2. Map, filter and reduce #

Created Thursday 25 March 2021

Why use map, filter or reduce? #

• Map, filter and reduce help us to make our code functionally pure(deterministic).

1. Map #

• Purpose - store f(ele) for each element, in a map object.
• Syntax - arr.map(f); where f can be a function in any form.
• Assume an argument when defining f.
• f(ele) is computed and stored for each element. undefined is stored if f returns nothing.
• The map object is index subscriptable.

2. Filter #

• Purpose - returns an iterable of elements for which f(ele) is true.
• Syntax - same as map.

3. Reduce #

• Purpose - Accumulates the result by running f(accum,ele) for each element.
• Syntax - arr.reduce(f, accum_start=0)
• You must have accumulator and element as parameters of f, in order.

const arr = [1, 2, 3]
const res = arr.reduce(((accum, ele)=> accum + ele), accum_init_value); // 0 if unspecified
// same as doing accum = accum + ele each time

• Remember to update accum inside f().

Note:

• All the quick for functions - forEach, map, filter and reduce work sequentially by default. They can be made parallel, however.
• Iteration number is available all forEach, map and **filter **- as the second parameter of f. This is optional.

Gotchas #

• map and filter return an array as output, while reduce returns a single value (usually).
• Use these anytime we need loops for small things.
• forEach is different, it returns nothing.