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
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
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.
const iterable = [10, 20, 30];
for (let value of iterable) {
value += 50;
console.log(value);
}
// 60
// 70
// 80