How to check if a variable is undefined or NULL in JavaScript

To check if a variable is undefined or null in JavaScript, either use the equality operator == or strict equality operator ===.

In JavaScript, a variable is considered undefined if it is declared but not assigned a value. Whereas the null is a special assignment value that signifies 'no value' or nonexistence of any value:

let name
let age = null

console.log(name)    // undefined
console.log(age)     // null

The equality operator == (also called loose equality operator) provides a more concise way to check whether the variable is undefined or null:

let name // undefined variable

if (name == null) {
  console.log(`Name is either undefined or null`)
} else {
  console.log(`Name is NOT undefined or null`)
}

// Output => Name is either undefined or null

The condition name == null evaluates to true because the equality operator treats both undefined and null as the same:

console.log(null == undefined) // true

Since the strict equality operator === (also called identity operator) does not treat undefined and null as the same, we have to use the || (or) operator to check if either of the two conditions is met:

let name

if (name === undefined || name === null) {
  console.log(`Name is either undefined or null`)
} else {
  console.log(`Name is NOT undefined or null`)
}

// Output => Name is either undefined or null

The strict equality operator === is more explicit but less concise as compared to the loose equality operator == for checking if the variable is undefined or null in JavaScript.

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