JS: Null-Coalescing Operator: Modern JavaScript allows ?? for null & undefined | JavaScript
JavaScript ?? Null-Coalescing Operator
In modern versions of JavaScript, Null-Coalescing Operator ?? results the next operand if the previous operand is either null or undefined.?? overcomes the backdraws of || in older versions of JavaScript to achieve ?? functionality.?? takes two arguments and can be nested.
var x = null;
var y = 3;
var z = x ?? y;
console.log(z);
3
?? reulsts 2nd (alternate) operand y because the first operand null.
var x;
var y = 'y-value';
var z = x ?? y;
console.log(z);
y-value
multiple expressions can be given in a single statement for ?? operator. the first non-null or non-undefined value is considered.
var x = null;
var y = undefined;
var z = x ?? y ?? 'other-value';
console.log(z);
other-value
Since x and y are null and undefined, respectively, so 'other-value' is assigned to z.
falsy other than null & undefined
falsy statement other than null & undefined are considered i.e. 0, false and '' etc.
var x = 0;
var y = undefined;
var z = x ?? y;
console.log(z);
0
var x = false;
var y = null;
var z = x ?? y;
console.log(z);
false
var x = ''; //Empty string
var y = null;
var z = x ?? y;
console.log(z);
var x = NaN; //Not a Number
var y = null;
var z = x ?? y;
console.log(z);
NaN
?? alternate in older versions of JavaScript
Null-Coalescing Operator (??) is not available in older versions of JavaScript. This functionality can be achieved with || upto some extent with some drawbacks.
var x = null;
var y = 3;
var z = x || y;
Demerits of Null-Coalescing using ||
In JavaScript, || acts as OR operator, since || is a conditional operator and returns the next operand (value) when first fails.
In case, if the first operand is falsy i.e. (zero 0, empty string '', false, null, undefined etc.), instead of returning first operand (value) returns the second.
Please have a look at falsy chapter of this JavaScript tutorial.
If the code expect other falsy values, then replacing the following line may work as required.
var z = x == null ? y : x;
Summary
- JavaScript
||may work as??of C# for with some limitations - Falsy expressions may cause buggy code, need for write code carefully