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