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function* - JavaScript | MDN

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function* 宣告式（function 關鍵字後面跟著一個星號）定義了一個生成器函式（generator function），他會回傳一個生成器（Generator）物件。

function* generator(i) {
  yield i;
  yield i + 10;
}

const gen = generator(10);

console.log(gen.next().value);
// Expected output: 10

console.log(gen.next().value);
// Expected output: 20

你可以透過 GeneratorFunction 建構式來定義生成器函式。

function* name([param[, param[, ... param]]]) {
   statements
}

name

函式名稱。

param

要被傳入函式的引數名稱，一個函式最多可以擁有 255 個引數。

statements

statements 構成了函式內容的陳述式。

生成器是可以離開後再次進入的函式。在兩次進入之間，生成器的執行狀態（變數綁定狀態）會被儲存。

呼叫生成器函式並不會讓裡面的程式碼立即執行，而是會回傳一個針對該函式的迭代器（iterator）物件。當呼叫迭代器的 next() 方法時，生成器函式將會執行到遭遇的第一個 yield 運算式，該運算式給定的值將從迭代器中回傳，如果是 yield* 則會交給另一個生成器函式處理。next() 方法回傳一個物件，該物件有 value 屬性，包含了產生的數值，還有 done 屬性，為布林值，指出該生成器是否產出最後的數值。呼叫 next() 方法如果帶有一個參數，將會讓先前暫停的生成器函式恢復執行，以該參數值取代先前暫停的 yield 陳述式。

生成器中的 return 陳述式被執行時，會讓生成器 done 狀態為真。若有數值被返回的動作帶回，將是放在 value 傳回的。已返回的生成器不會再產生任何數值。

function* idMaker() {
  var index = 0;
  while (index < index + 1) yield index++;
}

var gen = idMaker();

console.log(gen.next().value); // 0
console.log(gen.next().value); // 1
console.log(gen.next().value); // 2
console.log(gen.next().value); // 3
// ...

function* anotherGenerator(i) {
  yield i + 1;
  yield i + 2;
  yield i + 3;
}

function* generator(i) {
  yield i;
  yield* anotherGenerator(i);
  yield i + 10;
}

var gen = generator(10);

console.log(gen.next().value); // 10
console.log(gen.next().value); // 11
console.log(gen.next().value); // 12
console.log(gen.next().value); // 13
console.log(gen.next().value); // 20

function* logGenerator() {
  console.log(0);
  console.log(1, yield);
  console.log(2, yield);
  console.log(3, yield);
}

var gen = logGenerator();

// the first call of next executes from the start of the function
// until the first yield statement
gen.next(); // 0
gen.next("pretzel"); // 1 pretzel
gen.next("california"); // 2 california
gen.next("mayonnaise"); // 3 mayonnaise

function* yieldAndReturn() {
  yield "Y";
  return "R";
  yield "unreachable";
}

var gen = yieldAndReturn();
console.log(gen.next()); // { value: "Y", done: false }
console.log(gen.next()); // { value: "R", done: true }
console.log(gen.next()); // { value: undefined, done: true }

function* f() {}
var obj = new f(); // throws "TypeError: f is not a constructor"

const foo = function* () { yield 10; yield 20; };
const bar = foo();console.log(bar.next()); // {value: 10, done: false}

Older Firefox versions implement an older version of the generators proposal. In the older version, generators were defined using a regular function keyword (without an asterisk) among other differences. See Legacy generator function for further information.

Starting with Gecko 29, the completed generator function no longer throws a TypeError "generator has already finished". Instead, it returns an IteratorResult object like { value: undefined, done: true } (Firefox bug 958951).

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