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

TypeError

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, Objects 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);  // []