How to Fix the "Unexpected strict mode reserved word 'yield'" Error in JavaScript

Tari Ibaba
Last updated on August 05, 2023
How to Fix the "Unexpected strict mode reserved word 'yield'" Error in JavaScript

The "Unexpected strict mode reserved word 'yield'" syntax error happens in JavaScript when you use the yield keyword outside a generator function.

The unexpected strict mode reserved word 'yield' error happening in JavaScript

Here's an example of the error happening:

function* numberGen() {
  // ❌ SyntaxError: Unexpected strict mode reserved word 'yield'
  yield 2;
  yield 4;
}

const gen = numberGen();

console.log(gen.next().value);
console.log(gen.next().value);

Note: As this is a syntax error, we don't need to call the function for it to happen, and no part of the code runs until we fix it.

To fix the "SyntaxError unexpected strict mode reserved word 'yield'" error, ensure that the innermost enclosing function containing the yield keyword is a generator function.

function* numberGen() {
  // ✅ Runs successfully - no error
  yield 2;
  yield 4;
}

const gen = numberGen();

console.log(gen.next().value); // 2
console.log(gen.next().value); // 4

We create generator functions with function*; the * must be there.

Make sure innermost function is generator function

A common reason why the "Unexpected strict mode reserved word 'yield'" happens is using yield in an inner function that's in a generator function.

This could be a callback or closure.

function* numberGen() {
  return () => {
    for (let i = 0; i < 30; i += 10) {
      // ❌ SyntaxError: Unexpected strict mode reserved word 'yield'
      yield i;
    }
  }
}

const gen = numberGen()();

console.log(gen.next().value);
console.log(gen.next().value);

You may think the outer function* will let the yield work, but nope - it doesn't!

You have to make the innermost function a generator:

function numberGen() {
  return function* () {
    for (let i = 0; i < 30; i += 10) {
      // ✅ Runs successfully - no error
      yield i;
    }
  };
}

const gen = numberGen()();

console.log(gen.next().value);  // 10
console.log(gen.next().value);  // 20

Note we removed the * from the outer one, so it acts like a normal function and returns the generator.

Also we change the arrow function a normal one, because arrow functions can't be generators.

Here's another example, seen here:

function* generator() {
  const numbers = [1, 2, 3, 4, 5];
  // ❌ SyntaxError: Unexpected strict mode reserved word
  numbers.map((n) => yield(n + 1));
}

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

Here the goal was to consume the generator in the for loop, printing out each number one by one.

But the callback is the innermost function that has the yield, and it's not a generator. Let's fix this:

function* generator() {
  const numbers = [1, 2, 3, 4, 5];
  // ✅ Runs successfully - no error
  yield* numbers.map((n) => n + 1);
}

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

yield* keyword

Wondering about the * in the yield*? It's a shorter way of looping through the array and yielding each item.

A shorter way of this:

function* generator() {
  const numbers = [1, 2, 3, 4, 5];
  // ✅ Runs successfully - no error
  for (const num of numbers.map((n) => n + 1)) {
    yield num;
  }
}

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

Key takeaways

The "Unexpected strict mode reserved word 'yield'" error can happen in JavaScript when you mistakenly use the yield keyword outside a generator function.

To fix this error, make sure that the innermost enclosing function containing the yield keyword is actually a generator function.

One common reason for meeting this error is using yield in an inner function that's not a generator.

Remember, the yield* syntax lets us easily loop through an array and yield each item. No need for a traditional for loop with yield statements.

By understanding and correctly applying these concepts, you can avoid the "Unexpected reserved word 'yield'" error and ensure smooth execution of your generator functions in JavaScript. Happy coding!

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