Generator: return inside a generator function

return in a generator function is special

the returned value is an IteratorResult (just like any value returned via yield)

returns an IteratorResult (an object with the properties value and done)

function* generatorFunction() { return 1; } const returned = generatorFunction().next(); const propertyNames = []; assert.deepEqual(propertyNames, Object.keys(returned));

the property value is the returned value

function* generatorFunction() { return; } const {value} = generatorFunction().next(); assert.equal(value, 23);

the property done is true

function* generatorFunction() { yield 42; } const {done} = generatorFunction().next(); assert.equal(done, true);

NOTE: yield does not return done=true but done=false!

function* generatorFunction() { return 1; } const returned = generatorFunction().next(); assert.deepEqual(returned, {value: 1, done: false});

a missing return returns {value: undefined, done: true}

function* generatorFunction() { yield; } const returned = generatorFunction().next(); assert.deepEqual(returned, {value: void 0, done: true});

mixing return and yield

is possible

function* generatorFunctionWithYieldAndReturn() { yield 1; } const iterator = generatorFunctionWithYieldAndReturn(); const values = [ iterator.next(), iterator.next() ]; assert.deepEqual(values, [{value: 1, done: false}, {value: 2, done: true}]);

the mix behaves different to two yields

function* generatorFunctionWithYieldAndReturn() { yield 1; } const iterator = generatorFunctionWithYieldAndReturn(); const values = [1, 2]; assert.deepEqual(values, Array.from(iterator));

two yields returning values

function* generatorFunctionWithTwoYields() { } assert.deepEqual(Array.from(generatorFunctionWithTwoYields()), [1, 2]);

return a yielded value by "chaining" return and yield

function* generatorFunction() { return 1; } const iterator = generatorFunction(); const values = [ iterator.next().value, iterator.next(2).value ]; assert.deepEqual(values, [1, 2]);

Links

spec docs Description of the IteratorResult interface.
docs mdn Describes how a return statement behaves inside a generator.

Required Knowledge

• `const` declaration (Block scope)
• object (Destructuring)
• `Array.from()` (Array API)
• protocol (Iterator)
• creation (Generator)
• iterator (Generator)
• yield expressions (Generator)
• send value to a generator (Generator)

Related Katas

Generator

• creation
• iterator
• yield expressions
• send value to a generator
• send function to a generator
• return inside a generator function

Difficulty Level

ADVANCED

First Published

25 September 2015

Stats

9 tests to solve