Update

JavaScript now supports the nullish coalescing operator (??). It returns its right-hand-side operand when its left-hand-side operand is null or undefined, and otherwise returns its left-hand-side operand.

Old Answer

Please check compatibility before using it.

The JavaScript equivalent of the C# null coalescing operator (??) is using a logical OR (||):

var whatIWant = someString || "Cookies!";

There are cases (clarified below) that the behaviour won't match that of C#, but this is the general, terse way of assigning default/alternative values in JavaScript.

Clarification

Regardless of the type of the first operand, if casting it to a Boolean results in false, the assignment will use the second operand. Beware of all the cases below:

alert(Boolean(null)); // false
alert(Boolean(undefined)); // false
alert(Boolean(0)); // false
alert(Boolean("")); // false
alert(Boolean("false")); // true -- gotcha! :)

This means:

var whatIWant = null || new ShinyObject(); // is a new shiny object
var whatIWant = undefined || "well defined"; // is "well defined"
var whatIWant = 0 || 42; // is 42
var whatIWant = "" || "a million bucks"; // is "a million bucks"
var whatIWant = "false" || "no way"; // is "false"

Yes, it is coming soon. See proposal here and implementation status here.

It looks like this:

x ?? y

Example

const response = {
  settings: {
    nullValue: null,
    height: 400,
    animationDuration: 0,
    headerText: '',
    showSplashScreen: false
  }
};

const undefinedValue = response.settings?.undefinedValue ?? 'some other default'; // result: 'some other default'
const nullValue = response.settings?.nullValue ?? 'some other default'; // result: 'some other default'
const headerText = response.settings?.headerText ?? 'Hello, world!'; // result: ''
const animationDuration = response.settings?.animationDuration ?? 300; // result: 0
const showSplashScreen = response.settings?.showSplashScreen ?? true; // result: false

If || as a replacement of C#'s ?? isn't good enough in your case, because it swallows empty strings and zeros, you can always write your own function:

 function $N(value, ifnull) {
    if (value === null || value === undefined)
      return ifnull;
    return value;
 }

 var whatIWant = $N(someString, 'Cookies!');

Logical nullish assignment, 2020+ solution

A new operator is currently being added to the browsers, ??=. This combines the null coalescing operator ?? with the assignment operator =.

NOTE: This is not common in public browser versions yet. Will update as availability changes.

??= checks if the variable is undefined or null, short-circuiting if already defined. If not, the right-side value is assigned to the variable.

Basic Examples

let a          // undefined
let b = null
let c = false

a ??= true  // true
b ??= true  // true
c ??= true  // false

Object/Array Examples

let x = ["foo"]
let y = { foo: "fizz" }

x[0] ??= "bar"  // "foo"
x[1] ??= "bar"  // "bar"

y.foo ??= "buzz"  // "fizz"
y.bar ??= "buzz"  // "buzz"

x  // Array [ "foo", "bar" ]
y  // Object { foo: "fizz", bar: "buzz" }

Browser Support Jan '22 - 89%

Mozilla Documentation

Yes, and its proposal is Stage 4 now. This means that the proposal is ready for inclusion in the formal ECMAScript standard. You can already use it in recent desktop versions of Chrome, Edge and Firefox, but we will have to wait for a bit longer until this feature reaches cross-browser stability.

Have a look at the following example to demonstrate its behavior:

// note: this will work only if you're running latest versions of aforementioned browsers
const var1 = undefined;
const var2 = "fallback value";

const result = var1 ?? var2;
console.log(`Nullish coalescing results in: ${result}`);

Previous example is equivalent to:

const var1 = undefined;
const var2 = "fallback value";

const result = (var1 !== null && var1 !== undefined) ?
    var1 :
    var2;
console.log(`Nullish coalescing results in: ${result}`);

Note that nullish coalescing will not threat falsy values the way the || operator did (it only checks for undefined or null values), hence the following snippet will act as follows:

// note: this will work only if you're running latest versions of aforementioned browsers
const var1 = ""; // empty string
const var2 = "fallback value";

const result = var1 ?? var2;
console.log(`Nullish coalescing results in: ${result}`);

For TypesScript users, starting off TypeScript 3.7, this feature is also available now.

After reading your clarification, @Ates Goral's answer provides how to perform the same operation you're doing in C# in JavaScript.

@Gumbo's answer provides the best way to check for null; however, it's important to note the difference in == versus === in JavaScript especially when it comes to issues of checking for undefined and/or null.

There's a really good article about the difference in two terms here. Basically, understand that if you use == instead of ===, JavaScript will try to coalesce the values you're comparing and return what the result of the comparison after this coalescence.

beware of the JavaScript specific definition of null. there are two definitions for "no value" in javascript. 1. Null: when a variable is null, it means it contains no data in it, but the variable is already defined in the code. like this:

var myEmptyValue = 1;
myEmptyValue = null;
if ( myEmptyValue === null ) { window.alert('it is null'); }
// alerts

in such case, the type of your variable is actually Object. test it.

window.alert(typeof myEmptyValue); // prints Object

2. Undefined: when a variable has not been defined before in the code, and as expected, it does not contain any value. like this:

   if ( myUndefinedValue === undefined ) { window.alert('it is undefined'); }
   // alerts
   

if such case, the type of your variable is 'undefined'.

notice that if you use the type-converting comparison operator (==), JavaScript will act equally for both of these empty-values. to distinguish between them, always use the type-strict comparison operator (===).

?? vs || vs &&

None of the other answers compares all three of these. Since Justin Johnson's comment has so many votes, and since double question mark vs && in javascript was marked a duplicate of this one, it makes sense to include && in an answer.

First in words, inspired by Justin Johnson's comment:

• || returns the first "truey" value, else the last value whatever it is.

• && returns the first "falsey" value, else the last value whatever it is.

• ?? returns the first non-null, non-undefined value, else the last value, whatever it is.

Then, demonstrated in live code:

let F1,
    F2 = null,
    F3 = 0,
    F4 = '',
    F5 = parseInt('Not a number (NaN)'),
    T1 = 3,
    T2 = 8

console.log( F1 || F2 || F3 || F4 || F5 || T1 || T2 ) // 3 (T1)
console.log( F1 || F2 || F3 || F4 || F5 )             // NaN (F5)

console.log( T1 && T2 && F1 && F2 && F3 && F4 && F5 ) // undefined (F1)
console.log( T1 && T2 )                               // 8 (T2)

console.log( F1 ?? F2 ?? F3 ?? F4 ?? F5 ?? T1 )       // 0 (F3)
console.log( F1 ?? F2)                                // null (F2)

Note that React's create-react-app tool-chain supports the null-coalescing since version 3.3.0 (released 5.12.2019). From the release notes:

Optional Chaining and Nullish Coalescing Operators

We now support the optional chaining and nullish coalescing operators!

// Optional chaining
a?.(); // undefined if `a` is null/undefined
b?.c; // undefined if `b` is null/undefined

// Nullish coalescing
undefined ?? 'some other default'; // result: 'some other default'
null ?? 'some other default'; // result: 'some other default'
'' ?? 'some other default'; // result: ''
0 ?? 300; // result: 0
false ?? true; // result: false

This said, in case you use create-react-app 3.3.0+ you can start using the null-coalesce operator already today in your React apps.

It will hopefully be available soon in Javascript, as it is in proposal phase as of Apr, 2020. You can monitor the status here for compatibility and support - https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Operators/Nullish_coalescing_operator

For people using Typescript, you can use the nullish coalescing operator from Typescript 3.7

From the docs -

You can think of this feature - the ?? operator - as a way to “fall back” to a default value when dealing with null or undefined. When we write code like

let x = foo ?? bar();

this is a new way to say that the value foo will be used when it’s “present”; but when it’s null or undefined, calculate bar() in its place.

There are two items here:

1. Logical OR

const foo = '' || 'default string';

console.log(foo); // output is 'default string'

2. Nullish coalescing operator

const foo = '' ?? 'default string';

console.log(foo); // output is empty string i.e. ''

The nullish coalescing operator (??) is a logical operator that returns its right-hand side operand when its left-hand side operand is null or undefined, and otherwise returns its left-hand side operand.

https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Operators/Nullish_coalescing_operator

Need to support old browser and have a object hierarchy

body.head.eyes[0]  //body, head, eyes  may be null 

may use this,

(((body||{}) .head||{}) .eyes||[])[0] ||'left eye'

ECMAScript 2021 enabled two new features:

1. Nullish coalescing operator (??) which is a logical operator that returns its right-hand side operand when its left-hand side operand is either null or undefined, and otherwise returns its left-hand side operand.

let b = undefined ?? 5;

console.log(b); // 5

2. Logical nullish assignment (x ??= y) operator which only assigns if x has a nullish value (null or undefined).

const car = {speed : 20};
car.speed ??= 5;
console.log(car.speed);
car.name ??= "reno";
console.log(car.name);

More about Logical nullish assignment can be found here https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Operators/Logical_nullish_assignment

More about Nullish coalescing operator can be found here https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Operators/Nullish_coalescing_operator

Now it has full support in latest version of major browsers like Chrome, Edge, Firefox , Safari etc. Here's the comparison between the null operator and Nullish Coalescing Operator

const response = {
        settings: {
            nullValue: null,
            height: 400,
            animationDuration: 0,
            headerText: '',
            showSplashScreen: false
        }
    };
    /* OR Operator */
    const undefinedValue = response.settings.undefinedValue || 'Default Value'; // 'Default Value'
    const nullValue = response.settings.nullValue || 'Default Value'; // 'Default Value'
    const headerText = response.settings.headerText || 'Hello, world!'; //  'Hello, world!'
    const animationDuration = response.settings.animationDuration || 300; //  300
    const showSplashScreen = response.settings.showSplashScreen || true; //  true
    /* Nullish Coalescing Operator */
    const undefinedValue = response.settings.undefinedValue ?? 'Default Value'; // 'Default Value'
    const nullValue = response.settings.nullValue ?? ''Default Value'; // 'Default Value'
    const headerText = response.settings.headerText ?? 'Hello, world!'; // ''
    const animationDuration = response.settings.animationDuration ?? 300; // 0
    const showSplashScreen = response.settings.showSplashScreen ?? true; //  false

Those who are using Babel, need to upgrade to the latest version to use nullish coalescing (??):

Babel 7.8.0 supports the new ECMAScript 2020 features by default: you don't need to enable individual plugins for nullish coalescing (??), optional chaining (?.) and dynamic import() anymore with preset-env

From https://babeljs.io/blog/2020/01/11/7.8.0