338d7ee8a7
* change es6 challenges rest operator to param * fix rest parameter typos * change rest operator to parameter in guide * fix casing * change rest operator to rest parameter in guide * change rest operator to rest parameter in curriculum * remove extra whitespace * remove whitespaces * remove whitespace * fix: removed arabic file * fix: removed chinese file
118 lines
3.3 KiB
Markdown
118 lines
3.3 KiB
Markdown
---
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title: Arrow Functions
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---
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Arrow functions are a new ES6 syntax for writing JavaScript function expressions. The shorter syntax saves time and simplifies the function scope.
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## What are arrow functions?
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An arrow function expression is a more concise syntax for writing function expressions using a "fat arrow" token (`=>`).
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### The basic syntax
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Below is a basic example of an arrow function:
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```javascript
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// ES5 syntax
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var multiply = function(x, y) {
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return x * y;
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};
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// ES6 arrow function
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var multiply = (x, y) => { return x * y; };
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// Or even simpler
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var multiply = (x, y) => x * y;
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```
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You no longer need the `function` and `return` keywords, or even the curly brackets.
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```javascript
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// everything included
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const multiply = function(x, y) => { return x * y };
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// remove "function"
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const multiply = (x, y) => { return x * y };
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// remove curly brackets and "return" ==> this way it returns implicitly
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const multiply = (x, y) => x * y;
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// if you only have one argument/parameter
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const multiplyBy2 = x => x * 2;
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// if you need to concisely return an object, you can wrap the {} inside the () to avoid syntax conflicts
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const getSumProductObject = (x, y) => ({sum : x + y, product: x * y});
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// combined with the ternary operator, but note it's not a looker!
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const addOrMultiply = (x, y, mathOperator) => mathOperator.toLowerCase() === 'add' ? x + y : x * y;
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```
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### A simplified `this`
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Before arrow functions, new functions defined their own `this` value. To use `this` inside a traditional function expression, we have to write a workaround like so:
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```javascript
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// ES5 syntax
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function Person() {
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// we assign `this` to `self` so we can use it later
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var self = this;
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self.age = 0;
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setInterval(function growUp() {
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// `self` refers to the expected object
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self.age++;
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}, 1000);
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}
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```
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An arrow function doesn't define its own `this` value, it inherits `this` from the enclosing function:
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```javascript
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// ES6 syntax
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function Person(){
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this.age = 0;
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setInterval(() => {
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// `this` now refers to the Person object, brilliant!
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this.age++;
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}, 1000);
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}
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var p = new Person();
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```
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An arrow function does not have its own `arguments` object. For example, if you do not know the number of arguments passed to a function, instead of using `arguments` you can use the `rest` parameter:
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```javascript
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const myFunc = (...n) => {
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console.log('The first argument is', n[0]);
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}
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myFunc(10,20,30,40,40); // output: The first argument is 10
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```
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Because of this, an arrow function cannot be used as a constructor, hence there's no need for a `prototype` property.
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```javascript
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(() => {}).hasOwnProperty('prototype'); // false
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```
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### No arguments object
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Arrow functions provide no arguments object. Therefore, if the arguments object is used in an arrow function it will reference the arguments of the enclosing scope. For example,
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```
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const arguments = ["arg1", "arg2", "arg3"];
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const arrow = () => arguments[1];
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console.log(arrow("innerarg1", "innerarg2", "innerarg3"));//arg2-ignores local arguments goes to enclosed scope.
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function regular() {
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console.log(arguments[1]);
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}
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console.log(regular("innerarg1", "innerarg2", "innerarg3"));//innerarg2-uses local arguments.
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```
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#### Further Reading
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<a href='https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Functions/Arrow_functions' target='_blank' rel='nofollow'>MDN link</a>
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