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 (var i = 0 of iterable)
|. This is not allowed in for-of loops.
SyntaxError: for-of loop head declarations cannot have an initializer (Edge) SyntaxError: a declaration in the head of a for-of loop can't have an initializer (Firefox) SyntaxError: for-of loop variable declaration may not have an initializer. (Chrome)
The head of a
for...of
loop contains an initializer expression. That is, a variable is declared and assigned a value |
for (var i = 0 of iterable)
|. This is not allowed in for-of loops. You might want a
for
loop that does allow an initializer.
for-of
loop
let iterable = [10, 20, 30];
for (let value = 50 of iterable) {
console.log(value);
}
// SyntaxError: a declaration in the head of a for-of loop can't
// have an initializer
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.
let iterable = [10, 20, 30];
for (let value of iterable) {
value += 50;
console.log(value);
}
// 60
// 70
// 80
for...of
for...in
– disallows an initializer in strict mode as well (
SyntaxError: for-in loop head declarations may not have initializers
)
for
– allows to define an initializer when iterating.