js-operator-double-question-mark.md

Style

• Normal style:

result = (a !== null && a !== undefined) ? a : b;

• Compact style:

result = a ?? b

The result of a ?? b is:

• if a is defined, then a,
• if a isn’t defined, then b.

• The OR operator

result = a || b

Question

When a || b  equals to a ?? b

Ex:

let height = 0;

alert(height || 100); // 100
alert(height ?? 100); // 0

Precedence

let height = null;
let width = null;
let area1 = height ?? 100 * width ?? 50;
let area2 = (height ?? 100) * (width ?? 50);
let area3 = height ?? (100 * width) ?? 50;

Notice:

|| returns the first truthy value.
?? returns the first defined value.