Promise(), not promise

a min to read page

Quick test: what do these code snippets do?

const a = new Promise(function (a) {
  a
})

const b = new Promise(function (b) {
  b("success")
})

const c = new Promise(function (a, b) {
  b("failure")
})

const d = Promise.resolve("success")

const e = Promise.reject("failure")

const f = new Promise(function (a, b) {
  a("success")
  b("failure")
})

const g = new Promise(function (a, b) {
  b("failure")
  a("success")
})

const h = new Promise(function (a, b) {
  throw new Error()
})

const i = new Promise(function (a, b) {
  a()
})

const j = new Promise(function (a, b) {
  b()
})

const k = (number) => {
  return new Promise(function (a, b) {
    a(number * 2)
  })
}

Check by opening the console in your browser and test with e.g.

x.then(console.log).catch(alert)

Some points that were consolidated for me after the exercise:

  • “Promise” is not the same as “promise”. What I need to reach for is the “Promise()” constructor: “promise” is just the conventional name assigned to the promise object.

  • Promise() can only be constructed with new. Attempting to call it without new throws a TypeError. — MDN

  • Promise() takes an executor as its paramater. The executor receives two functions as parameters: resolveFunc (1st parameter) and rejectFunc (2nd parameter).

Sources: