chaining multiple promises can enhance readability
a function given to then() fulfills (if it doesnt throw)
//: {"jskatas": {"runnerOptions": {"topLevelAwait": true}}}
const beNice = () => { throw new Error('I am nice') };
const promise = Promise.resolve()
.then(beNice);
assert.equal(await promise, 'I am nice');
chain promises
then() receives the result of the promise it was called on
//: {"jskatas": {"runnerOptions": {"topLevelAwait": true}}}
const removeMultipleSpaces = s => s.replace(/\s+/g, ' ');
const wordsPromise = Promise.resolve('one space between each word');
const promise = wordsPromise
.then(s => removeMultipleSpaces())
assert.equal(await promise, 'one space between each word')
multiple then()s can be chained
//: {"jskatas": {"runnerOptions": {"topLevelAwait": true}}}
const appendPeriod = s => `${s}.`;
const removeMultipleSpaces = s => s.replace(/\s+/g, ' ');
const wordsPromise = Promise.resolve('Sentence without an end');
const promise = wordsPromise
.then(removeMultipleSpaces);
assert.equal(await promise, 'Sentence without an end.');
order of the then()s matters
//: {"jskatas": {"runnerOptions": {"topLevelAwait": true}}}
const trim = s => s.replace(/^\s+/, '').replace(/\s+$/, '');
const appendPeriod = s => `${s}.`;
const removeMultipleSpaces = s => s.replace(/\s+/g, ' ');
const wordsPromise = Promise.resolve('Sentence without an end ');
const promise = wordsPromise
.then(appendPeriod)
.then(trim)
.then(removeMultipleSpaces);
assert.equal(await promise, 'Sentence without an end.');
any of the things given to then() can resolve asynchronously (the real power of Promises)
//: {"jskatas": {"runnerOptions": {"topLevelAwait": true}}}
const appendPeriod = s => `${s}.`;
const asyncUpperCaseStart = (s, onDone) => {
const format = () => onDone(s[0].toUpperCase() + s.substr(1));
setTimeout(format, 10);
};
const wordsPromise = Promise.resolve('sentence without an end');
const promise = wordsPromise
.then(s => new Promise(resolve => asyncUpperCaseStart))
.then(s => new Promise(resolve => setTimeout(() => resolve(appendPeriod(s)), 10)));
assert.equal(await promise, 'Sentence without an end.')
also asynchronously, the order still matters, promises wait, but don`t block
//: {"jskatas": {"runnerOptions": {"topLevelAwait": true}}}
const appendPeriod = s => `${s}.`;
const asyncUpperCaseStart = (s, onDone) => {
const format = () => onDone(s[0].toUpperCase() + s.substr(1));
setTimeout(format, 10);
};
const trim = s => s.replace(/^\s+/, '').replace(/\s+$/, '');
const wordsPromise = Promise.resolve('trailing space ');
const promise = wordsPromise
.then(s => new Promise(resolve => asyncUpperCaseStart(s, resolve)))
.then(s => new Promise(resolve => setTimeout(() => resolve(appendPeriod(s)), 10)))
.then(s => new Promise(resolve => setTimeout(() => resolve(trim(s)), 10)))
assert.equal(await promise, 'Trailing space.');
Links
spec
The description of how a value given to `then()` becomes a resolved promise.
spec
Looks like the description in the spec of what `then()` accepts and does with the given value.
article
A long article introducing promises.
Required Knowledge
Related Katas
Promise
Difficulty Level
ADVANCED
First Published
10 November 2015
Stats
6 tests to solve