Unexpected strict mode reserved word 'yield' in JavaScript

# Unexpected strict mode reserved word 'yield' in JavaScript

The "SyntaxError: Unexpected strict mode reserved word yield" occurs when the yield keyword is used outside of a generator function.

To solve the error, mark the function as a generator by appending an asterisk to the function keyword, e.g. function* generator() {}.

Here's an example of how the error occurs.

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function example() {
  // ⛔️ SyntaxError: Unexpected strict mode reserved word 'yield'
  yield 10;
  yield 15;
}

We used the yield keyword inside of a function that we didn't mark as a generator.

# Add an asterisk to mark the function as a generator

Add an asterisk after the function keyword to mark a function as a generator.

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// ✅ works
function* generator() {
  yield 10;
  yield 15;
}

const gen = generator();

console.log(gen.next().value); // 👉️ 10
console.log(gen.next().value); // 👉️ 15

# The directly enclosing function has to be marked as a generator

The directly enclosing function has to be a generator for you to be able to use the yield keyword.

Here's an example of nested functions.

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function generator() {
  return function* inner() {
    yield 10;
    yield 15;
  };
}

const gen = generator()();

console.log(gen.next().value); // 👉️ 10
console.log(gen.next().value); // 👉️ 15

Notice that we marked the inner function as a generator and not the outer one.

This is because we use the yield keyword inside of the inner function.

To be able to use the yield keyword, declare the directly enclosing function as a generator.