.reduce(function(a, b) { return a && b;})


Can anyone please explain to me a bit what exactly this “.reduce(function(a, b) { return a && b;})” do here? I don’t quite understand it.

  // filter the collection
  return collection.filter(function(obj) {
    return srcKeys
      .map(function(key) {
        return obj.hasOwnProperty(key) && obj[key] === source[key];
      .reduce(function(a, b) {  // what does this ".reduce" do here?
        return a && b;

It is from solution 3 of this challenge:

Thank you.

the reduce method takes the elements of an array and reduce them to a single value using the callback, you can read about it here:


Specifically concerning this challenge:

The reduce method receives an array from the map method, an array that will contain a number of Booleans (depending on the length of your srcKeys).

reduce will now iterate over that array, and in each step, it will compare the accumulator (the a parameter) and the current element (the b parameter) with &&, so it will only return true if both are true. The return value will go into reduce again in the next step as the new a parameter. So in the end, it will only return true if all values in that array were true. If there’s just one false, a will become false and the comparison a && b will always be false for all following steps.

In this particular case, you could also get the same result by using the every method on the array instead of reduce.


Thank you very much for the detailed explanation.