What is the reason behind continuously receiving the error message stating "Not all code paths return a value here"?

Can someone help me understand why I am consistently encountering this error message from typescript?

PS. I am aware that in this scenario I could simply use a boolean and not create a function, but my focus here is on typescript.

I keep receiving the error: "Not all code paths return a value."

Here is the code snippet:

const isCompleted = (completed: boolean) => {
  if(completed) {
    return true
  } else if(!completed) {
    return false
  }
}

Interestingly, I do not receive an error when using the following code snippet, however, ideally, I should not be getting any errors either way.

const isCompleted = (completed: boolean) => {
  if(completed) {
    return true
  } else {
    return false
  }
}

Answer №1

Using a conditional statement in your code splits the current execution into different branches, creating multiple possible paths.

For instance, the first conditional if (completed) {...} leads to either returning true or continuing with the remaining lines. The following conditional else if (!completed) {...} further splits into two more paths: one where you return false and another where no value is returned (implicitly resulting in undefined in JavaScript).

While as a human you can easily see that the condition for each path is based on the same value of completed (a boolean with only two states), the TypeScript compiler may not infer this relationship automatically. Therefore, it's important to explicitly define the code paths when using conditionals.

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