Leaving out the return statement in an asynchronous typed function

Question

Leaving out the return statement in an asynchronous typed function

Consider this example:

export async function foo(): Promise<string>{
  await bar()
  return;
}

No errors are thrown during compilation.

However, if you change the code to:

export async function foo(): Promise<string>{
  await bar()
}

You will encounter an error. The specific error message is:

A function whose declared type is neither 'void' nor 'any' must return a value

So why does this happen?

It seems to be related to https://github.com/Microsoft/TypeScript/blob/master/doc/spec.md#63-function-implementations, as well as the complexity introduced by using async functions and TypeScript typing.

UPDATE:

For your information, having return;, return, or even return undefined; in a function implies the same semantics perspective, correct?

javascript typescript promise async-await return

Answer 1

Answer №1

This topic isn't directly related to the concept of async/await; when an empty return statement is used, it is essentially considered equivalent to return undefined. This means that the following code snippet also functions correctly:

function foo(): string{
    return; // same as return undefined;
}

If you have strictNullChecks enabled, you will encounter an error since undefined cannot be assigned to a type of string

If you completely omit the return statement, the compiler will infer the return type as void, which would result in a type mismatch with the declared type of string and trigger an error. The same principle applies if the function is marked as async and returns a Promise

Answer 2

This topic isn't directly related to the concept of async/await; when an empty return statement is used, it is essentially considered equivalent to return undefined. This means that the following code snippet also functions correctly:

function foo(): string{
    return; // same as return undefined;
}

If you have strictNullChecks enabled, you will encounter an error since undefined cannot be assigned to a type of string

If you completely omit the return statement, the compiler will infer the return type as void, which would result in a type mismatch with the declared type of string and trigger an error. The same principle applies if the function is marked as async and returns a Promise

Answer 3

Answer №2

Providing a specific return type in TypeScript other than Void or Any is crucial for maintaining type safety. It signals to the compiler that you intend to return a certain type of value, prompting it to enforce this requirement. This rule is outlined in section 6.1 of the TypeScript specification here:

An explicitly typed function with a non-void, non-any return type must include at least one return statement within its body.

In regards to the usage of return, your observation is correct.

Both return; and return undefined; yield the same outcome as per section 13.10 of the ECMAScript specification here:

If no Expression is specified, the return value defaults to undefined.

The absence of a return statement also results in the return of undefined according to section 9.2.1. Step 11 states that if no explicit return occurs (Step 9) and no exceptions are thrown (Step 10), then undefined is returned by default.

Although these different examples may lead to identical outcomes in JavaScript (returning undefined), they hold distinct semantic implications for the TypeScript compiler (the second example indicates no return value).

Answer 4

Providing a specific return type in TypeScript other than Void or Any is crucial for maintaining type safety. It signals to the compiler that you intend to return a certain type of value, prompting it to enforce this requirement. This rule is outlined in section 6.1 of the TypeScript specification here:

An explicitly typed function with a non-void, non-any return type must include at least one return statement within its body.

In regards to the usage of return, your observation is correct.

Both return; and return undefined; yield the same outcome as per section 13.10 of the ECMAScript specification here:

If no Expression is specified, the return value defaults to undefined.

The absence of a return statement also results in the return of undefined according to section 9.2.1. Step 11 states that if no explicit return occurs (Step 9) and no exceptions are thrown (Step 10), then undefined is returned by default.

Although these different examples may lead to identical outcomes in JavaScript (returning undefined), they hold distinct semantic implications for the TypeScript compiler (the second example indicates no return value).

Answer 5

Answer №3

If one desires stringent type checking, it is recommended to configure the following setting in the tsconfig.json file:

    "strictNullChecks": true

For instance, consider a function like this:

async function (): Promise<string> {
   return; // equivalent to returning undefined
}

If the `strictNullChecks" option is not enabled in your tsconfig.json, the code will compile without any errors.

Refer to docs/handbook/compiler-options.html for more information.

Answer 6

If one desires stringent type checking, it is recommended to configure the following setting in the tsconfig.json file:

    "strictNullChecks": true

For instance, consider a function like this:

async function (): Promise<string> {
   return; // equivalent to returning undefined
}

If the `strictNullChecks" option is not enabled in your tsconfig.json, the code will compile without any errors.

Refer to docs/handbook/compiler-options.html for more information.

Answer 7

Answer №4

To avoid returning the Promise, it's necessary to utilize Promise<void>.

Answer 8

To avoid returning the Promise, it's necessary to utilize Promise<void>.

Answer 9

Answer №5

When you use the return type annotation (: Promise<string>), you are indicating that the function will eventually return a Promise that resolves to a string. Therefore, if a function does not have a return statement, it goes against this specified annotation/intent.

If you want to signify that the function does not return anything, you can use the annotation : void. However, be aware that this may lead to an error related to coercing undefined to a Promise. In such cases, using : Promise<void> might be more suitable.

In the context of your example, having an empty return expression is acceptable because in an async function, non-promise values returned will automatically be wrapped in a promise. This means that the return; statement remains valid from the compiler's perspective.

Answer 10

When you use the return type annotation (: Promise<string>), you are indicating that the function will eventually return a Promise that resolves to a string. Therefore, if a function does not have a return statement, it goes against this specified annotation/intent.

If you want to signify that the function does not return anything, you can use the annotation : void. However, be aware that this may lead to an error related to coercing undefined to a Promise. In such cases, using : Promise<void> might be more suitable.

In the context of your example, having an empty return expression is acceptable because in an async function, non-promise values returned will automatically be wrapped in a promise. This means that the return; statement remains valid from the compiler's perspective.

Leaving out the return statement in an asynchronous typed function

Answer №1

Answer №2

Answer №3

Answer №4

Answer №5

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