't forget to use strict mode." ], "challengeSeed": [ "// change code below this line", "var magic = function() {", " return new Date();", "}", "// change code above this line", "// test your code", "console.log(magic());" ], "tests": [ "// Test user did replace var keyword", "// Test magic is const", "// Test magic is a function", "// Test magic() returns the correct date", "// Test function keyword was not used", "// Test arrow => was used" ], "type": "waypoint", "releasedOn": "Feb 17, 2017", "challengeType": 1, "translations": {} }, { "id": "587d7b88367417b2b2512b44", "title": "Write Arrow Functions with Parameters", "description": [ "Just like a normal function, you can pass arguments into arrow functions.", "
// doubles input value and returns it
const doubler = (item) => item * 2;
", "You can pass more than one argument into arrow functions as well.", "
", "Rewrite the myConcat function which appends contents of arr2 to arr1 so that the function uses arrow function syntax.", "Note", "Don't forget to use strict mode." ], "challengeSeed": [ "// change code below this line", "var myConcat = function(arr1, arr2) {", " return arr1.concat(arr2);", "}", "// change code above this line", "// test your code", "console.log(myConcat([1, 2], [3, 4, 5]));" ], "tests": [ "// Test user did replace var keyword", "// Test myConcat is const", "assert(typeof myConcat === \"function\", 'message: myConcat should be a function');", "// Test myConcat() returns the correct array", "// Test function keyword was not used", "// Test arrow => was used" ], "type": "waypoint", "releasedOn": "Feb 17, 2017", "challengeType": 1, "translations": {} }, { "id": "587d7b88367417b2b2512b45", "title": "Write Higher Order Arrow Functions", "description": [ "It's time we see how powerful arrow functions are when processing data.", "Arrow functions work really well with higher order functions, such as map(), filter(), and reduce(), that take other functions as arguments for processing collections of data.", "Read the following code:", "
FBPosts.filter(function(post) {
return post.thumbnail !== null && post.shares > 100 && post.likes > 500;
})
", "We have written this with filter() to at least make it somewhat readable. Now compare it to the following code which uses arrow function syntax instead:", "
FBPosts.filter((post) => post.thumbnail !== null && post.shares > 100 && post.likes > 500)
", "This code is more succinct and accomplishes the same task with fewer lines of code.", "
", "Use arrow function syntax to compute the square of only the positive integers (fractions are not integers) in the array realNumberArray and store the new array in the variable squaredIntegers.", "Note", "Don't forget to use strict mode." ], "challengeSeed": [ "const realNumberArray = [4, 5.6, -9.8, 3.14, 42, 6, 8.34];", "// change code below this line", "var squaredIntegers = realNumberArray;", "// change code above this line", "// test your code", "console.log(squaredIntegers);" ], "tests": [ "// Test user did replace var keyword", "// Test squaredIntegers is const", "assert(Array.isArray(squaredIntegers), 'message: squaredIntegers should be an array');", "assert(squaredIntegers[0] === 16 && squaredIntegers[1] === 1764 && squaredIntegers[2] === 36, 'message: squaredIntegers should be [16, 1764, 36]');", "// Test function keyword was not used", "// Test arrow => was used", "assert(!code.match(/(for)|(while)/g), 'message: loop should not be used');", "assert(code.match(/map/g) && code.match(/filter/g), 'message: map and filter should be used');" ], "type": "waypoint", "releasedOn": "Feb 17, 2017", "challengeType": 1, "translations": {} }, { "id": "587d7b88367417b2b2512b46", "title": "Set Default Parameters for Your Functions", "description": [ "In order to help us create more flexible functions, ES6 introduces default parameters for functions.", "Check out this code:", "
function greeting(name = \"Anonymous\") {
return \"Hello \" + name;
}
console.log(greeting(\"John\")); // Hello John
console.log(greeting()); // Hello Anonymous
", "The default parameter kicks in when the argument is not specified (it is undefined). As you can see in the example above, the parameter name will receive its default value \"Anonymous\" when you do not provide a value for the parameter. You can add default values for as many parameters as you want.", "
", "Modify the function increment by adding default parameters so that it will add 1 to number if value is not specified.", "Note
Don't forget to use strict mode." ], "challengeSeed": [ "function increment(number, value) {", " return number + value;", "}", "console.log(increment(5, 2)); // returns 7", "console.log(increment(5)); // returns NaN" ], "tests": [ "assert(increment(5, 2) === 7, \"The result of increment(5, 2) should be 7\");", "assert(increment(5) === 6, \"The result of increment(5) should be 6\");", "// Test default parameter was used for 'value'" ], "type": "waypoint", "releasedOn": "Feb 17, 2017", "challengeType": 1, "translations": {} }, { "id": "587d7b88367417b2b2512b47", "title": "Use the Rest Operator with Function Parameters", "description": [ "In order to help us create more flexible functions, ES6 introduces the rest operator for function parameters. With the rest operator, you can create functions that take a variable number of arguments. These arguments are stored in an array that can be accessed later from inside the function.", "Check out this code:", "
function howMany(...args) {
return \"You have passed \" + args.length + \" arguments.\";
}
console.log(howMany(0, 1, 2)); // You have passed 3 arguments
console.log(howMany(\"string\", null, [1, 2, 3], { })); // You have passed 4 arguments.
", "The rest operator eliminates the need to check the args array and allows us to apply map(), filter() and reduce() on the parameters array.", "
", "Modify the function sum so that is uses the rest operator and it works in the same way with any number of parameters.", "Note
Don't forget to use strict mode." ], "challengeSeed": [ "function sum(x, y, z) {", " const array = [ x, y, z ];", " return array.reduce((a, b) => a + b, 0);", "}", "console.log(sum(1, 2, 3)); // 6" ], "tests": [ "assert(sum(0,1,2) === 3, 'The result of sum(0,1,2) should be 3');", "assert(sum(1,2,3,4) === 10, 'The result of sum(1,2,3,4) should be 10');", "assert(sum(5) === 5, 'The result of sum(5) should be 5');", "assert(sum() === 0, 'The result of sum() should be 0');" ], "type": "waypoint", "releasedOn": "Feb 17, 2017", "challengeType": 1, "translations": {} }, { "id": "587d7b89367417b2b2512b48", "title": "Use the Spread Operator to Evaluate Arrays In-Place", "description": [ "ES6 introduces the spread operator, which allows us to expand arrays and other expressions in places where multiple parameters or elements are expected.", "The ES5 code below uses apply() to compute the maximum value in an array:", "
var arr = [6, 89, 3, 45];
var maximus = Math.max.apply(null, arr); // returns 89
", "We had to use Math.max.apply(null, arr) because Math.max(arr) returns NaN. Math.max() expects comma-separated arguments, but not an array.", "The spread operator makes this syntax much better to read and maintain.", "
const arr = [6, 89, 3, 45];
const maximus = Math.max(...arr); // returns 89
", "...arr returns an unpacked array. In other words, it spreads the array.", "However, the spread operator only works in-place, like in an argument to a function or in an array literal. The following code will not work:", "
const spreaded = ...arr; // will throw a syntax error
", "
", "Copy all contents of arr1 into another array arr2 using the spread operator." ], "challengeSeed": [ "const arr1 = ['JAN', 'FEB', 'MAR', 'APR', 'MAY'];", "const arr2 = []; // change this line", "arr1.push('JUN');", "console.log(arr2); // arr2 should not be affected" ], "tests": [ "// Test arr2 is correct copy of arr1", "// Test arr1 has changed", "// Test spread operator was used" ], "type": "waypoint", "releasedOn": "Feb 17, 2017", "challengeType": 1, "translations": {} }, { "id": "587d7b89367417b2b2512b49", "title": "Use Destructuring Assignment to Assign Variables from Objects", "description": [ "We earlier saw how spread operator can effectively spread, or unpack, the contents of the array.", "We can do something similar with objects as well. Destructuring assignment is special syntax for neatly assigning values taken directly from an object to variables.", "Consider the following ES5 code:", "
var voxel = {x: 3.6, y: 7.4, z: 6.54 };
var x = voxel.x; // x = 3.6
var y = voxel.y; // y = 7.4
var z = voxel.z; // z = 6.54
", "Here's the same assignment statement with ES6 destructuring syntax:", "
const { x, y, z } = voxel; // x = 3.6, y = 7.4, z = 6.54
", "If instead you want to store the values of voxel.x into a, voxel.y into b, and voxel.z into c, you have that freedom as well.", "
const { x : a, y : b, z : c } = voxel // a = 3.6, b = 7.4, c = 6.54
", "You may read it as \"get the field x and copy the value into a,\" and so on.", "
", "Use destructuring to obtain the length of the string greeting" ], "challengeSeed": [ "const greeting = 'itadakimasu';", "// change code below this line", "const length = 0; // change this", "// change code above this line", "console.log(length); // should be using destructuring" ], "tests": [ "// Test len is 11", "// Test destructuring was used" ], "type": "waypoint", "releasedOn": "Feb 17, 2017", "challengeType": 1, "translations": {} }, { "id": "587d7b89367417b2b2512b4a", "title": "Use Destructuring Assignment to Assign Variables from Nested Objects", "description": [ "We can similarly destructure nested objects into variables.", "Consider the following code:", "
const a = {
start: { x: 5, y: 6},
end: { x: 6, y: -9 }
};
const { start : { x: startX, y: startY }} = a;
console.log(startX, startY); // 5, 6
", "In the example above, the variable start is assigned the value of a.start, which is also an object.", "
", "Use destructuring assignment to obtain max of forecast.tomorrow and assign it to maxOfTomorrow." ], "challengeSeed": [ "const forecast = {", " today: { min: 72, max: 83 },", " tomorrow: { min: 73.3, max: 84.6 }", "};", "// change code below this line", "const maxOfTomorrow = undefined; // change this line", "// change code above this line", "console.log(maxOfTomorrow); // should be 84.6" ], "tests": [ "// Test maxOfTomorrow to be 84.6", "// Test destructuring was used" ], "type": "waypoint", "releasedOn": "Feb 17, 2017", "challengeType": 1, "translations": {} }, { "id": "587d7b89367417b2b2512b4b", "title": "Use Destructuring Assignment to Assign Variables from Arrays", "description": [ "ES6 makes destructuring arrays as easy as destructuring objects.", "One key difference between the spread operator and array destructuring is that the spread operator unpacks all contents of an array into a comma-separated list. Consequently, you cannot pick or choose which elements you want to assign to variables.", "Destructuring an array lets us do exactly that:", "
const [a, b] = [1, 2, 3, 4, 5, 6];
console.log(a, b); // 1, 2
", "The variable a is assigned the first value of the array, and b is assigned the second value of the array.", "We can also access the value at any index in an array with destructuring by using commas to reach the desired index:", "
const [a, b,,, c] = [1, 2, 3, 4, 5, 6];
console.log(a, b, c); // 1, 2, 5
", "
", "Use destructuring assignment to swap the values of a and b so that a receives the value stored in b, and b receives the value stored in a." ], "challengeSeed": [ "let a = 8, b = 6;", "// change code below this line", "", "// change code above this line", "console.log(a); // should be 6", "console.log(b); // should be 8" ], "tests": [ "assert(a === 6, 'message: Value of a should be 6, after swapping.');", "assert(b === 8, 'message: Value of b should be 8, after swapping.');", "// assert(/\\[\\s*(\\w)\\s*,\\s*(\\w)\\s*\\]\\s*=\\s*\\[\\s*\\2\\s*,\\s*\\1\\s*\\]/g.test(code), 'message: Use array destructuring to swap a and b.');" ], "type": "waypoint", "releasedOn": "Feb 17, 2017", "challengeType": 1, "translations": {} }, { "id": "587d7b8a367417b2b2512b4c", "title": "Use Destructuring Assignment with the Rest Operator to Reassign Array Elements", "description": [ "In some situations involving array destructuring, we might want to collect the rest of the elements into a separate array.", "The result is similar to Array.prototype.slice(), as shown below:", "
const [a, b, ...arr] = [1, 2, 3, 4, 5, 7];
console.log(a, b); // 1, 2
console.log(arr); // [3, 4, 5, 7]
", "Variables a and b take the first and second values from the array. After that, because of rest operator's presence, arr gets rest of the values in the form of an array.", "The rest element only works correctly as the last variable in the list. As in, you cannot use the rest operator to catch a subarray that leaves out last element of the original array.", "
", "Use destructuring assignment with the rest operator to perform an effective Array.prototype.slice() so that arr is a sub-array of the original array source with the first two elements ommitted." ], "challengeSeed": [ "const source = [1,2,3,4,5,6,7,8,9,10];", "// change code below this line", "const arr = source; // change this", "// change code below this line", "console.log(arr); // should be [3,4,5,6,7,8,9,10]", "console.log(source); // should be [1,2,3,4,5,6,7,8,9,10];" ], "tests": [ "// Test arr is [3,4,5,6,7,8,9,10];", "// Test source is [1,2,3,4,5,6,7,8,9,10];", "// Test destructuring was used", "// Test slice was not used" ], "type": "waypoint", "releasedOn": "Feb 17, 2017", "challengeType": 1, "translations": {} }, { "id": "587d7b8a367417b2b2512b4d", "title": "Use Destructuring Assignment to Pass an Object as a Function's Parameters", "description": [ "In some cases, you can destructure the object in a function argument itself.", "Consider the code below:", "
const profileUpdate = (profileData) => {
const { name, age, nationality, location } = profileData;
// do something with these variables
}
", "This effectively destructures the object sent into the function. This can also be done in-place:", "
const profileUpdate = ({ name, age, nationality, location }) => {
/* do something with these fields */
}
", "This removes some extra lines and makes our code look neat.", "This has the added benefit of not having to manipulate an entire object in a function; only the fields that are needed are copied inside the function.", "
", "Use destructuring assignment within the argument to the function half to send only max and min inside the function." ], "challengeSeed": [ "const stats = {", " max: 56.78,", " standard_deviation: 4.34,", " median: 34.54,", " mode: 23.87,", " min: -0.75,", " average: 35.85", "};", "// change code below this line", "const half = (stats) => ((stats.max + stats.min) / 2.0); // use function argument destructurung", "// change code above this line", "console.log(stats); // should be object", "console.log(half(stats)); // should be 28.015" ], "tests": [ "// Test stats is an object", "// Test half is 28.015", "// Test destructuring was used" ], "type": "waypoint", "releasedOn": "Feb 17, 2017", "challengeType": 1, "translations": {} }, { "id": "587d7b8a367417b2b2512b4e", "title": "Create Strings using Template Literals", "description": [ "A new feature of ES6 is the template literal. This is a special type of string that allows you to use string interpolation features to create strings.", "Consider the code below:", "
const person = {
name: \"Zodiac Hasbro\",
age: 56
};

// string interpolation
const greeting = `Hello, my name is ${person.name}!
I am ${person.age} years old.`;

console.log(greeting); // prints
// Hello, my name is Zodiac Hasbro!
// I am 56 years old.
", "A lot of things happened there.", "Firstly, the ${variable} syntax used above is a place holder. Basically, you won't have to use concatenation with the + operator anymore. To add variables to strings, you just drop the variable in a template string and wrap it with ${ and }.", "Secondly, the example uses backticks (`), not quotes (' or \"), to wrap the string. Notice that the string is multi-line.", "This new way of creating strings gives you more flexibility to create robust strings.", "
", "Use template literal syntax with backticks to display each entry of the result object's failure array. Each entry should be wrapped inside an li element with the class attribute text-warning." ], "challengeSeed": [ "const result = {", " success: [\"max-length\", \"no-amd\", \"prefer-arrow-functions\"],", " failure: [\"no-var\", \"var-on-top\", \"linebreak\"],", " skipped: [\"id-blacklist\", \"no-dup-keys\"]", "};", "// change code below this line", "const resultDisplay = null;", "// change code above this line", "console.log(resultDisplay);", "/**", " * should look like this", " *
  • no-var
    • ", " *
  • var-on-top
    • ", " *
  • linebreak
    • ", " **/" ], "tests": [ "// Test resultDisplay is a string", "// Test resultDisplay is the desired output", "// Test template strings were used" ], "type": "waypoint", "releasedOn": "Feb 17, 2017", "challengeType": 1, "translations": {} }, { "id": "587d7b8a367417b2b2512b4f", "title": "Write Concise Object Literal Declarations Using Simple Fields", "description": [ "ES6 adds some nice support for easily definining object literals.", "Consider the following code:", "
    const getMousePosition = (x, y) => ({
    x: x,
    y: y
    });
    ", "getMousePosition is a simple function that returns an object containing two fields.", "ES6 provides the syntactic sugar to eliminate the redundancy of having to write x: x. You can simply write x once, and it will be converted tox: x (or something equivalent) under the hood.", "Here is the same function from above rewritten to use this new syntax:", "
    const getMousePosition = (x, y) => ({ x, y });
    ", "
    ", "Use simple fields with object literals to create and return a Person object." ], "challengeSeed": [ "// change code below this line", "const createPerson = (name, age, gender) => {", " return {", " name: name,", " age: age,", " gender: gender", " };", "};", "// change code above this line", "console.log(createPerson(\"Zodiac Hasbro\", 56, \"male\")); // returns a proper object" ], "tests": [ "// Test the output is {name: \"Zodiac Hasbro\", age: 56, gender: \"male\"}", "// Test no : was present" ], "type": "waypoint", "releasedOn": "Feb 17, 2017", "challengeType": 1, "translations": {} }, { "id": "587d7b8b367417b2b2512b50", "title": "Write Concise Declarative Functions with ES6", "description": [ "When defining functions within objects in ES5, we have to use the keyword function as follows:", "
    const person = {
    name: \"Taylor\",
    sayHello: function() {
    return `Hello! My name is ${this.name}.`;
    }
    };
    ", "With ES6, You can remove the function keyword and colon altogether when defining functions in objects. Here's an example of this syntax:", "
    const person = {
    name: \"Taylor\",
    sayHello() {
    return `Hello! My name is ${this.name}.`;
    }
    };
    ", "
    ", "Refactor the function setGear inside the object bicycle to use the shorthand syntax described above." ], "challengeSeed": [ "// change code below this line", "const bicycle = {", " gear: 2,", " setGear: function(newGear) {", " this.gear = newGear;", " }", "};", "// change code above this line", "bicycle.setGear(3);", "console.log(bicycle.gear);" ], "tests": [ "// Test the output is Sending request to Yanoshi Mimoto", "// Test no : was present" ], "type": "waypoint", "releasedOn": "Feb 17, 2017", "challengeType": 1, "translations": {} }, { "id": "587d7b8b367417b2b2512b53", "title": "Use class Syntax to Define a Constructor Function", "description": [ "ES6 provides a new syntax to help create objects, using the keyword class.", "This is to be noted, that the class syntax is just a syntax, and not a full-fledged class based implementation of object oriented paradigm, unlike in languages like Java, or Python, or Ruby etc.", "In ES5, we usually define a constructor function, and use the new keyword to instantiate an object.", "
    var SpaceShuttle = function(targetPlanet){
    this.targetPlanet = targetPlanet;
    }
    var zeus = new spaceShuttle('Jupiter');
    ", "The class syntax simply replaces the constructor function creation:", "
    class SpaceShuttle {
    constructor(targetPlanet){
    this.targetPlanet = targetPlanet;
    }
    }
    const zeus = new spaceShuttle('Jupiter');
    ", "Notice that the class keyword declares a new function, and a constructor was added, which would be invoked when new is called - to create a new object.", "
    ", "Use class keyword and write a proper constructor to create the Vegetable class.", "The Vegetable lets you create a vegetable object, with a property name, to be passed to constructor." ], "challengeSeed": [ "/* Alter code below this line */", "const Vegetable = undefined;", "/* Alter code above this line */", "const carrot = new Vegetable('carrot');", "console.log(carrot.name); // => should be 'carrot'" ], "tests": [ "// Test the Vegetable is a class", "// Test that class keyword was used", "// Test that other objects could be created with the class" ], "type": "waypoint", "releasedOn": "Feb 17, 2017", "challengeType": 1, "translations": {} }, { "id": "587d7b8c367417b2b2512b54", "title": "Use getters and setters to Control Access to an Object", "description": [ "You can obtain values from an object, and set a value of a property within an object.", "These are classically called getters and setters.", "Getter functions are meant to simply return (get) the value of an object's private variable to the user without the user directly accessing the private variable.", "Setter functions are meant to modify (set) the value of an object's private variable based on the value passed into the setter function. This change could involve calculations, or even overwriting the previous value completely.", "
    class Book {
    constructor(author) {
    this._author = author;
    }
    // getter
    get writer(){
    return this._author;
    }
    // setter
    set writer(updatedAuthor){
    this._author = updatedAuthor;
    }
    }
    const lol = new Book('anonymous');
    console.log(lol.writer);
    lol.writer = 'wut';
    console.log(lol.writer);
    ", "Notice the syntax we are using to invoke the getter and setter - as if they are not even functions.", "Getters and setters are important, because they hide internal implementation details.", "
    ", "Use class keyword to create a Thermostat class. The constructor accepts Farenheit temperature.", "Now create getter and setter in the class, to obtain the temperature in Celsius scale.", "Remember that F = C * 9.0 / 5 + 32, where F is the value of temperature in Fahrenheit scale, and C is the value of the same temperature in Celsius scale", "Note", "When you implement this, you would be tracking the temperature inside the class in one scale - either Fahrenheit or Celsius.", "This is the power of getter or setter - you are creating an API for another user, who would get the correct result, no matter which one you track.", "In other words, you are abstracting implementation details from the consumer." ], "challengeSeed": [ "/* Alter code below this line */", "const Thermostat = undefined;", "/* Alter code above this line */", "const thermos = new Thermostat(76); // setting in Farenheit scale", "let temp = thermos.temperature; // 24.44 in C", "thermos.temperature = 26;", "temp = thermos.temperature; // 26 in C" ], "tests": [ "// Test the Thermostat is a class", "// Test that class keyword was used", "// Test that other objects could be created with the class" ], "type": "waypoint", "releasedOn": "Feb 17, 2017", "challengeType": 1, "translations": {} }, { "id": "587d7b8c367417b2b2512b55", "title": "Understand the Differences Between import and require", "description": [ "In the past, the function require() would be used to import the functions and code in external files and modules. While handy, this presents a problem: some files and modules are rather large, and you may only need certain code from those external resources.", "ES6 gives us a very handy tool known as import. With it, we can choose which parts of a module or file to load into a given file, saving time and memory.", "Consider the following example. Imagine that math_array_functions has about 20 functions, but I only need one, countItems, in my current file. The old require() approach would force me to bring in all 20 functions. With this new import syntax, I can bring in just the desired function, like so:", "
    import { countItems } from \"math_array_functions\"
    ", "A description of the above code:", "
    import { function } from \"file_path_goes_here\"
    // We can also import variables the same way!
    ", "There are a few ways to write an import statement, but the above is a very common use-case.", "Note
    the whitespace surrounding the function inside the curly braces is a best practice - it makes it easier to read the import statement.", "Note
    The lessons in this section handle non-browser features. import, and the statements we introduce in the rest of these lessons, won't work on a browser directly, However, we can use various tools to create code out of this to make it work in browser.", "
    ", "Add the appropriate import statement that will allow the current file to use the capitalizeString function. The file where this function lives is called \"string_functions\", and it is in the same directory as the current file." ], "challengeSeed": [ "capitalizeString(\"hello!\");" ], "tests": [ "assert(code.match(/import\\s+\\{\\s?capitalizeString\\s?\\}\\s+from\\s+\"string_functions\"/ig)" ], "type": "waypoint", "releasedOn": "Feb 17, 2017", "challengeType": 1, "translations": {} }, { "id": "587d7b8c367417b2b2512b56", "title": "Use export to Reuse a Code Block", "description": [ "In the previous challenge, you learned about import and how it can be leveraged to import small amounts of code from large files. In order for this to work, though, we must utilize one of the statements that goes with import, known as export. When we want some code - a function, or a variable - to be usable in another file, we must export it in order to import it into another file. Like import, export is a non-browser feature.", "The following is what we refer to as a named export. With this, we can import any code we export into another file with the import syntax you learned in the last lesson. Here's an example:", "
    const capitalizeString = (string) => {
    return string.charAt(0).toUpperCase() + string.slice(1);
    }
    export { capitalizeString } //How to export functions.
    export const foo = \"bar\"; //How to export variables.
    ", "Alternatively, if you would like to compact all your export statements into one line, you can take this approach:", "
    const capitalizeString = (string) => {
    return string.charAt(0).toUpperCase() + string.slice(1);
    }
    const foo = \"bar\";
    export { capitalizeString, foo }
    ", "Either approach is perfectly acceptable.", "
    ", "Below are two variables that I want to make available for other files to use. Utilizing the first way I demonstrated export, export the two variables." ], "challengeSeed": [ "const foo = \"bar\";", "const boo = \"far\";" ], "tests": [ "assert(code.match(/export\\s+const\\s+foo\\s+=+\\s\"bar\"/ig))", "assert(code.match(/export\\s+const\\s+boo\\s+=+\\s\"far\"/ig))" ], "type": "waypoint", "releasedOn": "Feb 17, 2017", "challengeType": 1, "translations": {} }, { "id": "587d7b8c367417b2b2512b57", "title": "Use * to Import Everything from a File", "description": [ "Suppose you have a file that you wish to import all of its contents into the current file. This can be done with the import * syntax.", "Here's an example where the contents of a file named \"math_functions\" are imported into a file in the same directory:", "
    import * as myMathModule from \"math_functions\"
    myMathModule.add(2,3);
    myMathModule.subtract(5,3);
    ", "And breaking down that code:", "
    import * as object_with_name_of_your_choice from \"file_path_goes_here\"
    object_with_name_of_your_choice.imported_function
    ", "You may use any name following the import * as portion of the statement. In order to utilize this method, it requires an object that receives the imported values. From here, you will use the dot notation to call your imported values.", "
    ", "The code below requires the contents of a file, \"capitalize_strings\", found in the same directory as it, imported. Add the appropriate import * statement to the top of the file, using the object provided." ], "challengeSeed": [ "myStringModule.capitalize(\"foo\");", "myStringModule.lowercase(\"Foo\");" ], "tests": [ "assert(code.match(/import\\s+\\*\\s+as\\s+myStringModule\\s+from\\s+\"capitalize_strings\"/ig))" ], "type": "waypoint", "releasedOn": "Feb 17, 2017", "challengeType": 1, "translations": {} }, { "id": "587d7b8c367417b2b2512b58", "title": "Create an Export Fallback with export default", "description": [ "In the export lesson, you learned about the syntax referred to as a named export. This allowed you to make multiple functions and variables available for use in other files.", "There is another export syntax you need to know, known as export default. Usually you will use this syntax if only one value is being exported from a file. It is also used to create a fallback value for a file or module.", "Here is a quick example of export default:", "
    export default function add(x,y) {
    return x + y;
    }
    ", "Note: Since export default is used to declare a fallback value for a module or file, you can only have one value be a default export in each module or file. Additionally, you cannot use export default with var, let, or const", "
    ", "The following function should be the fallback value for the module. Please add the necessary code to do so." ], "challengeSeed": [ "function subtract(x,y) {return x - y;}" ], "tests": [ "assert(code.match(/export\\s+default\\s+function\\s+subtract\\(x,y\\)\\s+{return\\s+x\\s-\\s+y;}/ig))" ], "type": "waypoint", "releasedOn": "Feb 17, 2017", "challengeType": 1, "translations": {} }, { "id": "587d7b8d367417b2b2512b59", "title": "Import a Default Export", "description": [ "In the last challenge, you learned about export default and its uses. It is important to note that, to import a default export, you need to use a different import syntax.", "In the following example, we have a function, add, that is the default export of a file, \"math_functions\". Here is how to import it:", "
    import add from \"math_functions\";
    add(5,4); //Will return 9
    ", "The syntax differs in one key place - the imported value, add, is not surrounded by curly braces, {}. Unlike exported values, the primary method of importing a default export is to simply write the value's name after import.", "
    ", "In the following code, please import the default export, subtract, from the file \"math_functions\", found in the same directory as this file." ], "challengeSeed": [ "subtract(7,4);" ], "tests": [ "assert(code.match(/import\\s+subtract\\s+from\\s+\"math_functions\"/ig))" ], "type": "waypoint", "releasedOn": "Feb 17, 2017", "challengeType": 1, "translations": {} } ] }