typescript check if null

Is there a way to check for both `null` and `undefined`?

stackoverflow.com › questions › 28975896 › is-there-a-way-to-check-for-both-null-and-undefined

Using a juggling-check, you can test both null and undefined in one hit:

if (x == null) {

If you use a strict-check, it will only be true for values set to null and won't evaluate as true for undefined variables:

if (x === null) {

You can try this with various values using this example:

var a: number;
var b: number = null;

function check(x, name) {
    if (x == null) {
        console.log(name + ' == null');
    }

    if (x === null) {
        console.log(name + ' === null');
    }

    if (typeof x === 'undefined') {
        console.log(name + ' is undefined');
    }
}

check(a, 'a');
check(b, 'b');

Output

"a == null"

"a is undefined"

"b == null"

"b === null"

Answer from Fenton on Stack Overflow

Stack Overflow

stackoverflow.com › questions › 28975896 › is-there-a-way-to-check-for-both-null-and-undefined

typescript - Is there a way to check for both `null` and `undefined`? - Stack Overflow

Top answer

1 of 16

705

Using a juggling-check, you can test both null and undefined in one hit:

if (x == null) {

If you use a strict-check, it will only be true for values set to null and won't evaluate as true for undefined variables:

if (x === null) {

You can try this with various values using this example:

var a: number;
var b: number = null;

function check(x, name) {
    if (x == null) {
        console.log(name + ' == null');
    }

    if (x === null) {
        console.log(name + ' === null');
    }

    if (typeof x === 'undefined') {
        console.log(name + ' is undefined');
    }
}

check(a, 'a');
check(b, 'b');

Output

"a == null"

"a is undefined"

"b == null"

"b === null"

2 of 16

482

if( value ) {
}

will evaluate to true if value is not:

null
undefined
NaN
empty string ''
0
false

typescript includes javascript rules.

W3Schools

w3schools.com › typescript › typescript_null.php

TypeScript Null & Undefined

Nullish coalescing is another JavaScript feature that also works well with TypeScript's null handling. It allows writing expressions that have a fallback specifically when dealing with null or undefined. This is useful when other falsy values can occur in the expression but are still valid. It can be used with the ?? operator in an expression, similar to using the && operator. function printMileage(mileage: number | null | undefined) { console.log(`Mileage: ${mileage ??

Videos

04:07

YouTube

Using Null & Undefined | strictNullChecks | TypeScript Tutorial ...

May 25, 2022

youtube.com

Best Way to Check 'Null', 'Undefined' or 'Empty' in JavaScript ...

JavaScript Fundamentals: Checking for null and undefined - YouTube

November 14, 2019

10.8K

youtube.com

TypeScript: The Null and Undefined Types

View all

TypeScript

typescriptlang.org › docs › handbook › release-notes › typescript-2-0.html

TypeScript: Documentation - TypeScript 2.0

In practical terms, strict null checking mode requires that all files in a compilation are null- and undefined-aware. TypeScript 2.0 implements a control flow-based type analysis for local variables and parameters. Previously, the type analysis performed for type guards was limited to if statements and ?: conditional expressions and didn’t include effects of assignments and control flow constructs such as return and break statements.

Omarileon

omarileon.me › blog › typescript-null-undefined

mari. | How to Detect Null and Undefined in Your TypeScript Code

February 27, 2024 - The most straightforward way of checking is of course the equality operator. const myValue = maybeNullOrUndefined(); if (myValue === null) console.log('Null!'); if (myValue === undefined) console.log('Undefined!');

GeeksforGeeks

geeksforgeeks.org › typescript › how-to-check-null-and-undefined-in-typescript

How to check null and undefined in TypeScript ? - GeeksforGeeks

July 23, 2025 - We can use typeof or '==' or '===' to check if a variable is null or undefined in typescript.

TutorialsPoint

tutorialspoint.com › javascript-typescript-object-null-check

JavaScript/ Typescript object null check?

In this example below, we've assigned ... need to run the above code in node.js terminal to get the output. var value=null; if(!value) { console.log("This is null."); } else { console.log("This is not null."); }...

Rampatra

blog.rampatra.com › null-vs-undefined-in-typescript-or-javascript-how-to-check-for-both-at-once

!== null vs !== undefined in Typescript or Javascript, how to check for both at once?

You want to check for both null and undefined together, since null and undefined are considered equal in loose equality (==). Commonly used when you don’t care about the specific type of “empty.” · if (value != null) { // Do something if value is not null or undefined }

Bobby Hadz

bobbyhadz.com › blog › typescript-check-if-null

How to correctly check for Null in TypeScript | bobbyhadz

To check for null in TypeScript, use a comparison to check if the value is equal or is not equal to `null`.

Find elsewhere

Google Bing Mojeek

reddit.com › r/reactjs › is null checking necessary in typescript for runtime safety?

r/reactjs on Reddit: Is Null Checking Necessary in TypeScript for Runtime Safety?

August 13, 2024 -

TypeScript types are only enforced at compile time and do not exist at runtime. Given this, if I define a TypeScript function that accepts a string parameter, is it necessary to perform null checks? This question arises because if the API returns null, it could lead to runtime errors.

Consider the following code snippet:

const capitalise = (val: string) => {
   // Question: Is this check redundant due to TypeScript's type checking?
   if (!val) {
      return '';
   }

   return val.toUpperCase();
};

// Assume 'name' is fetched from an API or another source that could return null
const name: string = /* fetch from API or another source */;

capitalise(name);

What are the best practices in such scenarios to ensure runtime safety in TypeScript applications?

Top answer

1 of 9

Is your program going to work with a null value in that spot? <- focus on this

2 of 9

I would say that the type-checking isn't necessary in the capitalise function. It might in fact make it harder to discover the error later. I would suggest that the best course of action would be to perform a null check on all the values that can't be null on the data received after doing the API-call. Assuming the API-call is its own function (which is should be), as a part of that function, check each field in the data received for null or undefined values where you don't want a null or undefined value. If one is found, either throw an error or revert to default values ('' for string, -1 for int, and so on)

Upmostly

upmostly.com › home › typescript › how to detect null and undefined

How to Detect Null and Undefined in Your TypeScript Code - Upmostly

March 28, 2023 - Null might mean the search has been completed, but the data wasn’t found, whereas undefined might indicate that the fetch was never completed successfully. Another way to check for null or undefined is to use the nullish coalescing operator (??), ...

Sandro Maglione

sandromaglione.com › articles › software › isnotnullable - check if a variable is not nullable in typescript

isNotNullable - Check if a variable is not nullable in Typescript | Sandro Maglione

July 24, 2022 - const isNotNullable = <T>(element: T | null | undefined): element is T => typeof element !== "undefined" && element !== null; const func = (item: { name: string } | null | undefined) => { if (isNotNullable(item)) { // Here `item` is of type `{ name: string }`, guaranteed! return item.name; } return "Nullable!"; } console.log(func({ name: 'Typescript' })); // Typescript console.log(func(null)); // Nullable!

reddit.com › r/typescript › how do i let typescript know that object is not null.

r/typescript on Reddit: How do i let Typescript know that object is not null.

May 27, 2021 -

given

let formEditor = document.getElementById('formeditor');

Problem is getElementById return type is HTMLElement|null

I know that formeditor always exists. And I do not want to do "formEditor!." every time.

is this my only option

let formEditor = document.getElementById('formeditor') as HTMLElement;

Top answer

1 of 7

Simple option. Replace as ... with ! Better option, use an if statement to handle the null case.

2 of 7

If you prefer an early return pattern, you can also use typeguards and throw an error(or exit the function) if the value of 'formeditor' is null. if(!formEditor) { return; // or throw new Error(...); } // formEditor is non null here and will not show the error

GitBook

basarat.gitbook.io › typescript › recap › null-undefined

Null vs. Undefined | TypeScript Deep Dive

Recommend == null to check for both undefined or null. You generally don't want to make a distinction between the two. ... function foo(arg: string | null | undefined) { if (arg != null) { // arg must be a string as `!=` rules out both null ...

Angular Wiki

angularjswiki.com › angular › how-to-check-both-null-or-undefined-in-typescript-or-angular

How To Check Both null or undefined in TypeScript/Angular | Angular Wiki

different ways we can check both null or undefined in TypeScript or Angular. 1. By using simple if condition 2. By using TypeScript Nullish Coalescing & Optional chaining. 3. By using Array.include() function.

Scaler

scaler.com › topics › typescript › strictnullchecks

StrictNullChecks in TypeScript - Scaler Topics

February 8, 2023 - TypeScript 2.0 introduces this option. We activate it by adding strictNullChecks: true to the tsconfig.json file, which assures that the compiler will throw an error if we assign Null and Undefined to other types.

Honlsoft

honlsoft.com › blog › 2021-07-20-typescript-tips-null-coalescing

Typescript Tips: null(and undefined) Checking | Honlsoft

console.log(null || "hello") // Outputs: hello console.log(undefined || "hello") // Outputs: hello console.log("world" || "hello") // Outputs: world console.log("" || "hello") // Outputs: hello console.log(0 || "hello") // Outputs: hello · If you've been using Typescript for some time you'll likely have come across both of these ways to deal with nulls.

Tektutorialshub

tektutorialshub.com › home › typescript › null vs undefined in typescript

Null Vs Undefined in TypeScript - Tektutorialshub

March 15, 2023 - But they are neither false nor true. You can use typeof operator to check for undefined but not null as it returns “object”. You can use the == & === operator to check their values

TypeScript

typescriptlang.org › tsconfig › strictNullChecks.html

TypeScript: TSConfig Option: strictNullChecks

When strictNullChecks is false, null and undefined are effectively ignored by the language.

Stack Overflow

stackoverflow.com › questions › 39056898 › easiest-way-to-check-for-null-and-empty-string-on-a-typescript-number

Easiest way to check for null and empty string on a TypeScript number - Stack Overflow

Top answer

1 of 5

You can simply use typeof. It will check undefined, null, 0 and "" also.

if(typeof RetailPrice!='undefined' && RetailPrice){
   return this.RetailPrice;
}

2 of 5

To excludes blank strings as well

if(this.retailPrice && this.retailPrice.trim()){
   //Implement your logic here
}

Gitbooks

hamednourhani.gitbooks.io › typescript-book › content › docs › tips › null.html

null is bad · typescript-book

When creating your own APIs its okay to use null in this case for consistency. In all sincerity for your own APIs you should look at promises, in that case you actually don't need to bother with absent error values (you handle them with .then vs. .catch). ... function toInt(str: string): { valid: boolean, int?: number } { const int = parseInt(str); if (isNaN(int)) { return { valid: false }; } else { return { valid: true, int }; } }