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| Declare Variables |
Declare Variables
JavaScript variable declarations can be sorted into three distinct components: the variable type, the variable name, and the variable value.
var myName = "Rafael";
Let's break the above line of code into the pieces that make it up:
var/const/let
JavaScript variables can have three declaration types: var, const, and let. Var-type variables are global, if declared outside a function they can be accessed by any JS file (or the console), and if created within a function they are accessible regardless of block scope. Let-type variables are limited in scope to their block. See the example below for the difference.
function varTest() {
var x = 1;
if (true) {
var x = 2; // same variable!
console.log(x); // 2
}
console.log(x); // 2
}
function letTest() {
let x = 1;
if (true) {
let x = 2; // different variable
console.log(x); // 2
}
console.log(x); // 1
}
Const-type variables have the same scope as let variables (block scope), but are immutable. Whatever value a const-type variable is to be assigned, must happen when the variable is declared, and JavaScript will thrown an error if the variable is changed later.
const genre = "non-fiction";
console.log(genre); // "non-fiction";
genre = "fantasy"; // error
Now that we can determine what the variable type is, let's take a look at the name. JavaScript variable names are written in camel case format. An example of camel case is: camelCase. In the context of our example:
myName
The name is also we'll access the variable again later:
console.log(myName); // "Rafael"
Finally, our value:
"Rafael"
JavaScript is dynamically typed, which means any given variable can represent any given data type at any given time. For example:
var example = "This is an example";
example = [0, 1, 2, 3]
example = {test: "Result"}
example = 5
All those statements are perfectly valid - JavaScript variables can jump from string to array to object to integer.
Declare object as const
As mentioned above, const variable are immutable means value assigned to such variable at the time of declaration cannot be updated but there is a point to note in case object declaration with const. Object of type const also cannot be updated once defined but properties of object cab be. For example.
const Car1 = {
name: 'BMW',
model: 'X1',
color: 'black'
}
Here, we cannot update the object but we can update the properties by accessing through dot(.) operator as below.
Car1.color = 'Red';
console.log(Car1);
O/P - {name: "BMW", model: "X1", color: "Red"}
If we need to to make enitre object as immutable (including properties) then we have to use freeze method.