Light weight Promise polyfill

Promise.js

  const Promise = window.Promise || (function () {
    function PromiseLike (init) {
      let state = 'pending'
      let resolved, error
      const pendingResolve = []
      const pendingReject = []
      function resolve (value) {
        state = 'resolved'
        resolved = value
        pendingResolve.forEach(function (fn) { fn(value) })
      }
      function reject (err) {
        state = 'rejected'
        error = err
        pendingReject.forEach(function (fn) { fn(err) })
      }

      init(resolve, reject)

      return {
        then: function (onResolve) {
          if (state === 'rejected') return this
          return PromiseLike(function (resolve, reject) {
            const wrappedOnResolve = wrapPending(onResolve, resolve, reject)
            if (state === 'resolved') wrappedOnResolve(resolved)
            else pendingResolve.push(wrappedOnResolve)
          })
        },
        catch: function (onReject) {
          if (state === 'resolved') return this
          return PromiseLike(function (resolve, reject) {
            const wrappedOnReject = wrapPending(onReject, resolve, reject)
            if (state === 'rejected') wrappedOnReject(error)
            else pendingReject.push(wrappedOnReject)
          })
        }
      }
    }

    function wrapPending (pending, onResolve, onReject) {
      return function (value) {
        try {
          const pendingResolved = pending(value)
          if (pendingResolved && pendingResolved.then) {
            pendingResolved.then(function (value) { onResolve(value) })
                           .catch(function (err) { onReject(err) })
          } else {
            onResolve(pendingResolved)
          }
        } catch (err) {
          onReject(err)
        }
      }
    }

    PromiseLike.resolve = function (value) {
      return PromiseLike(function (resolve) { resolve(value) })
    }
    PromiseLike.reject = function (err) {
      return PromiseLike(function (resolve, reject) { reject(err) })
    }
    return PromiseLike
  })()