Rubys Any And All In Js feb 21, 2016

I wrote a simple implementation of Ruby’s Enumerable#all? and Enumerable#any? in Javascript, using Array.prototype.reduce. They are a nice way to check if a condition holds for all or any elements in an array, respectively. In Ruby, you would call the method on the object (e.g. [0,1].any? {|x| x == 1}), but my first JS implementation required that you pass the array and testing function as arguments to either function. Array.prototype.reduce takes 2 arguments:

1. a function to be applied to each element
2. an optional initial value for the array

The arguments to the reducing function are the stored value (the return value of the last iteration, or the initialization value), and the current element. Here is my first all implementation:

function any(ary, fn) {
  return ary.reduce(function(o, n) {
    return o || fn(n)
  }, false);
}

function all(ary, fn) {
  return ary.reduce(function(o, n) {
    return o && fn(n)
  }, true);
}

An example usage: any([0,1], function(x){ return x == 1 })

any is initialized with false, and will only return true if any of the elements caused the test function to return truthy. This is from the logical OR function, which returns true when any of operands are true (eg true OR false == true). all is initialized with true, and conversely uses AND to ensure the final value is true only when it returns truthy for every value in the array.

You can add these functions to the Array.prototype so that they are even more similar to the Ruby implementation (a method that belongs to a object/prototype, rather than a method that accepts one as arguments):

Array.prototype.any = function(fn) {
  return this.reduce(function(o, n) {
    return o || fn(n)
  }, false);
}

Array.prototype.all = function(fn) {
  return this.reduce(function(o, n) {
    return o && fn(n)
  }, true);
}

Prototyped example: [0,1].any(function(x){ return x == 1 })