JS: Array Prototype Methods

Learning Goals

  • Summarize various array prototype methods
  • Use array prototype methods
  • Determine if a given method is a mutator or accessor
  • Practice using MDN documentation
  • Identify learning strategies that are most helpful for you


  • Array Prototype - methods that are built specifically for arrays. They help us change the arrays themselves or get certain information out of them.
  • Mutator - methods that mutate, or change, the original array
  • Accessor - methods that do not mutate the original array, rather just give us some information about the array

Warm Up


In your notebook, brainstorm how you might complete the following challenges. Don’t use any code. Instead, write down your thought process as a set of instructions; this is sometimes referred to as ‘pseudocode’.

var instructors = ["Scott", "Heather", "Jeremiah"];

For the array above,

  • Oops! We’re missing an instructor in this array. We need to add “Kayla” to the list. What are the instructions you would want to tell the computer to take in order to add a new instructor to the array?
  • I need to know if a certain instructor named “Heather” is in the instructors array. What are the instructions you would want to tell the computer to take in order to find out if “Heather” is included in the array?
  • Extra: How would you programmatically find the number of elements in the array above?


  • Consider the processes you just thought through. Both of the challenges above are based on array prototype methods. One is based on a mutator method and one is based on an accessor method. Which do you think was which? Why? (Refer to the vocabulary terms above for a reminder on what Mutator and Accessor mean.)

Accessor Methods

Sometimes we need to access information about an array or certain elements in our array. Methods we use to do that are called accessor methods. They do not mutate or change the original array - they just give us information, or copies of parts of the array.

In the warmup, we wrote some pseudocode to check if an element exists inside of an array. There’s actually an Accessor Method that we can use to implement that functionality in JavaScript. It’s called includes. The includes method is an accessor method because it doesn’t modify or mutate the original array.

Implementation using the includes method might look like this:

var instructors = ["Scott", "Heather", "Jeremiah"];

var instructorExists = instructors.includes('Heather');

console.log(instructorExists); // logs true

Mutator Methods

In other cases, we will want to change the data our array holds. Mutator methods actually mutate, or change, the data in the array. If we needed to remove an instructor element from the instructors array, we’d need to use a mutator method.

In the warm up, we wrote some pseudocode to add an element to an array. There’s a mutator method we can use to implement this functionality in code! It’s called push and it is a mutator method because it mutates the original array.

var instructors = ["Scott", "Heather", "Jeremiah"];


console.log(instructors) // logs ["Scott", "Heather", "Jeremiah", "Kayla"];


You will be learning about the following array prototype methods:

  • push
  • pop
  • unshift
  • shift
  • slice
  • splice
  • concat
  • includes

During the course of your research, you will practice different modes of learning! This is in order to help you gain a better understanding of how you individually learn, and furnish you with some strategies you can fall back on when you get stuck!


For each array prototype method, follow this protocol carefully:

  1. Read the documentation on MDN.
  2. Rubber duck through your understanding of it.
  3. Visually represent the method using sticky notes, drawings in your notebook, or using an online tool like Jamboard
  4. Think: What is returned from the method?
  5. Think: Does the method mutate the original array?
  6. Try It: In a repl.it, try out the method. While poking around here, try to verify your predictions from steps 3 and 4.
  7. Back to documentation - verify your predictions from steps 3 and 4.
  8. Document any remaining questions!


In groups, discuss your experience.

Some questions to get you started:

  • What strategies helped you learn best?
  • What strategies were most difficult for you?
  • Which methods were accessor methods and which were mutator methods?
  • What questions do you have about these mutator and accessor methods?


Work through the exercises in this REPL.

What should you memorize?

Often times, especially when we are starting to code, we put pressure on ourselves to memorize every little thing. This is NOT necessary and not a good use of your brain space! There will be some methods you use so frequently that you just memorize them, but for the most part, the things you “memorize” should be bigger ideas and how things work together.

Some of the methods we learned today are the kind you’ll use so often that you’ll have down pat (including the syntax): push, pop, shift, unshift.

You should have enough of an understanding of arrays that you know you could google something like “remove a specific element from an array”, but maybe you won’t memorize exactly what method to use or the exact syntax for it.

What is not worth memorizing?

It’s not a good use of time to attempt to memorize every array prototype and its syntax.


  1. Which learning strategy helped you learn the best? Rubber ducking? Spending time in the docs? Playing with a method in repl.it? Visualizing it without code?
  2. Which array prototype method was most interesting to you? Why?
  3. Which array prototype method is the most confusing right now?
  4. Open your notebooks to the last page, and write “When I’m stuck, I will:” and then add the learning strategies that worked best for you!

Lesson Search Results

Showing top 10 results