51-generator-yield-expression.js

// 51: Generator - Yield Expressions
// To do: make all tests pass, leave the assert lines unchanged!

describe('generator - `yield` is used to pause and resume a generator function', () => {

  function* generatorFunction() {
    yield 'hello';
    yield 'world';
  }

  let generator;

  beforeEach(function() {
    generator = generatorFunction();
  });

  it('converting a generator to an array resumes the generator until all values are received', () => {
    let values = Array.from(generator);
    assert.deepEqual(values, ['hello', 'world']);
  });

  describe('after the first `generator.next()` call', function() {
    
    it('the value is "hello"', function() {
      const {value} = generator.next();
      assert.equal(value, 'hello');
    });
    
    it('and `done` is false', function() {
      const {done} = generator.next();
      assert.equal(done, false);
    });
    
  });

  describe('after the second `next()` call', function() {
    
    let secondItem;
    beforeEach(function() {
      generator.next();
      secondItem = generator.next();
    });
    
    it('`value` is "world"', function() {
      let {value} = secondItem;
      assert.equal(value, 'world');
    });
    
    it('and `done` is still false', function() {
      const done = secondItem.done;
      assert.equal(done, false);
    });
  });

  describe('after stepping past the last element, calling `next()` that often', function() {
    
    it('`done` property equals true, since there is nothing more to iterator over', function() {
      generator.next();
      generator.next();
      let {done} = generator.next();
      assert.equal(done, true);
    });
    
  });

});