The OR operator || uses the right value if left is falsy, while the nullish coalescing operator ?? uses the right value if left is null or undefined.
These operators are often used to provide a default value if the first one is missing.
But the OR operator || can be problematic if your left value might contain "" or 0 or false (because these are falsy values):
console.log(12 || "not found") // 12
console.log(0 || "not found") // "not found"
console.log("jane" || "not found") // "jane"
console.log("" || "not found") // "not found"
console.log(true || "not found") // true
console.log(false || "not found") // "not found"
console.log(undefined || "not found") // "not found"
console.log(null || "not found") // "not found"
In many cases, you might only want the right value if left is null or undefined. That's what the nullish coalescing operator ?? is for:
console.log(12 ?? "not found") // 12
console.log(0 ?? "not found") // 0
console.log("jane" ?? "not found") // "jane"
console.log("" ?? "not found") // ""
console.log(true ?? "not found") // true
console.log(false ?? "not found") // false
console.log(undefined ?? "not found") // "not found"
console.log(null ?? "not found") // "not found"
While the ?? operator isn't available in current LTS versions of Node (v10 and v12), you can use it with some versions of TypeScript or Node:
The ?? operator was added to TypeScript 3.7 back in November 2019.
And more recently, the ?? operator was included in ES2020, which is supported by Node 14 (released in April 2020).
When the nullish coalescing operator ?? is supported, I typically use it instead of the OR operator || (unless there's a good reason not to).
Videos
The OR operator || uses the right value if left is falsy, while the nullish coalescing operator ?? uses the right value if left is null or undefined.
These operators are often used to provide a default value if the first one is missing.
But the OR operator || can be problematic if your left value might contain "" or 0 or false (because these are falsy values):
console.log(12 || "not found") // 12
console.log(0 || "not found") // "not found"
console.log("jane" || "not found") // "jane"
console.log("" || "not found") // "not found"
console.log(true || "not found") // true
console.log(false || "not found") // "not found"
console.log(undefined || "not found") // "not found"
console.log(null || "not found") // "not found"
In many cases, you might only want the right value if left is null or undefined. That's what the nullish coalescing operator ?? is for:
console.log(12 ?? "not found") // 12
console.log(0 ?? "not found") // 0
console.log("jane" ?? "not found") // "jane"
console.log("" ?? "not found") // ""
console.log(true ?? "not found") // true
console.log(false ?? "not found") // false
console.log(undefined ?? "not found") // "not found"
console.log(null ?? "not found") // "not found"
While the ?? operator isn't available in current LTS versions of Node (v10 and v12), you can use it with some versions of TypeScript or Node:
The ?? operator was added to TypeScript 3.7 back in November 2019.
And more recently, the ?? operator was included in ES2020, which is supported by Node 14 (released in April 2020).
When the nullish coalescing operator ?? is supported, I typically use it instead of the OR operator || (unless there's a good reason not to).
In short
The Nullish Coalescing Operator ?? distinguishes between:
- nullish values (
null,undefined) - falsey but defined values (
false,0,''etc.)
|| (logical OR) treats both of these the same.
I created a simple graphic to illustrate the relationship of nullish and falsey values in JavaScript:
Further explanation:
let x, y
x = 0
y = x || 'default' // y = 'default'
y = x ?? 'default' // y = 0
As seen above, the difference between the operators ?? and || is that one is checking for nullish values and one is checking for falsey values. However, there are many instances where they behave the same. That is because in JavaScript every nullish value is also falsey (but not every falsey value is nullish).
Using what we learned above we can create a few examples for different behavior:
let y
y = false || 'default' // y = 'default'
y = false ?? 'default' // y = false
y = 0n || 'default' // y = 'default'
y = 0n ?? 'default' // y = 0n
y = NaN || 'default' // y = 'default'
y = NaN ?? 'default' // y = NaN
y = '' || 'default' // y = 'default'
y = '' ?? 'default' // y = ''
Since the new Nullish Coalescing Operator can differentiate between no value and a falsey value, it can be beneficial if you, for example, need to check if there is no string or an empty string. Generally, you probably want to use ?? instead of || most of the time.
Last and also least here are the two instances where they behave the same:
let y
y = null || 'default' // y = 'default'
y = null ?? 'default' // y = 'default'
y = undefined || 'default' // y = 'default'
y = undefined ?? 'default' // y = 'default'
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 (||):
Copyvar 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:
Copyalert(Boolean(null)); // false
alert(Boolean(undefined)); // false
alert(Boolean(0)); // false
alert(Boolean("")); // false
alert(Boolean("false")); // true -- gotcha! :)
This means:
Copyvar 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"
Copyfunction 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.