Variables
In JavaScript, variables must be declared to be used. Here is an example:
let firstName; // declare
firstName = "Ali"; // initialize
console.log(firstName); // use
let lastName = "Madooei"; // declare and initialize
console.log(lastName); // use
Unlike in languages like Java and C++ where you declare a variable by indicating its type, in JavaScript you must use a variable declaration keyword:
let
const
var
JavaScript is a dynamically-typed language which means a variable is assigned a type at runtime based on the variable's value at the time.
Dynamic typing means you can also change a variable's type at any time:
let num;
num = 2; // num is a Number
num = "two"; // now num is a String
num = { value: 2 }; // now num is an Object!
If you declare a variable but not give it a value, it's type and value are undefined
let num; console.log(num); console.log(typeof num);
You can use the
typeof
operator to check the type of a value.
let
A variable declared with let
behaves for most part the way you expect; for example:
- It throws an error if you use it before it is declared:
console.log(num); let num = 2;
- It throws an error if you redefine it:
let num = 2; let num = "two"; console.log(num);
- It has a block scope:
for (let num = 1; num < 10; num++) { // do something with num } console.log(num);
const
Declares a read-only named constant.
let firstName = "Ali";
firstName = "John"; // Ok
const lastName = "Madooei";
lastName = "Doe"; // Error!
Constants are block-scoped, like variables defined using the let
keyword. The value of a constant can't be changed through reassignment, and it can't be redeclared.
An initializer for a constant is required.
const lastName; // Error!
lastName = "Madooei";
The const declaration creates a read-only reference to a value. It does not mean the value it holds is immutable (just that the variable identifier cannot be reassigned).
const numbers = [1, 2, 7, 9];
numbers[2] = 4; // Ok
Prefer
const
overlet
: it provides an extra layer of protection against accidentally reassigning your variables.
var
The variable declaration keywords let
and const
are relatively new additions to JavaScript. The old way of declaring a variable in JavaScript is using var
:
var firstName = "Ali";
Variables declared with var
are not block scoped (although they are function scoped). Also, no error is thrown if you declare the same variable twice using var
.
Don't use
var
!
Undeclared Variables!
Technically, you can declare a variable in JavaScript without using a variable declaration keyword. Variables defined as such become global variables. This is another cruft from the early days of JavaScript and it must be avoided.
firstName = "Ali"; console.log(window.firstName);
Using an undeclared variable throws ReferenceError
under JavaScript's "strict mode", introduced in ECMAScript 5.