ES6
Introduction
Generators are processes that you can pause and resume. Generators are
defined with function*.
const foo = function* () {
yield 'a';
yield 'b';
yield 'c';
};
let str = '';
for (const val of foo()) {
str = str + val;
}
console.log(str);Generator Kinds
Generator function declarations
function* genFunc() { }
const genObj = genFunc();Generator function expressions
const genFunc = function* () { };
const genObj = genFunc();Generator method definitions in object literals
const obj = {
* generatorMethod() {
}
};
const genObj = obj.generatorMethod();Generator method definitions in class definitions
class MyClass {
* generatorMethod() {
}
}
const myInst = new MyClass();
const genObj = myInst.generatorMethod();Examples
Iterables
function* objectEntries(obj) {
const propKeys = Reflect.ownKeys(obj);
for (const propKey of propKeys) {
// `yield` returns a value and then pauses
// the generator. Later, execution continues
// where it was previously paused.
yield [propKey, obj[propKey]];
}
}
const jane = { first: 'Jane', last: 'Doe' };
for (const [key,value] of objectEntries(jane)) {
console.log(`${key}: ${value}`);
}
// Output:
// first: Jane
// last: DoeAsync Iterables
async function* createAsyncIterable(syncIterable) {
for (const elem of syncIterable) {
yield elem;
}
}Asynchronous code
const fetchJson = co.wrap(function* (url) {
try {
let request = yield fetch(url);
let text = yield request.text();
return JSON.parse(text);
}
catch (error) {
console.log(`ERROR: ${error.stack}`);
}
});
fetchJson('http://example.com/some_file.json')
.then(obj => console.log(obj));