Incorrectly assuming the data type of a variable once it has been given a value within a callback function

Question

Incorrectly assuming the data type of a variable once it has been given a value within a callback function

There seems to be a type inference issue with TypeScript in the if condition involving the 'title' variable

The inferred type of the variable title that is being passed to the trim method is never

let title: string | null = null;

const response = await fetch("https://example.com");

const newResponse = await new HTMLRewriter()
  .on("#title", {
    text({ text }) {
      title ??= "";
      title += text;
    },
  })
  .transform(response);

await newResponse.arrayBuffer();

if (title !== null) {
  title = title.trim();
}

export {};

typescript

Answer 1

Answer №1

When it comes to Typescript, it may not always perfectly align with the flow of your JavaScript code. The analysis of the code reveals that the variable title remains unchanged except within the on callback. Consequently, the compiler assumes that title has not changed at all and, knowing its value is null, it deems the check for non-nullity impossible, resulting in the if block labeling the type of title as never.

An alternative to using a variable is wrapping the logic in a function that takes in a parameter of type string | null, although this approach might not be suitable in your scenario.

A more straightforward solution involves using an assertion rather than a strict type definition. With an assertion, the compiler is left uncertain about the precise value, which is our desired outcome:

let title = null as string | null;

(async () => {
  await fetch('https://example.com').then(() => (title = 'test'));

  if (title !== null) {
    title = title.trim() // no error;
  }
})();

playground

Answer 2

When it comes to Typescript, it may not always perfectly align with the flow of your JavaScript code. The analysis of the code reveals that the variable title remains unchanged except within the on callback. Consequently, the compiler assumes that title has not changed at all and, knowing its value is null, it deems the check for non-nullity impossible, resulting in the if block labeling the type of title as never.

An alternative to using a variable is wrapping the logic in a function that takes in a parameter of type string | null, although this approach might not be suitable in your scenario.

A more straightforward solution involves using an assertion rather than a strict type definition. With an assertion, the compiler is left uncertain about the precise value, which is our desired outcome:

let title = null as string | null;

(async () => {
  await fetch('https://example.com').then(() => (title = 'test'));

  if (title !== null) {
    title = title.trim() // no error;
  }
})();

playground

Answer 3

Answer №2

It appears that TypeScript is having trouble recognizing the possibility of the title variable being updated asynchronously before the trim operation takes place. To address this issue and provide TypeScript with a clearer understanding, consider restructuring the code as follows:

let title: string | null = null;

const response = await fetch("https://example.com");

await new HTMLRewriter()
  .on("#title", {
    text({ text }) {
      // Check if 'title' is null and update accordingly
      if (title === null) {
        title = text;
      } else {
        title += text;
      }
    },
  })
  .transform(response).arrayBuffer()

// TypeScript should now recognize 'title' as potentially a string value
if (title !== null) {
  title = title.trim();
}

export {};

I have explicitly handled the case where title is null within the text method, allowing TypeScript to better understand the changes that may occur to title over time, especially after asynchronous operations. This way, when reaching the trimming part, it will be more evident to the compiler that title has been modified and can indeed be treated as a string.

Alternatively, you could assert the type like so:

  title = (title as string).trim();

Answer 4

It appears that TypeScript is having trouble recognizing the possibility of the title variable being updated asynchronously before the trim operation takes place. To address this issue and provide TypeScript with a clearer understanding, consider restructuring the code as follows:

let title: string | null = null;

const response = await fetch("https://example.com");

await new HTMLRewriter()
  .on("#title", {
    text({ text }) {
      // Check if 'title' is null and update accordingly
      if (title === null) {
        title = text;
      } else {
        title += text;
      }
    },
  })
  .transform(response).arrayBuffer()

// TypeScript should now recognize 'title' as potentially a string value
if (title !== null) {
  title = title.trim();
}

export {};

I have explicitly handled the case where title is null within the text method, allowing TypeScript to better understand the changes that may occur to title over time, especially after asynchronous operations. This way, when reaching the trimming part, it will be more evident to the compiler that title has been modified and can indeed be treated as a string.

Alternatively, you could assert the type like so:

  title = (title as string).trim();

Answer 5

Answer №3

One possible reason for this issue could be the asynchronous nature of callbacks conflicting with synchronous checks on title content. The callback may not have finished executing by the time the check is performed.

let title: string | null = null

const response = await fetch('https://example.com')

const newResponse = await new HTMLRewriter()
  .on('#title', () => {
    title ??= ''
    title += 't'
  })
  .transform(response)

await newResponse.arrayBuffer()

if (title !== undefined) {
  // There seems to be an issue here
  title = title.trim()
}

setTimeout(() => {
  if (title !== null) {
    // Since setTimeout runs asynchronously, TypeScript may assume a different value is present at that point 
    title = title.trim()
  }
}, 1000);

export {}

Avoiding the use of setTimeout as a workaround, it might serve as an indicator of a bug in the code. This is because the if statement will likely execute before the fetch response is received.

Answer 6

One possible reason for this issue could be the asynchronous nature of callbacks conflicting with synchronous checks on title content. The callback may not have finished executing by the time the check is performed.

let title: string | null = null

const response = await fetch('https://example.com')

const newResponse = await new HTMLRewriter()
  .on('#title', () => {
    title ??= ''
    title += 't'
  })
  .transform(response)

await newResponse.arrayBuffer()

if (title !== undefined) {
  // There seems to be an issue here
  title = title.trim()
}

setTimeout(() => {
  if (title !== null) {
    // Since setTimeout runs asynchronously, TypeScript may assume a different value is present at that point 
    title = title.trim()
  }
}, 1000);

export {}

Avoiding the use of setTimeout as a workaround, it might serve as an indicator of a bug in the code. This is because the if statement will likely execute before the fetch response is received.

Incorrectly assuming the data type of a variable once it has been given a value within a callback function

Answer №1

Answer №2

Answer №3

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