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.

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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.