Array.prototype.do proposal

This proposal introduces a new method to the Javascript Array prototype to support the common use case of performing side effects while iterating through an Array, while returning the original array paramenter. It is a pretty useful tool to help with the case where one finds themselves having to leave the chainability of function calls like .map, .filter and .reduce

Inspiration

RxJS is an awesome library that almost lets you treat asynchronous events as elements in an array (called an Observable). RxJS has a similarly named method on Observable which is pretty useful.

Introduction

The Array.prototype.do method is essentially synctactic sugar on a call to Array.prototyp.map that performs side effects before returning the first parameter of the callback.

For example:

myArray.do(console.log)

is equivalent to

myArray.map(x => {
  console.log(x);
  return x;
})

Proposed API

arr.do(function callback(currentValue, index, array) {
    // Return element for new_array
}[, thisArg])

Parameters

callback Function that performs the intended side effect, taking three arguments:

currentValue The current element being processed in the array.

index The index of the current element being processed in the array.

array The array do was called upon.

thisArg Optional. Value to use as this when executing callback.

Return value

The original array that .do was called on

Examples

Debugging (example inspired from RxJS docs

let clicks = ... // events
let positions = clicks
  .do(console.log)  // easy to add line when debugging
  .map(ev => ev.clientX)

Filtering function applications

let allFiles = ... // list of files
allFiles
  .do(someFunctionForAllFiles)
  .filter(s => s.endsWith('.js'))
  .do(someFunctionForJSFilesOnly)