next() is not a function unable to use generators in NodeJS

Question

I am using the following code in NodeJS 7.10.0:

function * gen(){
    yield 100;
    yield 200;
    yield 300
}
console.log(gen.next());

and I get this in return:

TypeError: gen.next is not a function
    at Object.<anonymous> (/Files/development/learning/node-bits/generators.js:15:17)
    at Module._compile (module.js:571:32)
    at Object.Module._extensions..js (module.js:580:10)
    at Module.load (module.js:488:32)
    at tryModuleLoad (module.js:447:12)
    at Function.Module._load (module.js:439:3)
    at Module.runMain (module.js:605:10)
    at run (bootstrap_node.js:427:7)
    at startup (bootstrap_node.js:151:9)
    at bootstrap_node.js:542:3

Robin James Kerrison · Accepted Answer · 2018-02-16 13:35:07Z

5

When you call a generator function, its body is not executed. Instead it returns you an iterator object. When you call iterator.next(), body is executed, you get the value and context is maintained.

You can do

let i= gen()
console.log(i.next())

and you will get the desired behaviour.

Refer this: https://developer.mozilla.org/en/docs/Web/JavaScript/Reference/Statements/function*

edited Feb 16, 2018 at 13:35

Robin James Kerrison

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answered May 8, 2017 at 9:12

Krrish Raj

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1 Comment

quinton Over a year ago

Thanks for that, sounds obvious when I read your post - maybe another time my brain will kick in. Thanks!

RickN · Accepted Answer · 2017-05-08 09:11:56Z

2

You should assign the output of gen to a variable and call next on the returned value:

const out = gen();
console.log(out.next());
console.log(out.next());
console.log(out.next());
console.log(out.next());

answered May 8, 2017 at 9:11

RickN

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1 Comment

quinton Over a year ago

Thanks for the answer, but krrish had a better explanation.

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