Daniel Schaffer Daniel Schaffer - 10 months ago 45
Javascript Question

Is there a "null coalescing" operator in JavaScript?

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?


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.


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"