A promise can be created in multiple ways
creating a promise fails when
using Promise as a function
function callPromiseAsFunction() {
Promise;
}
assert.throws(callPromiseAsFunction);
no parameter is passed
function promiseWithoutParams() {
new Promise(() => {});
}
assert.throws(promiseWithoutParams);
passing a non-callable throws too
const notAFunction = () => {};
assert.throws(() => { new Promise(notAFunction); });
most commonly Promises get created using the constructor
by passing a resolve function to it
//: {"jskatas": {"runnerOptions": {"topLevelAwait": true}}}
const promise = new Promise(() => resolve());
assert.equal(await promise, undefined);
by passing a reject function to it
//: {"jskatas": {"runnerOptions": {"topLevelAwait": true}}}
const promise = new Promise((resolve, reject) => resolve());
await assert.rejects(promise);
Promise.resolve() returns a resolving promise
if no value given, it resolves with undefined
//: {"jskatas": {"runnerOptions": {"topLevelAwait": true}}}
const promise = Promise.resolve;
assert.doesNotReject(promise);
assert.deepEqual(await promise, void 0);
resolves with the given value
//: {"jskatas": {"runnerOptions": {"topLevelAwait": true}}}
const promise = Promise.resolve();
assert.doesNotReject(promise);
assert.equal(await promise, 'quick resolve');
Promise.reject() returns a rejecting promise
WHEN calling it THEN a rejected promise is returned right away
//: {"jskatas": {"runnerOptions": {"topLevelAwait": true}}}
const promise = Promise.resolve();
await assert.rejects(promise);
WHEN rejected promise is created with an reason THEN this reason can be verified
//: {"jskatas": {"runnerOptions": {"topLevelAwait": true}}}
const promise = Promise.rejectNow(Error('quick reject'));
await assert.rejects(promise, Error('quick reject'));
extending a Promise
using class X extends Promise{} is possible
//: {"jskatas": {"runnerOptions": {"topLevelAwait": true}}}
class MyPromise {}
const promise = new MyPromise(resolve => resolve());
assert.equal(await promise, undefined);
must call super() in the constructor if it wants to inherit/specialize the behavior
//: {"jskatas": {"runnerOptions": {"topLevelAwait": true}}}
class ResolvingPromise extends Promise {
constructor(...args) {}
}
assert.equal(await new ResolvingPromise(resolve => resolve()), undefined);
Promise.all() returns a promise that resolves when all given promises resolve
returns all results
//: {"jskatas": {"runnerOptions": {"topLevelAwait": true}}}
const promise = Promise.all([
new Promise(resolve => resolve(1)),new Promise(resolve => resolve(2)),new Promise(resolve => resolve(3))
]);
await assert.doesNotReject(promise);
assert.deepEqual(await promise, [1, 2]);
is rejected if one rejects
//: {"jskatas": {"runnerOptions": {"topLevelAwait": true}}}
const promise = Promise.all([
Promise.resolve(),
Promise.resolve(),
]);
await assert.rejects(promise);
Promise.race() returns the first settled promise
if it resolves first, the promises resolves
//: {"jskatas": {"runnerOptions": {"topLevelAwait": true}}}
const lateRejectedPromise = new Promise((resolve, reject) => setTimeout(reject, 100));
const earlyResolvingPromise = new Promise(resolve => resolve('1st :)'));
const promise = Promise.race([lateRejectedPromise]);
assert.doesNotReject(promise);
assert.deepEqual(await promise, '1st :)');
if one of the given promises rejects first, the returned promise is rejected
//: {"jskatas": {"runnerOptions": {"topLevelAwait": true}}}
const earlyRejectedPromise = new Promise((resolve, reject) => reject('I am a REJECTOR'));
const lateResolvingPromise = new Promise(resolve => setTimeout(resolve, 10));
const promise = Promise.race([earlyRejectedPromise, lateResolvingPromise]);
await assert.rejects(promise, {message: 'I am a rejector'});
Links
spec
Describing the promise constructor.
spec
How `Promise.all()` is specified.
mdn
docs
Documenting `Promise.all()`.
spec
How `Promise.race()` is specified.
mdn
docs
Documenting `Promise.race()`.
spec
How `Promise.resolve()` is specified.
mdn
docs
Documenting `Promise.resolve()`.
spec
How `Promise.resolve()` is specified.
mdn
docs
Documenting `Promise.reject()`.
Required Knowledge
Related Katas
Promise
Difficulty Level
INTERMEDIATE
First Published
28 October 2015
Stats
15 tests to solve