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));
}

f         


        

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.