The Generator object is returned by a generator function and it conforms to both the iterable protocol and the iterator protocol.
Generator is a subclass of the hidden Iterator class.
Constructor
The Generator constructor is not available globally. Instances of Generator must be returned from generator functions:
function* generator() {
yield 1;
yield 2;
yield 3;
}
const gen = generator(); // "Generator { }"
console.log(gen.next().value); // 1
console.log(gen.next().value); // 2
console.log(gen.next().value); // 3
In fact, there's no JavaScript entity that corresponds to the Generator constructor. There's only a hidden object which is the prototype object shared by all objects created by generator functions. This object is often stylized as Generator.prototype to make it look like a class, but it should be more appropriately called GeneratorFunction.prototype.prototype, because GeneratorFunction is an actual JavaScript entity.
Instance properties
These properties are defined on Generator.prototype and shared by all Generator instances.
Generator.prototype.constructor
: The constructor function that created the instance object. For
Generatorinstances, the initial value isGeneratorFunction.prototype.Note:
Generatorobjects do not store a reference to the generator function that created them.
Generator.prototype[@@toStringTag]- : The initial value of the
@@toStringTagproperty is the string"Generator". This property is used in Object.prototype.toString.
- : The initial value of the
Instance methods
Also inherits instance methods from its parent Iterator.
- Generator.prototype.next
- : Returns a value yielded by the yield expression.
- Generator.prototype.return
- : Acts as if a
returnstatement is inserted in the generator's body at the current suspended position, which finishes the generator and allows the generator to perform any cleanup tasks when combined with atry...finallyblock.
- : Acts as if a
- Generator.prototype.throw
- : Acts as if a
throwstatement is inserted in the generator's body at the current suspended position, which informs the generator of an error condition and allows it to handle the error, or perform cleanup and close itself.
- : Acts as if a
Examples
An infinite iterator
With a generator function, values are not evaluated until they are needed. Therefore a generator allows us to define a potentially infinite data structure.
function* infinite() {
let index = 0;
while (true) {
yield index++;
}
}
const generator = infinite(); // "Generator { }"
console.log(generator.next().value); // 0
console.log(generator.next().value); // 1
console.log(generator.next().value); // 2
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