TypeScript fails to recognize when variable has already undergone undefined or null value check

Consider the following scenario: TypeScript fails to recognize that parameters.a and parameters.b have been checked for undefined values, leading to a potential issue where transformValue(parameters.a) line might return an undefined value:

type Scenario = {
    a?: string,
    b?: string
}

function scenario(parameters: {
    a?: string,
    b?: string,
    skipA?: boolean,
    skipB?: boolean
}): Scenario {

    const shouldReturnA = typeof parameters.a !== "undefined" && parameters.skipA !== false;
    const shouldReturnB = typeof parameters.b !== "undefined" && parameters.skipB !== false;

    return {
        ...shouldReturnA ? { a: transformValue(parameters.a) } : {},
        ...shouldReturnB ? { b: transformValue(parameters.b) } : {}
    }
}

function transformValue(targetValue: string): string {
    return targetValue + "~~~";
}

Argument of type 'string | undefined' is not assignable to parameter of type 'string'.
  Type 'undefined' is not assignable to type 'string'.(2345)

Argument of type 'string | undefined' is not assignable to parameter of type 'string'.
  Type 'undefined' is not assignable to type 'string'.(2345)

🌎 Fiddle

To simplify this synthetic example, it can be rewritten as follows:

function scenario(parameters: {
    a?: string,
    b?: string,
    skipA?: boolean,
    skipB?: boolean
}): Scenario {
  return {
      ...typeof parameters.a !== "undefined" && parameters.skipA !== false ? { 
        a: transformValue(parameters.a) 
      } : {},
      ...shouldReturnB = typeof parameters.b !== "undefined" && parameters.skipB !== false ? { 
        b: transformValue(parameters.b) 
      } : {}
  }
}

However, when looking at a live example like the one below:

public static normalizeRawConfig(
  {
    pickedFromConsoleInputConfig,
    rawValidConfigFromFile
  }: {
    pickedFromConsoleInputConfig: ProjectBuilderRawConfigNormalizer.PickedFromConsoleInputConfig;
    rawValidConfigFromFile: ProjectBuilderRawValidConfigFromFile;
  }
): ProjectBuilderNormalizedConfig {

  const markupPreprocessingConfigNormalizingIsRequired: boolean =
      !isUndefined(rawValidConfigFromFile[ProjectBuilderTasksIDsForConfigFile.markupPreprocessing]) &&
      (
          isUndefined(pickedFromConsoleInputConfig.tasksAndSourceFilesSelection) ||
          Object.keys(pickedFromConsoleInputConfig.tasksAndSourceFilesSelection)
              .includes(ProjectBuilderTasksIDsForConfigFile.markupPreprocessing)
      );

  const stylesPreprocessingConfigNormalizingIsRequired: boolean =
      !isUndefined(rawValidConfigFromFile[ProjectBuilderTasksIDsForConfigFile.stylesPreprocessing]) &&
      (
          isUndefined(pickedFromConsoleInputConfig.tasksAndSourceFilesSelection) ||
          Object.keys(pickedFromConsoleInputConfig.tasksAndSourceFilesSelection)
              .includes(ProjectBuilderTasksIDsForConfigFile.stylesPreprocessing)
      );

  return {
    ...markupPreprocessingConfigNormalizingIsRequired ? {
      markupPreprocessing: MarkupPreprocessingRawSettingsNormalizer.getNormalizedSettings(
          rawValidConfigFromFile.markupPreprocessing, commonSettings__normalized
      )
    } : {},
    ...stylesPreprocessingConfigNormalizingIsRequired ? {} : {
      stylesPreprocessing: StylesPreprocessingRawSettingsNormalizer.getNormalizedSettings(
          rawValidConfigFromFile.stylesPreprocessing, commonSettings__normalized
      )
    }
  };
}

If we remove 

markupPreprocessingConfigNormalizingIsRequired
and 
stylesPreprocessingConfigNormalizingIsRequired
, the code becomes more complex:

return {
  ...!isUndefined(rawValidConfigFromFile[ProjectBuilderTasksIDsForConfigFile.markupPreprocessing]) &&
    (
        isUndefined(pickedFromConsoleInputConfig.tasksAndSourceFilesSelection) ||
        Object.keys(pickedFromConsoleInputConfig.tasksAndSourceFilesSelection)
            .includes(ProjectBuilderTasksIDsForConfigFile.markupPreprocessing)
    ) ? {
    markupPreprocessing: MarkupPreprocessingRawSettingsNormalizer.getNormalizedSettings(
        rawValidConfigFromFile.markupPreprocessing, commonSettings__normalized
    )
  } : {},
  ...!isUndefined(rawValidConfigFromFile[ProjectBuilderTasksIDsForConfigFile.stylesPreprocessing]) &&
    (
        isUndefined(pickedFromConsoleInputConfig.tasksAndSourceFilesSelection) ||
        Object.keys(pickedFromConsoleInputConfig.tasksAndSourceFilesSelection)
            .includes(ProjectBuilderTasksIDsForConfigFile.stylesPreprocessing)
    ) ? {
    stylesPreprocessing: StylesPreprocessingRawSettingsNormalizer.getNormalizedSettings(
        rawValidConfigFromFile.stylesPreprocessing, commonSettings__normalized
    )
  } : {}
};

This approach can lead to more complicated conditions in the future.

Are there any best practices in dealing with such scenarios in TypeScript?

Update

After trying to compare XXX !== undefined instead of typeof XXX !== "undefined", similar results were obtained.

https://i.sstatic.net/rAV2f.png

typescript

Answer №1

Why isn't the undefined check being recognized?

The issue causing the TypeScript compiler to overlook the checks for variable a and b relates to how "type guards" function. In your code, you have a type guard condition parameters.a !== undefined. When utilized like this:

if (parameters.a !== undefined) {
  transformValue(parameters.a)
}

TypeScript will understand this because the transformValue call is within the if statement block. This is how type guards narrow down variable types within specific blocks. However, TypeScript fails to realize that you narrowed the type when setting the value for shouldReturnA. To TypeScript, this narrowing of types becomes irrelevant by the time it reaches the ...shouldReturnA ? code.

Alternative Approach

This solution may not be as quick as using ! after the variable, but it offers better logic organization and avoids repetition in my opinion.

type Example = {
    a?: string,
    b?: string
}

function example(parameters: {
    a?: string,
    b?: string,
    skipA?: boolean,
    skipB?: boolean
}): Example {

    const validatedProp = (prop: string | undefined, shouldSkip: boolean | undefined) => 
      !shouldSkip && typeof prop === "string" ? { a: transformValue(prop) } : {}

    return {
        ...validatedProp(parameters.a, parameters.skipA),
        ...validatedProp(parameters.b, parameters.skipB),
    }
}

function transformValue(targetValue: string): string {
    return targetValue + "~~~";
}

TS Playground There's still room for additional improvements through further refactoring.

My View on typeof undefined vs 'undefined'

I suggest following ESLint on this matter. The typeof keyword returns a string "undefined," not the actual value undefined. To make your type guard work, ensure you use the string form for comparison (even though I compare against "string").

Answer №2

According to the information provided in the Typescript handbook, there exists a concept known as the non-null assertion operator:

A new post-fix expression operator, denoted by !, can be used to assert that its operand is non-null and not undefined in situations where the type checker cannot infer this information. Essentially, the expression x! generates a value of the same type as x but without including null or undefined. Similar to type assertions such as <T>x and x as T, the ! non-null assertion operator is eliminated in the resulting JavaScript code.

type Example = {
    a?: string,
    b?: string
}

function example(parameters: {
    a?: string,
    b?: string,
    skipA?: boolean,
    skipB?: boolean
}): Example {

    const shouldReturnA = parameters.a !== undefined && parameters.skipA !== false;
    const shouldReturnB = parameters.b !== undefined && parameters.skipB !== false;

    return {
        ...shouldReturnA ? { a: transformValue(parameters.a!) } : {},
        ...shouldReturnB ? { b: transformValue(parameters.b as string) } : {} // another way
    }
}

function transformValue(targetValue: string): string {
    return targetValue + "~~~";
}

For further insights on the non-null assertion operator, you may refer to this informative article: https://medium.com/better-programming/cleaner-typescript-with-the-non-null-assertion-operator-300789388376

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