The Double question mark is called the nullish coalescing operator in JavaScript. It allows you to provide a default value to use when a variable expression evaluates to null or undefined.
leftExpr ?? rightExprIf firstValue is null or undefined
console.log(firstValue ?? secondValue) // secondValueIf firstValue isn’t null or undefined
console.log(firstValue ?? secondValue) // firstValueA double question mark in JavaScript
Simple example code using a double question mark (??). It returns the expression on the right side of the mark when the expression on the left side is null or undefined.
<!DOCTYPE html>
<html>
<body>
<script>
let firstName = null;
let username = firstName ?? "Rokcy";
console.log(username);
//or
let name = undefined ?? "Rokcy";
console.log(name);
</script>
</body>
</html>
Output:
JavaScript evaluates an empty string to false as in the code below:
let firstName = ""; // empty string evaluates to false in JavaScript
let username = firstName ?? "Guest";
console.log(username); // ""More examples
const foo = null ?? 'default string';
console.log(foo);
// output: "default string"
const baz = 0 ?? 42;
console.log(baz);
// output: 0Do comment if you have any doubts or suggestions on this JS basic code.
Note: The All JS Examples codes are tested on the Firefox browser and the Chrome browser.
OS: Windows 10
Code: HTML 5 Version
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