Is there a "null coalescing" operator?

03/27/2020 03:00:01

Is there a null coalescing operator in Javascript?

For example, in C#, I can do this:

String someString = null;
var whatIWant = someString ?? "Cookies!";

The best approximation I can figure out for Javascript is using the conditional operator:

var someString = null;
var whatIWant = someString ? someString : 'Cookies!';

Which is sorta icky IMHO. Can I do better?

Verified Answer (2101 Votes)

01/24/2009 23:25:31

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"
2101

Answer #2 (76 Votes)

03/08/2014 10:24:46
function coalesce() {
    var len = arguments.length;
    for (var i=0; i<len; i++) {
        if (arguments[i] !== null && arguments[i] !== undefined) {
            return arguments[i];
        }
    }
    return null;
}

var xyz = {};
xyz.val = coalesce(null, undefined, xyz.val, 5);

// xyz.val now contains 5

this solution works like the SQL coalesce function, it accepts any number of arguments, and returns null if none of them have a value. It behaves like the C# ?? operator in the sense that "", false, and 0 are considered NOT NULL and therefore count as actual values. If you come from a .net background, this will be the most natural feeling solution.

76

Answer #3 (59 Votes)

05/07/2018 14:32:28

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
59
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