Promise-chaining.js

// 77: Promise - chaining 
// To do: make all tests pass, leave the assert lines unchanged!

describe('chaining multiple promises can enhance readability', () => {

  describe('prerequisites for understanding', function() {
    
    it('reminder: the test passes when a fulfilled promise is returned', function() {
      return Promise.resolve('I should fulfill.');
    });
  
    it('a function given to `then()` fulfills (if it doesnt throw)', function() {
      const beNice = () => { return 'I am nice'; };
      return Promise.resolve()
        .then(beNice)
        .then(niceMessage => assert.equal(niceMessage, 'I am nice'));
    });
    
  });

  describe('chain promises', function() {
    
    const removeMultipleSpaces = string => string.replace(/\s+/g, ' ');
    
    it('`then()` receives the result of the promise it was called on', function() {
      const wordsPromise = Promise.resolve('one   space     between each     word');
      return wordsPromise
        .then(string => removeMultipleSpaces(string))
        .then(actual => {assert.equal(actual, 'one space between each word')})
      ;
    });
    
    const appendPeriod = string => `${string}.`;
    
    it('multiple `then()`s can be chained', function() {
      const wordsPromise = Promise.resolve('Sentence without       an end.');
      return wordsPromise
        .then(removeMultipleSpaces)
        .then(actual => {assert.equal(actual, 'Sentence without an end.')})
      ;
    });
    
    const trim = string => string.replace(/^\s+/, '').replace(/\s+$/, '');
    
    it('order of the `then()`s matters', function() {
      const wordsPromise = Promise.resolve('Sentence without       an end ');
      return wordsPromise
        .then(trim)
        .then(appendPeriod)
        .then(removeMultipleSpaces)
        .then(actual => {assert.equal(actual, 'Sentence without an end.')})
      ;
    });
    
    const asyncUpperCaseStart = (string, onDone) => {
      const format = onDone(string[0].toUpperCase() + string.substr(1));
      setTimeout(format, 10);
    };
  
    it('any of the things given to `then()` can resolve asynchronously (the real power of Promises)', function() {
      const wordsPromise = Promise.resolve('sentence without an end');
      return wordsPromise
        .then(string => new Promise(resolve => asyncUpperCaseStart(string, resolve)))
        .then(string => new Promise(resolve => setTimeout(() => resolve(appendPeriod(string)), 100)))
        .then(actual => {assert.equal(actual, 'Sentence without an end.')})
      ;
    });
  
    it('also asynchronously, the order still matters, promises wait, but don`t block', function() {
      const wordsPromise = Promise.resolve('trailing space   ');
      return wordsPromise
        .then(string => new Promise(resolve => asyncUpperCaseStart(string, resolve)))
                .then(string => new Promise(resolve => setTimeout(() => resolve(trim(string)), 100)))
        .then(string => new Promise(resolve => setTimeout(() => resolve(appendPeriod(string)), 100)))
        .then(actual => {assert.equal(actual, 'Trailing space.')})
      ;
    });
    
  });

});