Accessing a non-existent array index

JavaScriptJunior~2 min read

If you access a non-existent array index, JavaScript returns undefined and no error is thrown. This is not a bug but a consequence of arrays being objects, and reading a missing property of an object in JavaScript is always safe.

Theory

TL;DR

Reading a non-existent index yields undefined, no exception is thrown.
The reason: an array is an object, indexes are string keys, and a missing property returns undefined.
undefined here means "the value is absent", not "an error occurred".
A TypeError appears only when the array itself does not exist: null or undefined.
You can check for presence with index in arr or Object.hasOwn(arr, index).
An empty slot (a hole) also yields undefined, but in returns false for it.

Quick example

javascript

const arr = ['apple', 'banana', 'cherry'];

console.log(arr[0]);  // 'apple'
console.log(arr[3]);  // undefined, there is no such element
console.log(arr[10]); // undefined, and again no error

None of these reads stops the execution of the code.

Why it works this way

In JavaScript an array is an object in which indexes are string keys:

javascript

// roughly equivalent
const arr = { '0': 'apple', '1': 'banana', '2': 'cherry', length: 3 };

When you access a non-existent property, JavaScript returns undefined instead of throwing, unlike Python with its IndexError or Java with ArrayIndexOutOfBoundsException.

The same holds for negative and fractional indexes: they simply become ordinary string keys that the object does not have.

javascript

const arr = [1, 2, 3];

console.log(arr[-1]);  // undefined, this is the key '-1', not "the last element"
console.log(arr[1.5]); // undefined
console.log(arr.at(-1)); // 3, this is how you take the last element

When you do get an error

undefined means "the value is absent", not "an error". But if you try to access an index of an array that does not exist at all, that is, the array itself is undefined or null, then you do get an exception:

javascript

const arr = undefined;
console.log(arr[0]); // TypeError: Cannot read properties of undefined

Optional chaining keeps such a read safe:

javascript

const arr = undefined;

console.log(arr?.[0]);      // undefined, no error
console.log(arr?.[0] ?? 0); // 0, a default value

How to check that an element exists

javascript

const arr = [1, 2, 3];

console.log(2 in arr); // true, the element at index 2 exists
console.log(5 in arr); // false, there is no such element

A more modern and safer alternative is Object.hasOwn():

javascript

console.log(Object.hasOwn(arr, 2)); // true
console.log(Object.hasOwn(arr, 5)); // false

The check arr[i] !== undefined is not suitable here: it cannot tell a missing element from a stored undefined value.

javascript

const values = [1, undefined, 3];

console.log(values[1] !== undefined); // false, even though the element exists
console.log(1 in values);             // true

An example with holes (empty slots)

javascript

const arr = [1, , 3]; // a skipped element
console.log(arr[1]);   // undefined
console.log(1 in arr); // false, the cell really is not there

So arr[1] gives undefined, yet the slot does not exist: it is an empty cell, not a stored undefined value.

A short table

Situation	Result	Error?
The index exists	returns the value	No
There is no such index	`undefined`	No
The array does not exist (`null` or `undefined`)	`TypeError`	Yes
An empty slot (a hole)	`undefined`, but `index in arr` gives `false`	No

Summary: accessing a non-existent array index returns undefined but does not raise an error. That behaviour is part of the flexibility of JavaScript.

Common mistakes

Expecting an exception, as in other languages. Going out of bounds is silent in JavaScript, so you will see the failure much later, already as an undefined.
Confusing "no element" with "the value is undefined". A comparison against undefined gives no answer; use in or Object.hasOwn().
Using arr[-1] as the last element. That is just the key '-1'; for the last element use arr.at(-1) or arr[arr.length - 1].
Forgetting to check the array itself. If the data came from the network, the variable may be undefined, and then the read throws a TypeError; guard it with arr?.[i].
Chaining reads without a guard. arr[10].name throws a TypeError, because undefined has no properties; write arr[10]?.name.
Writing to a large index. arr[100] = 1 does not throw, it stretches length to 101 and creates holes.

Short Answer

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