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"

if( value ) {
}

will evaluate to true if value is not:

• null
• undefined
• NaN
• empty string ''
• 0
• false

typescript includes javascript rules.

In TypeScript 3.7 we have now Optional chaining and Nullish Coalescing to check null and undefined in the same time, example:

let x = foo?.bar.baz();

this code will check if foo is defined otherwise it will return undefined

old way :

if(foo != null && foo != undefined) {
   x = foo.bar.baz();
} 

this:

let x = (foo === null || foo === undefined) ? undefined : foo.bar();

if (foo && foo.bar && foo.bar.baz) { // ... }

With optional chaining will be:

let x = foo?.bar();

if (foo?.bar?.baz) { // ... }

another new feature is Nullish Coalescing, example:

let x = foo ?? bar(); // return foo if it's not null or undefined otherwise calculate bar

old way:

let x = (foo !== null && foo !== undefined) ?
foo :
bar();

BONUS

Does TypeScript has dedicated function or syntax sugar for this

TypeScript fully understands the JavaScript version which is something == null.

TypeScript will correctly rule out both null and undefined with such checks.

More

https://basarat.gitbook.io/typescript/recap/null-undefined

SIMPLE ANSWER

Although Typescript is a strongly typed language, it has the same problems with pointers and variables initialization inherited from Javascript.
Javascript doesn't check whether a variable exists in the context, the so common undefined status.

to evaluate if value ISN'T null,undefined,0,false,"", or NaN:

if ( value )
or
if ( !!value )

for negative conditional, check if the value is null,undefined,0,false,"",or NaN:

if ( !value )

to test if is null or undefined:

if ( value == null )

to test only null:

if ( value === null )

to test only undefined:

if ( value === undefined )

MORE DETAILED ANSWER

1- It will evaluate to true if value is not: null, undefined, NaN, empty string '', 0, false
If the value is null,undefined,NaN,empty string,0, or false, will go to the else condition.

if ( value ) {
  console.log('value is something different from 0, "", false, NaN, null, undefined');
} else {
  console.log('value is 0, "", false, NaN, null or undefined');
}
if ( !!value ) {
  console.log('value is something different from 0, "", false, NaN, null, undefined');
} else {
  console.log('value is 0, "", false, NaN, null or undefined');
}

2- If you want a negative condition, then you'll need to use:

if ( !value ) {
  console.log('value is 0, "", false, NaN, null or undefined');
} else {
  console.log('value is something different from 0, "", false, NaN, null, undefined');
}

3- It will evaluate if value is null or undefined

if ( value == null ) {
  console.log('is null or undefined');
} else {
  console.log('it isnt null neither undefined');
}

4- Using a test with boolean values doesn't work.
It will NOT evaluate to true neither to false if value is null, undefined, 0, empty string, NaN
Both conditions will always go to the else condition.
With the exception if value is a boolean variable.

if ( value==true ) {
} else { 
}
if ( value==false ) {
} else { 
}

I did different tests on the typescript playground:

http://www.typescriptlang.org/play/

let a;
let b = null;
let c = "";
var output = "";

if (a == null) output += "a is null or undefined\n";
if (b == null) output += "b is null or undefined\n";
if (c == null) output += "c is null or undefined\n";
if (a != null) output += "a is defined\n";
if (b != null) output += "b is defined\n";
if (c != null) output += "c is defined\n";
if (a) output += "a is defined (2nd method)\n";
if (b) output += "b is defined (2nd method)\n";
if (c) output += "c is defined (2nd method)\n";

console.log(output);

gives:

a is null or undefined
b is null or undefined
c is defined

so:

• checking if (a == null) is right to know if a is null or undefined
• checking if (a != null) is right to know if a is defined
• checking if (a) is wrong to know if a is defined

You may want to try

if(!!someValue)

with !!.

Explanation

The first ! will turn your expression into a boolean value.

Then !someValue is true if someValue is falsy and false if someValue is truthy. This might be confusing.

By adding another !, the expression is now true if someValue is truthy and false if someValue is falsy, which is much easier to manage.

Discussion

Now, why do I bother myself with if (!!someValue) when something like if (someValue) would have give me the same result?

Because !!someValue is precisely a boolean expression, whereas someValue could be absolutely anything. This kind of expression will now alow to write functions (and God we need those) like:

isSomeValueDefined(): boolean {
  return !!someValue
}

instead of:

isSomeValueDefined(): boolean {
  if(someValue) {
    return true
  }
  return false
}

I hope it helps.

For Typescript 2.x.x you should do it in a following way(using type guard):

tl;dr

function isDefined<T>(value: T | undefined | null): value is T {
  return <T>value !== undefined && <T>value !== null;
}

Why?

In this way isDefined() will respect variable's type and the following code would know take this check in account.

Example 1 - basic check:

function getFoo(foo: string): void { 
  //
}

function getBar(bar: string| undefined) {   
  getFoo(bar); //ERROR: "bar" can be undefined
  if (isDefined(bar)) {
    getFoo(bar); // Ok now, typescript knows that "bar' is defined
  }
}

Example 2 - types respect:

function getFoo(foo: string): void { 
  //
}

function getBar(bar: number | undefined) {
  getFoo(bar); // ERROR: "number | undefined" is not assignable to "string"
  if (isDefined(bar)) {
    getFoo(bar); // ERROR: "number" is not assignable to "string", but it's ok - we know it's number
  }
}

I think this answer needs an update, check the edit history for the old answer.

Basically, you have three deferent cases null, undefined, and undeclared, see the snippet below.

// bad-file.ts
console.log(message)

You'll get an error says that variable message is undefined (aka undeclared), of course, the Typescript compiler shouldn't let you do that but REALLY nothing can prevent you.

// evil-file.ts
// @ts-gnore
console.log(message)

The compiler will be happy to just compile the code above. So, if you're sure that all variables are declared you can simply do that

if ( message != null ) {
    // do something with the message
}

the code above will check for null and undefined, BUT in case the message variable may be undeclared (for safety), you may consider the following code

if ( typeof(message) !== 'undefined' && message !== null ) {
    // message variable is more than safe to be used.
}

Note: the order here typeof(message) !== 'undefined' && message !== null is very important you have to check for the undefined state first atherwise it will be just the same as message != null, thanks @Jaider.

if(data){}

it's mean !data

• null
• undefined
• false
• ....

UPDATE (Sept 4, 2020)

You can now use the ?? operator to validate null and undefined "values" and set a default value. For example:

const foo = null;
const bar = foo ?? 'exampleValue';
console.log(bar); // This will print 'exampleValue' due to the value condition of the foo constant, in this case, a null value

As a verbose way, if you want to compare null and undefined values ONLY, use the following example code for reference:

const incomingValue : string = undefined;
const somethingToCompare : string = incomingValue; // If the line above is not declared, TypeScript will return an excepion

if (somethingToCompare == (undefined || null)) {
  console.log(`Incoming value is: ${somethingToCompare}`);
}

If incomingValue is not declared, TypeScript should return an exception. If this is declared but not defined, the console.log() will return "Incoming value is: undefined". Note we are not using the strict equals operator.

The "correct" way (check the other answers for details), if the incomingValue is not a boolean type, just evaluate if its value is true, this will be evaluated according to the constant/variable type. A true string have to be defined explicitly as string using the = '' assignation. If not, it will be evaluated as false. Let's check this case using the same context:

const incomingValue : string = undefined;
const somethingToCompare0 : string = 'Trumpet';
const somethingToCompare1 : string = incomingValue;

if (somethingToCompare0) {
  console.log(`somethingToCompare0 is: ${somethingToCompare0}`); // Will return "somethingToCompare0 is: Trumpet"
}

// Now, we will evaluate the second constant
if (somethingToCompare1) {
  console.log(`somethingToCompare1 is: ${somethingToCompare1}`); // Launched if incomingValue is defined
} else {
  console.log(`somethingToCompare1 is: ${somethingToCompare1}`); // Launched if incomingValue is undefined. Will return "somethingToCompare1 is: undefined"
}

If you want to pass tslint without setting strict-boolean-expressions to allow-null-union or allow-undefined-union, you need to use isNullOrUndefined from node's util module or roll your own:

// tslint:disable:no-null-keyword
export const isNullOrUndefined =
  <T>(obj: T | null | undefined): obj is null | undefined => {
    return typeof obj === "undefined" || obj === null;
  };
// tslint:enable:no-null-keyword

Not exactly syntactic sugar but useful when your tslint rules are strict.

If you are using TypeScript, it is a better approach to let the compiler check for nulls and undefineds (or the possibility thereof), rather than checking for them at run-time. (If you do want to check at run-time, then as many answers indicate, just use value == null).

Use the compile option strictNullChecks to tell the compiler to choke on possible null or undefined values. If you set this option, and then there is a situation where you do want to allow null and undefined, you can define the type as Type | null | undefined.

It can be done like this:

 // value is the value which you want to check
 [undefined, null].includes(value)

The simplest way is to use:

import { isNullOrUndefined } from 'util';

and than:

if (!isNullOrUndefined(foo))

There are several ways to do it. The most important is to be consistent across your code base. And consider all the pros and cons per case.

With !!, example:

if (!!value) {
  doSomething();
}

Using !! is the shortest (in terms of amount of symbols). But it has several serious flaws:

• It won't work with numbers and booleans. The same goes for javascript, so it's not typescript specific actually.
• It won't work with numeric enums. This one is more tricky and might lead to bugs. especially if enum type has bad naming. It's really easy to confuse it with an object, and voila, bug.
• It makes code less intuitive and less readable. Yes, any experienced js/ts dev will understand it. But I've had complaints about using it on code reviews from time to time, and couldn't disagree. With cross functional teams and multilingual projects this might be even more spot on.

With != null, example:

if (value != null) {
  doSomething();
}

Using != null is the best option which would check for null and undefined in one hit. It's more readable than !! but still has "not intuitive enough" flaw for cross functional teams. I would lean to this option, because it has the least amount of drawbacks.

Using lodash / other library's / self-written isDefined method, example:

if (isDefined(value)) {
  doSomething();
}

It's the most readable approach, and if done properly it won't increase your bundle size that much. But it also has it's flaws. Using it for single checks will certainly work. But what if you need to do it in cycle? It would be much slower than using a juggling-check.

Since TypeScript is a typed superset of ES6 JavaScript. And lodash are a library of javascript.

Using lodash to checks if value is null or undefined can be done using _.isNil().

_.isNil(value)

Arguments

value (*): The value to check.

Returns

(boolean): Returns true if value is nullish, else false.

Example

_.isNil(null);
// => true

_.isNil(void 0);
// => true

_.isNil(NaN);
// => false

Link

Lodash Docs

We use a helper hasValue that both checks for nulls/undefined and ensures via TypeScript that unnecessary checks are not performed. (The latter is similar to how TS would complain about if ("a" === undefined), since it is always false).

Using this consistently is always safe, unlike !val which matches empty strings, zero, etc. It also avoid the use of fuzzy == matching which is almost always a bad practice - no need to introduce an exception.

type NullPart<T> = T & (null | undefined);

// Ensures unnecessary checks aren't performed - only a valid call if 
// value could be nullable *and* could be non-nullable
type MustBeAmbiguouslyNullable<T> = NullPart<T> extends never
  ? never
  : NonNullable<T> extends never
  ? never
  : T;

export function hasValue<T>(
  value: MustBeAmbiguouslyNullable<T>,
): value is NonNullable<MustBeAmbiguouslyNullable<T>> {
  return (value as unknown) !== undefined && (value as unknown) !== null;
}

export function hasValueFn<T, A>(
  value: MustBeAmbiguouslyNullable<T>,
  thenFn: (value: NonNullable<T>) => A,
): A | undefined {
  // Undefined matches .? syntax result
  return hasValue(value) ? thenFn(value) : undefined;
}

May be to late! but you can use ?? operator in typescript. see https://mariusschulz.com/blog/nullish-coalescing-the-operator-in-typescript

You can do this easily with a ternary operator and the new nullish coalesce operator.

First: check to see if it is true using the ternary. If so return false so the if statement does not run.

Second: because you now know the value is falsey, you can use the nullish coalesce operator to return true if it is nullish. Since it will return itself for any other value, if it is not nullish it will fail the if statement correctly.

let x = true;
console.log("starting tests")

if (x?false:x ?? true){
  console.log(x,"is nullish")
}

x = false
if (x?false:x ?? true){
  console.log(x,"is nullish")
}

x = 0;
if (x?false:x ?? true){
  console.log(x,"is nullish")
}

x=1;
if (x?false:x ?? true){
  console.log(x,"is nullish")
}

x="";
if (x?false:x ?? true){
  console.log(x,"is nullish")
}

x="hello world";
if (x?false:x ?? true){
  console.log(x,"is nullish")
}

x=null;
if (x?false:x ?? true){
  console.log(x,"is nullish")
}

x=undefined;
if (x?false:x ?? true){
  console.log(x,"is nullish")
}

All,

The answer with the most votes, does not really work if you are working with an object. In that case, if a property is not present, the check will not work. And that was the issue in our case: see this sample:

var x =
{ name: "Homer", LastName: "Simpson" };

var y =
{ name: "Marge"} ;

var z =
{ name: "Bart" , LastName: undefined} ;

var a =
{ name: "Lisa" , LastName: ""} ;

var hasLastNameX = x.LastName != null;
var hasLastNameY = y.LastName != null;
var hasLastNameZ = z.LastName != null;
var hasLastNameA = a.LastName != null;



alert (hasLastNameX + ' ' + hasLastNameY + ' ' + hasLastNameZ + ' ' + hasLastNameA);

var hasLastNameXX = x.LastName !== null;
var hasLastNameYY = y.LastName !== null;
var hasLastNameZZ = z.LastName !== null;
var hasLastNameAA = a.LastName !== null;

alert (hasLastNameXX + ' ' + hasLastNameYY + ' ' + hasLastNameZZ + ' ' + hasLastNameAA);

Outcome:

true , false, false , true (in case of !=)
true , true, true, true (in case of !==) => so in this sample not the correct answer

plunkr link: https://plnkr.co/edit/BJpVHD95FhKlpHp1skUE

I had this issue and some of the answer work just fine for JS but not for TS here is the reason.

//JS
let couldBeNullOrUndefined;
if(couldBeNullOrUndefined == null) {
  console.log('null OR undefined', couldBeNullOrUndefined);
} else {
  console.log('Has some value', couldBeNullOrUndefined);
}

That is all good as JS has no Types

//TS
let couldBeNullOrUndefined?: string | null; // THIS NEEDS TO BE TYPED AS undefined || null || Type(string)

if(couldBeNullOrUndefined === null) { // TS should always use strict-check
  console.log('null OR undefined', couldBeNullOrUndefined);
} else {
  console.log('Has some value', couldBeNullOrUndefined);
}

In TS if the variable wasn't defined with null when you try to check for that null the tslint | compiler will complain.

//tslint.json
...
"triple-equals":[true],
...

 let couldBeNullOrUndefined?: string; // to fix it add | null

 Types of property 'couldBeNullOrUndefined' are incompatible.
      Type 'string | null' is not assignable to type 'string | undefined'.
        Type 'null' is not assignable to type 'string | undefined'.

Late to join this thread but I find this JavaScript hack very handy in checking whether a value is undefined

 if(typeof(something) === 'undefined'){
   // Yes this is undefined
 }

Usually I do the juggling-check as Fenton already discussed. To make it more readable, you can use isNil from ramda.

import * as isNil from 'ramda/src/isNil';

totalAmount = isNil(totalAmount ) ? 0 : totalAmount ;

careful if you're using local storage, you can end up with the string undefined rather than the value undefined:

localStorage.setItem('mykey',JSON.stringify(undefined));
localStorage.getItem('mykey') === "undefined"
true

People may find this useful: https://github.com/angular/components/blob/master/src/cdk/coercion/boolean-property.spec.ts

/**
 * @license
 * Copyright Google LLC All Rights Reserved.
 *
 * Use of this source code is governed by an MIT-style license that can be
 * found in the LICENSE file at https://angular.io/license
 */

/** Coerces a data-bound value (typically a string) to a boolean. */
export function coerceBooleanProperty(value: any): boolean {
  return value != null && `${value}` !== 'false';
}

import {coerceBooleanProperty} from './boolean-property';

describe('coerceBooleanProperty', () => {

  it('should coerce undefined to false', () => {
    expect(coerceBooleanProperty(undefined)).toBe(false);
  });

  it('should coerce null to false', () => {
    expect(coerceBooleanProperty(null)).toBe(false);
  });

  it('should coerce the empty string to true', () => {
    expect(coerceBooleanProperty('')).toBe(true);
  });

  it('should coerce zero to true', () => {
    expect(coerceBooleanProperty(0)).toBe(true);
  });

  it('should coerce the string "false" to false', () => {
    expect(coerceBooleanProperty('false')).toBe(false);
  });

  it('should coerce the boolean false to false', () => {
    expect(coerceBooleanProperty(false)).toBe(false);
  });

  it('should coerce the boolean true to true', () => {
    expect(coerceBooleanProperty(true)).toBe(true);
  });

  it('should coerce the string "true" to true', () => {
    expect(coerceBooleanProperty('true')).toBe(true);
  });

  it('should coerce an arbitrary string to true', () => {
    expect(coerceBooleanProperty('pink')).toBe(true);
  });

  it('should coerce an object to true', () => {
    expect(coerceBooleanProperty({})).toBe(true);
  });

  it('should coerce an array to true', () => {
    expect(coerceBooleanProperty([])).toBe(true);
  });
});

you can use

if(x === undefined)

A faster and shorter notation for null checks can be:

value == null ? "UNDEFINED" : value

This line is equivalent to:

if(value == null) {
       console.log("UNDEFINED")
} else {
    console.log(value)
}

Especially when you have a lot of null check it is a nice short notation.