How to check null and undefined in TypeScript ?

In this article let’s learn how to check if a variable is null or undefined in TypeScript. A variable is undefined when it’s not assigned any value after being declared. Null refers to a value that is either empty or doesn’t exist. null means no value. To make a variable null we must assign null value to it as by default in typescript unassigned values are termed undefined. We can use typeof or ‘==’ or ‘===’ to check if a variable is null or undefined in typescript.

By using typescript compiler tcs we transpile typescript code to javascript and then run the javascript file.

tcs name_of_the_typescript_file

run the javascript file in the terminal by using:

node name_of_the_js_file

Example 1: In this example, we demonstrate when variables are null and when they are null. 

Javascript

let s: string;

// Returns undefined
console.log(s);
let n: number;

// Assigned null value
n = null;
console.log(n);

Output:

undefined
null

Example 2: In this example, typeof operator returns the type of the variable we want to check. In the below example we check the variable. unassigned variable returns undefined and the value which is assigned null returns object as ‘null’ is also taken as a null object in javascript.

Javascript

let s: string;

// Returns undefined
console.log(typeof s);
let n: number;

// Assigned null value
n = null;
console.log(typeof n);

Output:

undefined
object

Example 3: In this example, ‘==’ the equals operator helps us check whether the variable is null or undefined but when we check if null == undefined it results ‘true’. it’s also known as equality check.

Javascript

let s: string;
let n: number;

// Assigned null value
n = null;
console.log(n == null); // Returns true
console.log(s == undefined); // Returns true
console.log(null == undefined); // Returns true

Output:

true
true
true

Example 4: In this example, just as in the previous example instead of ‘==’ , we use ‘===’ to check. this method is called strict equality check. when checked if null=== undefined, it returns false unlike the previous method.

Javascript

let s: string;
let n: number;

// Assigned null value
n = null;
console.log(n === null); // Returns true
console.log(s === undefined); // Returns true
console.log(null === undefined); // Returns false

Output:

true
true
false