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TypeError: 'x' is not iterable

The JavaScript exception "is not iterable" occurs when the value which is spread into an array or function call, given as the right-hand side of for...of, as argument of a function such as {{jsxref("Promise.all")}} or {{jsxref("Set/Set", "Set()")}}, or as the right-hand side of an array destructuring assignment, is not an iterable object. This error is also encountered when {{jsxref("Array.fromAsync()")}} or {{jsxref("Statements/for-await...of", "for await...of")}} is used with a non-async iterable.

Message​

TypeError: Spread syntax requires ...iterable[Symbol.iterator] to be a function (V8-based & Safari)
TypeError: %Array%.from requires that the property of the first argument, items[Symbol.iterator], when exists, be a function (V8-based & Safari)
TypeError: Array.fromAsync requires that the property of the first argument, items[Symbol.asyncIterator], when exists, be a function (V8-based & Safari)
TypeError: object is not iterable (cannot read property Symbol(Symbol.iterator)) (V8-based)
TypeError: x is not async iterable (V8-based)
TypeError: x is not iterable (V8-based & Firefox)
TypeError: undefined is not a function (near '...y of x...') (Safari)
TypeError: Array.from: no function (Safari)
TypeError: Type error (Safari)

Error type​

What went wrong?​

The value which is spread into an array or function call, given as the right-hand side of for...of, or as argument of a function such as {{jsxref("Promise.all")}} or {{jsxref("Set/Set", "Set()")}}, or as the right-hand side of an array destructuring assignment, is not an iterable object. An iterable can be a built-in iterable type such as {{jsxref("Array")}}, {{jsxref("String")}} or {{jsxref("Map")}}, a generator result, or an object implementing the iterable protocol.

const nonIterable1 = {};
const nonIterable2 = { [Symbol.iterator]: 1 };

[...nonIterable1];
Math.max(...nonIterable1);
for (const x of nonIterable1);
new Set(nonIterable1);
Array.from(nonIterable2);
new Int8Array(nonIterable2);
const [] = nonIterable1;

Examples​

Array destructuring a non-iterable​

const myobj = { arrayOrObjProp1: {}, arrayOrObjProp2: [42] };

const {
  arrayOrObjProp1: [value1],
  arrayOrObjProp2: [value2],
} = myobj; // TypeError: object is not iterable

console.log(value1, value2);

The non-iterable might turn to be undefined in some runtime environments.

Iterating over Object properties​

In JavaScript, {{jsxref("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.

const obj = { France: "Paris", England: "London" };
for (const p of obj) {
  // …
} // TypeError: obj is not iterable

Instead you have to use {{jsxref("Object.keys")}} or {{jsxref("Object.entries")}}, to iterate over the properties or entries of an object.

const obj = { France: "Paris", England: "London" };
// Iterate over the property names:
for (const country of Object.keys(obj)) {
  const 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 {{jsxref("Map")}}:

const map = new Map();
map.set("France", "Paris");
map.set("England", "London");
// Iterate over the property names:
for (const country of map.keys()) {
  const capital = map.get(country);
  console.log(country, capital);
}

for (const capital of map.values()) {
  console.log(capital);
}

for (const [country, capital] of map.entries()) {
  console.log(country, capital);
}

Iterating over a generator​

Generator functions are functions you call to produce an iterable object.

function* generate(a, b) {
  yield a;
  yield b;
}

for (const x of generate) {
  console.log(x);
} // TypeError: generate is not iterable

When they are not called, the {{jsxref("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 (const x of generate(1, 2)) {
  console.log(x);
}

Iterating over a custom iterable​

Custom iterables can be created by implementing the {{jsxref("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); // []

See also​