Brush up map, filter and reduce in 5 minutes

Map, reduce, and filter are all array methods in JavaScript. Each one iterates over an array and performs a transformation or computation and returns a new array based on the result of the function.

Map

map method is used to create a new array from an existing one by applying a function to each element of the existing array.

The callback function takes 3 arguments:

• current element
• index
• the actual array

Polyfill for map() :

Filter

filter method takes each element in an array and applies a conditional statement against it. If the condition is fulfilled, only then the element is pushed to the result array.
The callback function for filter takes the same 3 arguments as map();

Polyfill for filter() :

Reduce

reduce method reduces an array of elements down to a single output value. reduce() takes 2 arguments: the callback function & an initial value. If the initial value is not provided by the user, the first element of the array is taken as the initial value. The callback function takes 4 arguments:

• accumulator (result of previous computations)
• current element
• index
• the actual array

Polyfill for reduce() :

Example to sum it up: