Can I pass a parameter to ES6 generator function

Question

Here's a ES6 generator:

function *Gen() {
    var input1 = yield 'output1'
}

var gen = Gen()

gen.next('input1').value // return 'output1'

gen called 1st time, return output1 but the variable input1 doesn't equal 'input1' which passed in , the value of input actually is the 'input2', 2nd time next('input2')called

My question is how to access the input1 which is the first time next called, something like:

function *Gen() {
    var input 1 = param1
    var input2 = yield 'output1'
}

Answer 1

When you obtain the generator you don't have yields you can push value to (you are at the beginning of the generator function). You need to trigger the generator and reach the first yield by calling gen.next() without any arguments. At this point you have an yield at your disposal and you can push your value via gen.next('input1') effectively replacing the expression yield 'output1' with the value you passed to next - 'input1'. Then you need another yield or a return to provide your custom value to the consumer of the generator. Here is the code:

function *Gen() {
    var input1 = yield 'output1'
    return input1
}

var gen = Gen()

gen.next();
gen.next('input1').value // return 'input1'

Answer 2

If you want a parametrised generator function you can use a higher order function that returns generator:

function myGenerator(startFrom) {
    return (function *() {
        var i = startFrom;
        while (true) yield i++;
    })();
}

var gen = myGenerator(5);
console.log(gen.next().value) // 5
console.log(gen.next().value) // 6
console.log(gen.next().value) // 7

Higher order generators can be of use too:

function getGenerator(baseStartFrom, expStartFrom) {
    return (function *() {
        var a = baseStartFrom;
        while (true) {
          yield (function *() {
              var i = expStartFrom;
              while (true) yield Math.pow(a, i++);
          })();
          a++;
        }
    })();
}

var gen = getGenerator(2, 3);
var gen2 = gen.next().value; // generator yields powers of 2
  console.log(gen2.next().value); // 8
  console.log(gen2.next().value); // 16
  console.log(gen2.next().value); // 32
var gen3 = gen.next().value; // generator yields powers of 3
  console.log(gen3.next().value); // 27
  console.log(gen3.next().value); // 81
  console.log(gen3.next().value); // 243

The sample is most likely useless but the same approach can be used, for example, to generate random number generators.