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JavaScript - Yield Operator

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JavaScript Yield Operator

The yield operator in JavaScript is used to pause and resume the generator function asynchronously. In JavaScript, generator functions are the special functions that you can pause or resume while executing. The generator functions are defined with the 'function*' syntax. The yield keyword can only be used within the generator function that contains it.

The yield operator pauses the execution of generator function and returns its operand (expression) to the generator's caller.

Syntax

The syntax of the yield operator in JavaScript is as follows −

yield expression;

Paramter

  • expression − The value to yield from the generator function via the iterator protocol. 'undefined' is yielded, if expression is omitted.

Return value

It returns the optional value passed to the generator's next() method to resume its execution.

Yield Operator in Genrator Function

To understand the yield operator, let's understand fist the working of the generator function.

When a generator function is called, it returns a generator object. When the next() method this generator object is called it resumes the execution of the generator function. When a yield expression is encountered, it pauses the execution and returns the expression after yield keyword to the object's caller (the next() method).

The next() method of the generator object returns an iterator object with two properties – value and done. The value is the actual value of the expression and the done is a boolean value. The done property is true if the execution of the generator function is completely executed, else it if false.

Below is a complete example code of generator function with yield keyword (operator).

function* test() {
 // function code
 yield expression;
}
const genObj = test();
genObj.next();

In the above syntax, 'function*' is used to create a generator function named test, and the yield keyword is used to return 'expression' from the function.

The generator function test is called and assigned the returned generator object to the variable genObj. The next() method resumes the execution of the function and returns iterator object when encounters yield expression.

Let's execute the below JavaScript code snippet –

function* test() {
    console.log("I'm before yield expression");
    yield 20;
}
const genObj = test();
console.log(genObj.next());

Notice when we call next() method, it display the message in the console first and then display the iterator object.

I'm before yield expression
{ value: 20, done: false }

Example: Returning a value

In the example below, we have defined the test() generator function. We used the yield operator 3 times in the function to return a number, an array, and a string, respectively.

After that, we used the next() method four times to resume the execution of the function. Whenever the control flow finds the yield operator, it will stop execution and return the value.

In the output, you can observe that it returns the object containing the operand of the yield operator and boolean value.

function* test() {
    yield 20;
    yield [1,2,3];
    yield "Hello World";
}
let res = test();
console.log(res.next());
console.log(res.next());
console.log(res.next());
console.log(res.next());

Output

{ value: 20, done: false }
{ value: [ 1, 2, 3 ], done: false }
{ value: 'Hello World', done: false }
{ value: undefined, done: true }

Example: Returning undefined

When we omit the expression following the yield keyword, it will return undefined.

function* test() {
    yield;
}

let res = test();
console.log(res.next());
console.log(res.next());

Output

{ value: undefined, done: false }
{ value: undefined, done: true }

Example: Passing a value to the next() method

We can also pass a value to the next() method. In the below example, we have passed 30 to the second next() method. It evaluates yield to 30. The variable result is assigned the value of yield which is evaluated as 30.

function* test() {
    let result = yield 20;
    console.log("default value paased to next() method " + result);
}

let res = test();
console.log(res.next());
console.log(res.next(30));

Output

{ value: 20, done: false }
default value paased to next() method 30
{ value: undefined, done: true }

Example

In the below code, we used the loop, and in each operation, we stop the execution of the function using the yield operator. Afterward, we use the next() method to start the execution of the generator function.

// Generator function
function* test() {
    for (let p = 0; p < 6; p += 2) {
        yield p;
    }
}
let res = test();
console.log(res.next());
console.log(res.next());
console.log(res.next());
console.log(res.next());

Output

{ value: 0, done: false }
{ value: 2, done: false }
{ value: 4, done: false }
{ value: undefined, done: true }

In real-time development, programmers use the 'yield' operator for asynchronous operations, lazy evaluations, task scheduling, iterating through large datasets, creating custom iterators, etc.

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