check if function is a generator

Question

I played with generators in Nodejs v0.11.2 and I\'m wondering how I can check that argument to my function is generator function.

I found this way typeof f ===

Answer 1

In node 7 you can instanceof against the constructors to detect both generator functions and async functions:

const GeneratorFunction = function*(){}.constructor;
const AsyncFunction = async function(){}.constructor;

function norm(){}
function*gen(){}
async function as(){}

norm instanceof Function;              // true
norm instanceof GeneratorFunction;     // false
norm instanceof AsyncFunction;         // false

gen instanceof Function;               // true
gen instanceof GeneratorFunction;      // true
gen instanceof AsyncFunction;          // false

as instanceof Function;                // true
as instanceof GeneratorFunction;       // false
as instanceof AsyncFunction;           // true

This works for all circumstances in my tests. A comment above says it doesn't work for named generator function expressions but I'm unable to reproduce:

const genExprName=function*name(){};
genExprName instanceof GeneratorFunction;            // true
(function*name2(){}) instanceof GeneratorFunction;   // true

The only problem is the .constructor property of instances can be changed. If someone was really determined to cause you problems they could break it:

// Bad people doing bad things
const genProto = function*(){}.constructor.prototype;
Object.defineProperty(genProto,'constructor',{value:Boolean});

// .. sometime later, we have no access to GeneratorFunction
const GeneratorFunction = function*(){}.constructor;
GeneratorFunction;                     // [Function: Boolean]
function*gen(){}
gen instanceof GeneratorFunction;      // false