Solve JavaScript cannot read property push of undefined

If you’ve tried to use the push method on an array in JavaScript and gotten an error message like “cannot read property ‘push’ of undefined,” don’t worry, you’re not alone.

This problem can be caused by a number of things from calling the push method on a variable that stores an undefined value to forgetting to initialize properties in a class before calling the push method on them.

In this blog post, we’ll explore some of the most common causes of this error message and offer solutions for how to fix it. So if you’re having trouble getting your arrays to work properly in JavaScript, keep reading!

Reasons Why You Get The ‘cannot read property push of undefined’ Error

The ‘cannot read property push of undefined’ error can occur because of these 3 main reasons –

1. You Call The push Method On A Variable That Is Undefined

2. You Forget To Initialize A Property In A Class

3. You Use The push Method On The Index Of An Array

Let’s discuss each of these in detail below and look at some possible solutions.

1. You Call The push Method On A Variable That Is Undefined

This is the most common cause of the ‘cannot read property push of undefined’ error. It occurs when you try to call the push method on a variable that doesn’t exist or has an undefined value.

For example, consider the following code:

let myArray = [];

myArray.push("someValue"); // this will work fine

let anotherArray = undefined;

anotherArray.push("someValue"); // this will throw an error

In the above code, we first create a variable called myArray and initialize it with an empty array. We then call the push method on this variable and pass in ‘someValue’, so the push method will add the value ‘someValue’ to the end of our array.

Then we attempt to call the push method on anotherArray, which has an undefined value. Since the push method can only work on real JavaScript arrays and not variables that store undefined values, this will throw an error.

Solution: Make sure you’re calling the push method on a variable that has an actual array stored in it, instead of a variable with an undefined value.

You can use the Array.isArray method if you’re not sure whether or not a variable contains an array.

Here is an example:

let myArray = [];

if (Array.isArray(myArray)) {

myArray.push("someValue"); // this will work fine

} else {

console.log("myArray is not an array!");

}

You can also make sure that you initialize a variable to an empty array if you get that value from somewhere else, like a database.

const val = undefined;

const myArr = val || [];

myArr.push("someValue"); // this will work fine

Here, we have used the logical || operator to assign the value of val to myArr if val is undefined. If val has a defined value, then myArr will be assigned that value instead. But if val doesn’t have a defined value, we initialize it with an empty array.

2. You Forget To Initialize A Property In A Class

This second reason is a bit more specific and only applies if you’re trying to use the push method on an array that is a property of a class.

For example, consider the following code:

class MyClass {

constructor(values) {

this.values = values;

}

someMethod() {

this.myArray.push("someValue"); // this will throw an error

}

}

In the code above, we’ve created a class called myClass and we’ve initialized a property of the class called myArray. We then try to call the push method on this property, which will throw an error since it’s not an array.

var obj = new MyClass(["a","b"]);

obj.someMethod(); //TypeError: Cannot read property "push" of undefined

Solution: Make sure that you initialize a property in a class before you use it as an array. For example :

class MyClass {

constructor() {

this.myArray = []; // initialize the property here

}

someMethod() {

this.myArray.push("someValue"); // this will work fine

}

}

3. You Use The push Method On An Index Of An Array

The third and final reason for the ‘cannot read property push of undefined’ error is using the push method on an index of an array that doesn’t even exist.

For example, consider the following code:

let myArray = ["a","b","c"];

myArray[4].push("d"); // this will throw an error

In the code above, we’ve initialized myArray to be an array with three elements: ‘a’, ‘b’, and ‘c’. Then we try to call the push method on myArray’s fourth index, which doesn’t exist. This will cause JavaScript to throw an error since you can’t call the push method on a nonexistent index.

Solution: Don’t call the push function on an index of an array that doesn’t exist. For example:

let myArray = ["a","b","c"];

myArray.push("d"); // this will work fine

Note that you can still call the push function on myArray and pass in the index of ‘d’, since we’ve initialized myArray using real JavaScript arrays, which can have elements added to them.

Conclusion

The ‘cannot read property push of undefined’ error is caused by trying to call the push method on a variable that doesn’t store an array, or on an index of an array that doesn’t exist.

Make sure you only call the push function on actual JavaScript arrays, and initialize properties in your classes before using them as arrays. You can also use the Array.isArray method to check whether or not a variable contains an array.

If you continue having issues with this error, you may want to consult a JavaScript developer for help debugging your code. However, following these tips should help prevent this error from occurring in the future.