TypeError: Cannot read property 'forEach' of Undefined in JS

TypeError: Cannot read property 'forEach' of Undefined in JS #

The "Cannot read property 'forEach' of undefined" error occurs when the forEach() method is called on an undefined value. To solve the error, make sure to only call the forEach method on arrays, Set or Map objects.

Here is an example of how the error occurs.

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const arr = undefined;

// ⛔️ TypeError: Cannot read properties of undefined (reading 'forEach')
arr.forEach(element => {
  console.log(element);
});

We called the forEach method on an undefined value, so the error occurred.

Instead, initialize the value of the variable to the correct data type before calling the forEach method.

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const fromDb = undefined;

// ✅ Initialize to empty array if falsy
const arr = fromDb || [];

// ✅ Using optional chaining
arr?.forEach(element => {
  console.log(element);
});

// ✅ Check if truthy
if (arr) {
  arr.forEach(element => {
    console.log(element);
  });
}

// ✅ Check if Array
if (Array.isArray(arr)) {
  arr.forEach(element => {
    console.log(element);
  });
}

We used the logical OR (||) operator to provide a fallback value if the value to the left is falsy (e.g. undefined).

The next example shows how to use the optional chaining (?.) operator. The operator short-circuits returning undefined if the reference is equal to null or undefined.

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const fromDb = undefined;

const arr = fromDb || [];

arr?.forEach(element => {
  console.log(element);
});

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const fromDb = undefined;

const arr = fromDb || [];

if (arr) {
  arr.forEach(element => {
    console.log(element);
  });
}

The last example shows how to use the Array.isArray method to check if the variable stores an array.

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const fromDb = undefined;

const arr = fromDb || [];

if (Array.isArray(arr)) {
  arr.forEach(element => {
    console.log(element);
  });
}

The forEach() method is implemented by multiple objects:

• Array.forEach
• Map.forEach
• Set.forEach

If you use the method with arrays, use the optional chaining operator or the Array.isArray method to only call the forEach method on a valid array.

The "Cannot read property 'forEach' of undefined" error often occurs when trying to access an array index that doesn't exist.

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const arr = [];

// ⛔️ Cannot read properties of undefined (reading 'forEach')
arr[0].forEach(element => {
  console.log(element);
});

We accessed an empty array at index 0, which returns undefined, on which we called the forEach method and got the error.

If you're unsure whether the index exists in the array, use the optional chaining operator.

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const arr = [];

arr[0]?.forEach(element => {
  console.log(element);
});

The optional chaining (?.) operator won't invoke the forEach method because accessing the array at index 0 returns an undefined value.

Conclusion #

The "Cannot read property 'forEach' of undefined" error occurs when calling the forEach method on an undefined value.

To solve the error make sure to initialize the variable to the correct value and only call the forEach method on the correct data type.

Further Reading #

• Get an Object's Key by its Value using JavaScript
• How to get the Length of an Object in JavaScript
• Get Difference between two Arrays of Objects in JavaScript