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

The JavaScript strict mode-only exception "for-in loop head declarations may not have initializers" occurs when the head of a for...in contains an initializer expression, such as |for (var i = 0 in obj)|. This is not allowed in for-of loops in strict mode.

Message

SyntaxError: for-in loop head declarations cannot have an initializer (Edge)
SyntaxError: for-in loop head declarations may not have initializers (Firefox)
SyntaxError: for-in loop variable declaration may not have an initializer. (Chrome)

Error type

SyntaxError in strict mode only.

What went wrong?

The head of a for...in loop contains an initializer expression. That is, a variable is declared and assigned a value |for (var i = 0 in obj)|. In non-strict mode, this head declaration is silently ignored and behaves like |for (var i in obj)|. In strict mode, however, a SyntaxError is thrown.

Examples

This example throws a SyntaxError:

"use strict";

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

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

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

Valid for-in loop

You can remove the initializer (i = 0) in the head of the for-in loop.

"use strict";

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

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

Array iteration

The for...in loop shouldn't be used for Array iteration. Did you intend to use a for loop instead of a for-in loop to iterate an Array? The for loop allows you to set an initializer then as well:

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

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

// "c"