Arrays and Functions

Throughout the lesson we went over arrays being mutable, or changeable. Well what happens if we try to change an array inside a function? Does the array keep the change after the function call or is it scoped to inside the function?

Take a look at the following example where we call .push() on an array inside a function. Recall, the .push() method mutates, or changes, an array:

const flowers = ['peony', 'daffodil', 'marigold']; function addFlower(arr) { arr.push('lily'); } addFlower(flowers); console.log(flowers); // Output: ['peony', 'daffodil', 'marigold', 'lily']

Let’s go over what happened in the example:

  • The flowers array that has 3 elements.
  • The function addFlower() has a parameter of arr uses .push() to add a 'lily' element into arr.
  • We call addFlower() with an argument of flowers which will execute the code inside addFlower.
  • We check the value of flowers and it now includes the 'lily' element! The array was mutated!

So when you pass an array into a function, if the array is mutated inside the function, that change will be maintained outside the function as well. You might also see this concept explained as pass-by-reference since what we’re actually passing the function is a reference to where the variable memory is stored and changing the memory.



In main.js, there is an array concept. There is also a function changeArr that will assign the element in index 3 of an array to 'MUTATED'. The function changeArr was called with an argument of concept.

Underneath the function call, log concept to the console to check if this reassignment mutated the array.


Let’s double check what happens if we mutate an array using a built-in method inside a function.

Under the console.log() statement, define another function named removeElement that takes a parameter of newArr. Inside the function body call .pop() on newArr.


Call removeElement() with an argument of concept.


After calling removeElement(concept), check the value of concept by logging it to console.

Notice that in both cases, the change to the array was maintained outside of the function!

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