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Nullish Coalescing Operator

Nullish Coalescing Operator (??)

Introduction

The nullish coalescing operator ?? returns the right-hand operand only when the left-hand operand is null or undefined. Unlike ||, it doesn’t treat 0, '', or false as falsy.

Why Do We Need This?

The || operator treats 0, '', and false as falsy and replaces them with the default. But sometimes these are valid values — ?? solves this problem.

const count = 0;
count || 10;    // 10 — wrong! 0 is valid
count ?? 10;    // 0 — correct

Visual Explanation

flowchart TD
    Left["Left operand"]
    Check{"Is left null or undefined?"}
    Yes["Return right operand"]
    No["Return left operand"]

    Left --> Check
    Check -->|Yes| Yes
    Check -->|No| No

Examples

null ?? 'default';      // 'default'
undefined ?? 'default'; // 'default'
0 ?? 'default';         // 0 — preserved!
'' ?? 'default';        // '' — preserved!
false ?? 'default';     // false — preserved!

Cannot Chain with && or ||

null || undefined ?? 'default'; // SyntaxError!
// Use parentheses:
(null || undefined) ?? 'default'; // 'default'

Summary

?? checks for null/undefined only
Use instead of || when 0, '', false are valid values
Cannot mix with && or || without parentheses

Further Reading

MDN: Nullish Coalescing