Typescript is failing to infer the definition of an object, even after conducting a thorough check

Question

Typescript is failing to infer the definition of an object, even after conducting a thorough check

I am encountering an issue with the code structure below:

interface Data {
   isAvailable: boolean;
}

const foo = (data: Data | undefined, error: boolean) => {
   const hasError = error || !data;
   if (!hasError) {
      if (data.isAvailable) // do something here. this is where I am facing a problem
   }
   return hasError;
}

When running Typescript, it flags a Object is possibly undefined error on data in the if (data.isAvalable) check, even though I have verified hasError. A solution could be to add && data to the if (hasError), but this leads to duplicated code. Shouldn't Typescript be able to deduce that data is defined without the explicit check?

typescript

Answer 1

Answer №1

Exciting news for TypeScript 4.4 as it introduces support for control flow analysis of aliased conditions. This means that your tests will seamlessly work without any complicated modifications:

const foo = (data: Data | undefined, error: boolean) => {
    const hasError = error || !data;
    if (!hasError) {
        if (data.isAvailable) { } // everything runs smoothly
    }
    return hasError;
}

Here's the TS4.4 beta Playground link

If you are using versions of TypeScript older than 4.4, some adjustments in coding approach may be required to ensure clarity for the compiler. One possible solution is to incorporate the test directly instead of creating an alias:

const foo = (data: Data | undefined, error: boolean) => {
    if (!(error || !data)) {
        if (data.isAvailable) { } // all good
    }
    return error || !data;
}

Unfortunately, this might result in a bit of code duplication.

Check out TS4.3 on Playground

Answer 2

Exciting news for TypeScript 4.4 as it introduces support for control flow analysis of aliased conditions. This means that your tests will seamlessly work without any complicated modifications:

const foo = (data: Data | undefined, error: boolean) => {
    const hasError = error || !data;
    if (!hasError) {
        if (data.isAvailable) { } // everything runs smoothly
    }
    return hasError;
}

Here's the TS4.4 beta Playground link

If you are using versions of TypeScript older than 4.4, some adjustments in coding approach may be required to ensure clarity for the compiler. One possible solution is to incorporate the test directly instead of creating an alias:

const foo = (data: Data | undefined, error: boolean) => {
    if (!(error || !data)) {
        if (data.isAvailable) { } // all good
    }
    return error || !data;
}

Unfortunately, this might result in a bit of code duplication.

Check out TS4.3 on Playground

Answer 3

Answer №2

The function signature data: Data | undefined indicates that the variable data can either be of type Data or it can be undefined.

If at a certain point in time, data is undefined, TypeScript will not recognize the existence of the isAvailable field.

To handle this situation, you can use optional chaining like so:

if (data?.isAvailable) {

Reference: https://www.typescriptlang.org/docs/handbook/release-notes/typescript-3-7.html#optional-chaining

Check out the playground link for an example: Playground Link

Answer 4

The function signature data: Data | undefined indicates that the variable data can either be of type Data or it can be undefined.

If at a certain point in time, data is undefined, TypeScript will not recognize the existence of the isAvailable field.

To handle this situation, you can use optional chaining like so:

if (data?.isAvailable) {

Reference: https://www.typescriptlang.org/docs/handbook/release-notes/typescript-3-7.html#optional-chaining

Check out the playground link for an example: Playground Link

Typescript is failing to infer the definition of an object, even after conducting a thorough check

Answer №1

Answer №2

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