Array.prototype.reduce 的数组交集（集合论）

Question

I have two arrays [a,b,c,d] and [b,d,f,h] .

I want to get an array back with the common elements [b,d] .

I can achieve that with a combination of filter and indexOf :

[a,b,c,d].filter(el => [b,d,f,h].indexOf(el) !== -1)

but I was wondering if and how I can do the same with reduce .

I admit that, despite looking at many examples, reduce still is to me one of the most obscure JS methods, so I'd really appreciate some advice.

Answer 1

ES6, a propoosal with Array#includes<\/code><\/a>

Answer 2

Reduce is designed to return a single value from a list of items. So filter makes much more sense here.

A good use for reduce would be to return the total number of common elements. Check it out here: https://jsfiddle.net/c69vgzL4/

var a = ['a','b','c','d']
var b = ['b','d','f','h']

var number_of_common = b.reduce(function(prev, next) {
    return prev + (a.indexOf(next) + 1 ? 1 : 0)
}, 0)

$('body').html(number_of_common)

Answer 3

Not only two arrays but for an intersection of n arrays... Let's invent Array.prototype.intersect()<\/code>