Javascript generator function yield is not assigning value in variable

Question

I am facing a issue while using yield in javascript generator.Let's have a look of my code first-

function* gen(a, b) {
 let k = yield a + b;
 let m = yield a + b + k;
 yield a + b + k + m;
}
let x = gen(10, 20);
console.log(x.next());
console.log(x.next());
console.log(x.next());

In the above code,at the first time when I run next(),it gave me a value of 30.But when I tried second time,in the second next(),it gave me a NAN value.It means,yield didn't assign the value of a+b in k.Instead it derectly move to the next line.But I think,it should assign value to k first.

What is happening here?Please let me know if you know.

Answer 1

Inside a generator, the expression:

yield <someExpression>

evaluates to the value passed into the generator with .next, like this:

function* gen(a, b) {
 const valPassedIn = yield 'foo';
 console.log('valPassedIn', valPassedIn);
}
let x = gen(10, 20);
const containsFoo = x.next();
x.next(123);

In your code, since you aren't calling .next with anything, both k and m are undefined, so they can't be +d with anything (they'll produce NaN).

For what it sounds like you want, calculate the sum, yield it, and assign it to k separately.

function* gen(a, b) {
 let k = a + b;
 yield k;
 let m = a + b + k;
 yield m;
 yield a + b + k + m;
}
let x = gen(10, 20);
console.log(x.next());
console.log(x.next());
console.log(x.next());

Answer 2

k and m remain undefined as the assignment to them is after the yield has completed. Assign value to the respective variables before the yield

function* gen(a, b) {
 k = a + b;
 yield k;
 m = a + b + k
 yield m;
 yield a + b + k + m;
}
let x = gen(10, 20);
console.log(x.next());
console.log(x.next());
console.log(x.next());

Answer 3

Because k is undefined the first time we run next()