遞減運算子（--）
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遞減運算子（--）遞減（減一）它的運算元並將結果回傳。
嘗試一下
let x = 3;
const y = x--;
console.log(`x:${x}, y:${y}`);
// Expected output: "x:2, y:3"
let a = 3;
const b = --a;
console.log(`a:${a}, b:${b}`);
// Expected output: "a:2, b:2"
語法
js
x--
--x
詳細說明
若將遞減運算子作為後綴（例如 x--），則會先回傳原本的值，在進行遞減。
若作為前綴（例如 --x），則會先進行遞減，在將遞減後的結果回傳。
範例
遞減運算子置於後綴
js
let x = 3;
y = x--;
// y = 3
// x = 2
遞減運算子置於前綴
js
let a = 2;
b = --a;
// a = 1
// b = 1
規範
| Specification |
|---|
| ECMAScript® 2026 Language Specification # sec-postfix-decrement-operator |