How to solve join is not a function error in JavaScript

In this tutorial, we are going to learn about how to solve the TypeError: join is not a function in JavaScript

When we use a Array.join() method on a value which is not an data type array we will get the following error in our console.

Example:

const arr=  { };

arr.join('');

Output:

"TypeError: arr.join is not a function

In the example above, we are getting the error because we using the join() method on a object, but join() method is only available on arrays.

To solve the error, we need to make sure to call the join() method on a valid array but not on strings or objects.

Here is an example:

const arr = [1, 2, 3];

const result = arr.join('-');;
console.log(result); //Output -> "1-2-3"

Note: The join() method returns a new string by concatenating all elements from the array, separated by the specified separator or commas.

We can also check, if the given value is an type array or not before calling the join() method on it.So that, we can avoid the runtime errors.

var arr = [1, 2, 3];

if(Array.isArray(arr)){
    arr.join('-');
}else{
    console.log("Given data is not an array")
}

The Array.isArray() takes the variable as an argument and returns true, if the given variable is a valid array otherwise it will return false.

In the above example, we have passed a number to the Array.isArray() method. So, it returns false and executed the ‘else’ case and logs the “Given data is not an array” message in the console.

Conclusion

The “join is not a function” error occurs, when we call a join() method on a value which is not array. To solve the error convert the value to an array before calling the join() method on it or make sure to use the join() method on a valid arrays.