How to Define a Variable in JavaScript

A variable is a way for the program to "remember" a value so that it can be reused later.

There are three different ways to define a variable in JavaScript, using the keywords let, const, and var.

The let variables

let defines a mutable variable, meaning you can change the value after it has been declared. For example:

let a = 123;
 
a = null;
console.log(a); // -> null
 
a = 0;
console.log(a); // -> 0
 
a = "John Doe";
console.log(a); // -> John Doe

Remember that the equal sign (=) here doesn't really mean equal, it is the assignment operator that assigns a value to a variable.

Using let, you are allowed to declare a variable first, and assign a value later. Before the value is assigned, the variable will be automatically assigned undefined.

let a;
console.log(a); // -> undefined
 
a = 123;
console.log(a); // -> 123

The const variables

const, on the other hand, defines an immutable constant. If you try to change its value, an error will be returned.

const b = 123;
 
b = 100; // -> Error
console.log(b);

/Users/. . ./index.js:3
b = 100;
         ^
 
TypeError: Assignment to constant b.
    at Object.<anonymous> (/Users/. . ./index.js:3:10)
    . . .
 
Node.js v21.6.0

You cannot declare the variable first and then assign the value later using const.

const variable;
 
variable = 100;
console.log(variable);

/Users/. . ./index.js:1
const variable;
      ^^^^^^^^
 
SyntaxError: Missing initializer in const declaration
    at internalCompileFunction (node:internal/vm:77:18)
    . . .
 
Node.js v21.6.0

The var variables

The var keyword is the legacy method of defining a variable. In terms of mutability, it behaves just like let. You can change its value after being declared.

var variable = 123;
 
variable = 100;
console.log(variable);

100

The feature that makes var unique has to do with its scope. var can only be either function-scoped or global-scoped, but not block-scoped.

To better understand this, we'll discuss what is a scope later.