javascript - Why is ES6 "yield" a reserved word when called in this context?

Question

I am using node 4.1.1. When I run this code

"use strict";

function *generator() {
  let numbers = [1,2,3,4,5];
  numbers.map(n => yield (n + 1));
}

for (var n of generator()) {
  console.log(n);
}

I get this error

  numbers.map(n => yield (n + 1));
                   ^^^^^

SyntaxError: Unexpected strict mode reserved word

If I rearrange the code to be this

"use strict";

function *generator() {
  let numbers = [1,2,3,4,5];
  let higherNumbers = numbers.map(n => n + 1);
  for(let i=0;i<higherNumbers.length;i++) {
    yield higherNumbers[i];
  }
}

for (var n of generator()) {
  console.log(n);
}

I get the expected result.

Why does the second one work, and the first fail? And surely if a keyword is reserved, it's reserved in all contexts, not just when it's used in a arrow function?

Answer 1

It is because arrow functions are not generator functions. For example,

function temp() {
  yield 1;
}

Can we expect this to work? No. Because temp is not a generator function. The same is applicable to arrow functions as well.

---

FWIW, the usage of yield in an Arrow function is an early error as per the ECMAScript 2015 specification, as per this section,

ArrowFunction : ArrowParameters => ConciseBody

• It is a Syntax Error if ArrowParameters Contains YieldExpression is true.

• It is a Syntax Error if ConciseBody Contains YieldExpression is true.