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-in loops in strict mode. In addition, lexical declarations with initializers like for (const i = 0 in obj) are not allowed outside strict mode either.

Message

SyntaxError: for-in loop variable declaration may not have an initializer. (V8-based)
SyntaxError: for-in loop head declarations may not have initializers (Firefox)
SyntaxError: a lexical declaration in the head of a for-in loop can't have an initializer (Firefox)
SyntaxError: Cannot assign to the loop variable inside a for-in loop header. (Safari)

Error type

SyntaxError.

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. In addition, lexical declarations with initializers like for (const i = 0 in obj) are not allowed outside strict mode either, and will always produce a SyntaxError.

Examples

This example throws a 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

Valid for-in loop

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

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

for (const 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:

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

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

// "c"