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)

SyntaxError.

The head of afor...in loop contains an initializer expression.That is, a variable is declared and assigned a valuefor (var i = 0 in obj).In non-strict mode, this head declaration is silently ignored and behaves likefor (var i in obj).Instrict mode, however, aSyntaxError is thrown. In addition, lexical declarations with initializers likefor (const i = 0 in obj) are not allowed outside strict mode either, and will always produce aSyntaxError.

This example throws aSyntaxError:

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

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]);}

The for...in loopshouldn't be used for Array iteration.Did you intend to use afor loopinstead of afor-in loop to iterate anArray? Thefor 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"

• for...in
• for...of
• for