Message
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 instad 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"