Pass a value to the generator using next() method in JavaScript

Published August 13, 2020

The yield keyword in the generator function is commonly used to return a value from the generator function.

But what if I told you it can do more than that!

It can also accept a value to a generator function.

Let's understand this,

let's make a generator function that asks a question first and then takes a value and then yields a value based on that.

// Multiply by 2 Generator function
function* MultiplyBy2() {
  const number = yield "Which number would you like to be multiplied by 2 with?";

  yield number * 2;
}

// initializing genrator function
const multiplyBy2 = MultiplyBy2();

// First the question is returned from generator function
// We can show the returned or yielded value using the
// next() method and then using the value property
console.log(multiplyBy2.next().value); // the question is shown
  • Here we have a generator function which first gives a question Which number would you like to be multiplied by 2 with? to the user.

  • After initializing the generator function, then we are showing the first yield value which is the question to the user using the next() method and access the value using the value property.

  • Now to pass a value to the generator function we need to pass that value as an argument to the next() method like this.

// Multiply by 2 Generator function
function* MultiplyBy2() {
  const number = yield "Which number would you like to be multiplied by 2 with?";

  yield number * 2;
}

// initialising generator function
const multiplyBy2 = MultiplyBy2();

// First the question is returned from generator function
// We can show the returned or yielded value using the
// next() method and then using the value property
console.log(multiplyBy2.next().value); // the question is shown

// passing 4 as a value to the generator function
// as an argument to the next() method
// after that, the passed value is multiplied by 2
// and then the next yield value is
// returned from the generator function
console.log(multiplyBy2.next(4).value);
  • Now the passed value will be stored inside the number variable inside the generator function, and the next value of the yield is returned from the generator function (The value which is multiplied by 2)

  • 2 things have happened here, first we have passed 4 as a value to the next() method and the next yield value is returned from the generator function.

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