SyntaxError: a declaration in the head of a for-of loop can't have an initializer

The JavaScript exception "a declaration in the head of a for-of loop can't have an initializer" occurs when the head of a for...of loop contains an initializer expression such as for (const i = 0 of iterable). This is not allowed in for-of loops.

Message#

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

Error type#

{{jsxref("SyntaxError")}}

What went wrong?#

The head of a for...of loop contains an initializer expression. That is, a variable is declared and assigned a value for (const i = 0 of iterable). This is not allowed in for-of loops. You might want a for loop that does allow an initializer.

Examples#

Invalid for-of loop#

```js-nolint example-bad const iterable = [10, 20, 30];

for (const value = 50 of iterable) { console.log(value); }

// SyntaxError: a declaration in the head of a for-of loop can't // have an initializer

### Valid for-of loop

You need to remove the initializer (`value = 50`) in the head of the `for-of` loop. Maybe you intended to make 50 an offset value, in that case you could add it to the loop body, for example.

```js example-good
const iterable = [10, 20, 30];

for (let value of iterable) {
  value += 50;
  console.log(value);
}
// 60
// 70
// 80

See also#

• for...of
• for...in
• for