Fix the Cannot Read Property Push of Undefined Error in Javascript

Dec 20, 2023

2 mins read

Published in

Javascript

Understanding the Error:

To comprehend the error, it’s crucial to understand that JavaScript throws this when attempting to perform an operation on an undefined or null value. In the context of ‘push,’ it typically arises when trying to add an element to an array that hasn’t been properly instantiated.

Common Scenarios:

1. Forgetting to initialize an array:

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let myArray;
myArray.push("element"); // Error: Cannot read property 'push' of undefined

2. Trying to push to an undefined property:

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let myObject = {};
myObject.myArray.push("element"); // Error: Cannot read property 'push' of undefined

Solutions:

1. Initialize the Array:

Ensure that the array is initialized before attempting to use the ‘push’ method. This involves declaring the array using square brackets.

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let myArray = [];
myArray.push("element"); // No error

2. Check and Initialize Object Properties:

Verify if the object property exists before manipulating it. If it doesn’t, initialize it as an empty array.

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let myObject = {};
if (!myObject.myArray) {
  myObject.myArray = [];
}
myObject.myArray.push("element"); // No error

3. Use Default Values:

Modern JavaScript offers a concise way to set default values using the nullish coalescing operator (??). This ensures the property is initialized if it’s undefined or null.

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let myObject = { myArray: [] };
myObject.myArray = myObject.myArray ?? [];
myObject.myArray.push("element"); // No error

4. Leverage Optional Chaining:

Optional chaining (?.) is another ECMAScript feature that simplifies handling undefined or null values, reducing the risk of encountering the mentioned error.

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let myObject = {};
myObject.myArray?.push("element"); // No error, myArray is undefined but won't throw an error

Resolving the “Cannot Read Property ‘push’ of Undefined” error requires a keen understanding of variable initialization and handling in JavaScript.