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Computer Science Introduction to Data Structures Exploring Arrays Operations on Arrays

Alex Hipolito
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Alex Hipolito
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Operations on Arrays for JavaScript

Hey everyone, I found there wasn't an outline of Operations on Arrays for JavaScript. I thought I'd add my notes here in case anyone found it useful. You can try this out using the browser console in Chrome DevTools.

These notes are not comprehensive, please refer to the excellent JS courses on Treehouse and the MDN documentation for more information. Happy to take in any feedback as well.

JavaScript

Accessing Elements

Similar to Python, use the index operator on a list to access values using an index. Note that index values start at 0.

let numbers = [1, 2, 3, 4, 5, 6];
numbers[2]
// returns 3

Using an index that is out of bounds of the current range of values returns undefined.

numbers[10];
// returns undefined

List Length

To determine the number of items in a list, use the Array.length property:

numbers.length
// returns 6

To iterate of each item in the list:

for (let i = 0; i < numbers.length; i++) {
  console.log(numbers[i]);
}
/* returns
1
2
3
4
5
6
*/

Array Operations

To append to the array, use the Array.push() method:

numbers.push(7)
numbers
// returns (7) [1, 2, 3, 4, 5, 6, 7]

To concantenate one array with another, use Array.concat() method:

numbers = numbers.concat([8, 9, 10]);
// returns (10) [1, 2, 3, 4, 5, 6, 7, 8, 9, 10]

To insert an element to the array, use Array.splice():

numbers.splice(10, 0, 11)
numbers
// returns (11) [1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11]

To remove the last item from the array, use Array.pop() method:

numbers.pop()
numbers
// returns (10) [1, 2, 3, 4, 5, 6, 7, 8, 9, 10]

Or if you want to remove an item within the array, you can also use Array.splice() method for this purpose:

numbers.splice(2, 1)
numbers
// returns (9) [1, 2, 4, 5, 6, 7, 8, 9, 10]

Array Membership

To test for membership in a list, use Array.findIndex() method:

numbers.findIndex((number) => number === 5);
// returns 3
numbers.findIndex((number) => number === 30);
// returns -1

Arrays in JavaScript are also mutable, which means any element can be updated using the index operator along with index position.

numbers[0] = 22
numbers
// returns (9) [22, 2, 4, 5, 6, 7, 8, 9, 10]

Hope you find these useful!

1 Answer

Don't forget about shift() and unshift() to remove or add elements to the beginning of an array.

let numbers = [1, 2, 3, 4, 5, 6];
numbers.shift()
numbers
// returns (5) [2, 3, 4, 5, 6]
numbers.unshift(1)
numbers
// returns (6) [1, 2, 3, 4, 5, 6]

There are also some nice new methods for arrays, like .filter(), .map(), .reduce(), and .reverse() to name a few. W3Schools has a complete list here