Every arithmetic, shift and bitwise operator has a compound form: x op= y means x = x op y with x evaluated once. ES2021 added short-circuiting logical assignments: a ||= b assigns only if a is falsy, a &&= b only if truthy (Truthiness), a ??= b only if nullish; otherwise the right side never runs.
const config = { timeout: 0, retries: null, title: " Home " };
config.timeout ||= 5000; // 0 is falsy: overwritten
config.retries ??= 3; // null is nullish: filled in
config.title &&= config.title.trim(); // truthy: replaced
let s = "ab", x = 7;
s += "c"; x **= 2; x %= 10;
console.log(config, s, x);{ timeout: 5000, retries: 3, title: 'Home' } abc 9Prefer ??= for defaults whenever 0, "" or false are valid values.