deferred-finally.js

import $ from 'jquery'

// jQuery Deferreds don't have a prototype, so patching them is more complicated
// than the native Promises.

const origDeferred = $.Deferred

const sym = module.hot && module.hot.data
  ? module.hot.data
  : Symbol('deferred-finally')

// throws -- should only be called inside a .then handler
function execFinally (func, resolveValue, shouldThrow) {
  const p = func()
  // If p is Promise-like
  if (typeof p.then === 'function') {
    // Restores the original resolveValue, but only if p doesn't reject
    return p.then(postFinally)
  }
  return postFinally()

  function postFinally () {
    if (shouldThrow) {
      throw resolveValue
    }
    return resolveValue
  }
}

const finallyPrototype = {
  finally (func) {
    func = () => func()
    // Use .then instead of .always to ensure it behaves like Promise.finally
    // Rejection in the "catch" branch must always be propagated
    // Could use Promise.resolve(this).finally, but that would change the type of the result :(
    return this.then(
      (v) => execFinally(func, v, false),
      (e) => execFinally(func, e, true)
    )
  }
}

function patchDeferred (obj) {
  if (obj[sym]) {
    return obj
  }
  // $.Deferred basically calls self.promise(self) to add promise methods to self
  const origPromise = obj.promise
  // Patch .promise to run the original .promise, and patch the resulting promise as well
  // Because this modifies the parameter, it will work even when called in void context (like inside $.ajax)
  obj.promise = function promise (...args) {
    return patchDeferred(origPromise.call(this, ...args))
  }
  if (!obj.finally) {
    Object.assign(obj, finallyPrototype)
  }
  obj[sym] = true
  return obj
}

// Replace $.Deferred with a version that patches itself before running the
// callback or returning.
//
// Tested with jQuery 3.0.0.
$.Deferred = function Deferred (init = function () {}) {
  return origDeferred.call(this, /* @this */ function deferred () {
    patchDeferred(this)
    init.call(this, this)
  })
}

if (module.hot) {
  module.hot.accept()
  module.hot.dispose(() => {
    $.Deferred = origDeferred
    return sym
  })
}