TypeError: 'x' is not iterable
The JavaScript exception "is not iterable" occurs when the value which is given as the
right hand-side of for...of
or as argument of a function such as Promise.all
or TypedArray.from
,
is not an iterable object.
Message
TypeError: 'x' is not iterable (Firefox, Chrome)
TypeError: 'x' is not a function or its return value is not iterable (Chrome)
Error type
What went wrong?
The value which is given as the right hand-side of for...of
or as argument of a function such as Promise.all
or TypedArray.from
,
is not an iterable object.
An iterable can be a built-in iterable type such as
Array
, String
or Map
, a generator result, or
an object implementing the iterable protocol.
Examples
Iterating over Object properties
In JavaScript, Object
s are not iterable unless they implement
the iterable protocol.
Therefore, you cannot use for...of
to iterate over the properties of an object.
var obj = { 'France': 'Paris', 'England': 'London' };
for (let p of obj) { // TypeError: obj is not iterable
// …
}
Instead you have to use Object.keys
or Object.entries
, to
iterate over the properties or entries of an object.
var obj = { 'France': 'Paris', 'England': 'London' };
// Iterate over the property names:
for (let country of Object.keys(obj)) {
var capital = obj[country];
console.log(country, capital);
}
for (const [country, capital] of Object.entries(obj))
console.log(country, capital);
Another option for this use case might be to use a Map
:
var map = new Map;
map.set('France', 'Paris');
map.set('England', 'London');
// Iterate over the property names:
for (let country of map.keys()) {
let capital = map[country];
console.log(country, capital);
}
for (let capital of map.values())
console.log(capital);
for (const [country, capital] of map.entries())
console.log(country, capital);
Iterating over a generator
Generators are functions you call to produce an iterable object.
function* generate(a, b) {
yield a;
yield b;
}
for (let x of generate) // TypeError: generate is not iterable
console.log(x);
When they are not called, the Function
object corresponding to the
generator is callable, but not iterable. Calling a generator produces an iterable
object which will iterate over the values yielded during the execution of the
generator.
function* generate(a, b) {
yield a;
yield b;
}
for (let x of generate(1,2))
console.log(x);
Iterating over a custom iterable
Custom iterables can be created by implementing the
Symbol.iterator
method. You must be certain that your iterator method
returns an object which is an iterator, which is to say it must have a next method.
const myEmptyIterable = {
[Symbol.iterator]() {
return [] // [] is iterable, but it is not an iterator -- it has no next method.
}
}
Array.from(myEmptyIterable); // TypeError: myEmptyIterable is not iterable
Here is a correct implementation:
const myEmptyIterable = {
[Symbol.iterator]() {
return [][Symbol.iterator]()
}
}
Array.from(myEmptyIterable); // []