JavaScript

JavaScript Array Type

Map

JavaScript provides the map method to transform all of the elements of an array by applying a given function on them. map returns a new array with transformation applied. The transformation can be customized by specifying a callback function. The callback function should accept a value from the array and return a single transformed value back.

const c2f = c => (c * 9 / 5) + 32
const temps = [20, 16, 32]
const tempsF = temps.map(c2f)


console.log(temps)  // [20, 16, 32]
console.log(tempsF) // [68, 60.8, 89.6]

Code example: indexes

["a", "b", "c"].map((el, i) => `${i} : ${el}`)
// ["0 : a", "1 : b", "2 : c"]

The map method passes the index to the callback function as well, and we can use this in our transformations if we want to.

Code example: plus one

const nums = [1, 2, 3]
const oneMore = nums.map(n => n + 1)
console.log(nums) // [2, 3, 4]

Code example: re-implementation of map

const myMap = (arr, fn) => {
  const result = []
  arr.forEach(el => result.push(fn(el)))
  return result
}

The above re-implementation of map is not equivalent to the Array.prototype.map method, but it shows the basic concept.

Read more on MDN

Array.prototype.map()

Test yourself

Description

Define a new function with the name

sumOfSquares

that takes an integer array input parameter arr and returns a new array with the sum of the squares of all the elements.

Assumptions:

  • All elements of the input array are integer numbers.

Tips:

  • Transform the input array into an array of squares.
  • Calculate the sum of the array of squares by iterating through it.

Tests

  1. 1 You should define a function named sumOfSquares.
  2. 2sumOfSquares([]) should return 0.
  3. 3sumOfSquares([100,200,300]) should return 140000.
  4. 4sumOfSquares([1,2,3,4]) should return 30.
  5. 5sumOfSquares([-1,2,-3,4]) should return 30.
Editor
Run tests (ALT+Q)
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