Sometimes you need to append one or more new values at the end of an array. In this situation the push() method is what you need.

The push() method will add one or more arguments at the end of an array in JavaScript:

let arr = [0, 1, 2, 3];
arr.push(4);
console.log(arr); // [0, 1, 2, 3, 4]

This method accepts an unlimited number of arguments, and you can add as many elements as you want at the end of the array.

let arr = [0, 1, 2, 3];
arr.push(4, 5, 6, 7, 8, 9);
console.log(arr); // [0, 1, 2, 3, 4, 5, 6, 7, 8, 9]

The push() method also returns the new length of the array.

let arr = [0, 1, 2, 3];
let newLength = arr.push(4);
console.log(newLength); // 5

Examples of push in JavaScript and common errors

How to reassign the array

Reassigning the array with the output from push is a common error.

let arr = [0, 1, 2, 3];
arr = arr.push(4);
console.log(arr); // 5

To avoid this error you need to remember that push  changes the array, and returns the new length. If you reassign the variable with the return value from push() you are overwriting the array value.

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How to add the contents of one array to the end of another

If you want to add the content of an array to the end of another, push is a possible method to use. push will add as new elements whatever you use as an argument. This is the same also for another array, so the array has to be unpacked with the spread operator:

let arr1 = [0, 1, 2, 3];
let arr2 = [4, 5, 6, 7];
arr1.push(...arr2);
console.log(arr1); // [0, 1, 2, 3, 4, 5, 6, 7]
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How to use push on an array-like object

There are objects that are similar to arrays (like the arguments object – the object that allows access to all arguments of a function), but that do not have all methods that arrays have.

To be able to use push or other array methods on these, first they have to be converted to arrays.

function myFunc() {
   let args = [...arguments];
   args.push(4);
   returns args;
}

console.log(myFunc(0, 1, 2, 3)); // [0, 1, 2, 3, 4]

If you don't first change the array-like arguments object to an array, the code would stop with a TypeError: arguments.push is not a function.

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Conclusion

If you work with arrays, don't miss out on push. It adds one or more elements at the end of an array and returns the new length of the array.