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Showing content from https://developer.mozilla.org/zh-CN/docs/Web/JavaScript/Reference/Errors/Invalid_for-in_initializer below:

SyntaxError: for-in loop head declarations may not have initializers - JavaScript

SyntaxError: for-in loop head declarations may not have initializers (Firefox)

SyntaxError: for-in loop variable declaration may not have an initializer. (Chrome)

该 SyntaxError 只出现于严格模式下。

在 for...in 循环的头部存在初始化表达式。也就是存在变量声明并且被赋值，例如 |for (var i = 0 in obj)|。在非严格模式下，这种在循环头部的变量声明会被静默忽略，语句的表现形式与 |for (var i in obj)|相同。而在严格模式下，会报语法错误。

下面这个示例会报语法错误（SyntaxError）：

const obj = { a: 1, b: 2, c: 3 };

for (const i = 0 in obj) {
  console.log(obj[i]);
}

// SyntaxError: for-in loop head declarations may not have initializers

可以把初始化语句 (i = 0) 从 for-in 循环的头部移除。

"use strict";

var obj = { a: 1, b: 2, c: 3 };

for (var i in obj) {
  console.log(obj[i]);
}

for...in 循环不应该应用于数组迭代中。是否考虑使用 for 循环而不是 for-in 循环来遍历数组（Array）？在 for 循环中是允许使用初始化语句的：

var arr = ["a", "b", "c"];

for (var i = 2; i < arr.length; i++) {
  console.log(arr[i]);
}

// "c"

• for...in
• for...of – 无论是在严格模式下还是非严格模式下也都不允许使用初始化语句。
• for – 更适用于数组迭代，因为允许使用初始化语句。

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