Javascript: Processing collections with map, reduce, and filter

Jack Javascript

Map executes that callback on every element within it, returning a new array with all of the values that the callback returned. Map passes three arguments to your callback:

  • The current item in the array
  • The current index
  • The entire array you called map on

Filter takes an array, and filters out unwanted elements.Filter passes your callback three arguments:

  • The current item
  • The current index
  • The array you called filter on

Reduce takes all of the elements in an array, and reduces them into a single value. Reduce passes your callback four arguments:

  • The current value
  • The previous value
  • The current index
  • The array you called reduce on
    •  
 	 
var isEven = num => num % 2 === 0;
var square = num => num * num;
var add = (a, b) => a + b;
  
var arr = [1, 2, 3, 4, 5, 6, 7, 8, 9, 10];
  
arr.filter(isEven).map(square).reduce(add); 


Output: 220