Specify the object key type when using a `for-in` loop

Question

Specify the object key type when using a `for-in` loop

My current situation involves an object type:

interface ShortUrlParam {
    openid: string;
    avatar: string;
    nickname: string;
}

const param: ShortUrlParam = {
    openid: 'abc123',
    avatar: '',
    nickname: 'wenzi'
}

let query = '';
for(let key in param) {
    query += `&${key}=${encodeURIComponent(param[key])}`; // encountering an error here
}

When accessing param[key], I receive the following error message:

'Element implicitly has an 'any' type because expression of type 'string' can't be used to index type 'ShortUrlParam'. No index signature with a parameter of type 'string' was found on type 'ShortUrlParam'.ts(7053)

I have come up with two potential solutions, both not without flaws.

1. Redefining the interface ShortUrlParam

interface ShortUrlParam {
    openid: string;
    avatar: string;
    nickname: string;
    [key: string]: string;
}

2. Treating param as any

for(let key in param) {
    query += `&${key}=${encodeURIComponent((param as any)[key])}`;
}

My inquiry is whether there exists a more optimal solution to this issue?

typescript

Answer 1

Answer №1

Typescript does not allow type annotations for variables declared in a for..in loop due to the open nature of object types. This means that an object can have additional properties beyond what is expected, making it impossible to narrow down the type of a variable introduced in a loop.

However, there are workarounds available. One option is to declare the variable outside the loop with the desired type annotation:

let key: keyof ShortUrlParam;
for (key in param) {
  query += `&${key}=${encodeURIComponent(param[key])}`;
}

This approach ensures type safety, although caution should be taken if unexpected properties are added to the object later on.

An alternative method is to assert the type of the variable within the loop using type assertions:

for (let key in param) {
  query += `&${key}=${encodeURIComponent(param[key as keyof ShortUrlParam])}`;
}

Both approaches are more secure than using 'any', as they help catch errors where extra properties are added to the object. Hopefully, these suggestions prove helpful. Good luck!

Answer 2

Typescript does not allow type annotations for variables declared in a for..in loop due to the open nature of object types. This means that an object can have additional properties beyond what is expected, making it impossible to narrow down the type of a variable introduced in a loop.

However, there are workarounds available. One option is to declare the variable outside the loop with the desired type annotation:

let key: keyof ShortUrlParam;
for (key in param) {
  query += `&${key}=${encodeURIComponent(param[key])}`;
}

This approach ensures type safety, although caution should be taken if unexpected properties are added to the object later on.

An alternative method is to assert the type of the variable within the loop using type assertions:

for (let key in param) {
  query += `&${key}=${encodeURIComponent(param[key as keyof ShortUrlParam])}`;
}

Both approaches are more secure than using 'any', as they help catch errors where extra properties are added to the object. Hopefully, these suggestions prove helpful. Good luck!

Answer 3

Answer №2

Not quite as secure as jcalz's solution, but still handy for type hints when you don't have a specific type definition readily available (maybe because you didn't define an object literal right before). You can make an educated guess instead of resorting to any. Just stay away from any...

Furthermore, the JavaScript heap within a for loop gets reset after each iteration, so using const instead of let ensures that the key remains unchanged. If you do need to change the key being iterated over, stick with a traditional for-loop to keep your code clean.

Here is a general example:

for (const key in thing) {
  doThings(key as keyof typeof thing)
}

And here's an example relevant to your query:

for (const key in param) {
  query += `&${key}=${encodeURIComponent(param[key as keyof typeof param])}`
}

Answer 4

Not quite as secure as jcalz's solution, but still handy for type hints when you don't have a specific type definition readily available (maybe because you didn't define an object literal right before). You can make an educated guess instead of resorting to any. Just stay away from any...

Furthermore, the JavaScript heap within a for loop gets reset after each iteration, so using const instead of let ensures that the key remains unchanged. If you do need to change the key being iterated over, stick with a traditional for-loop to keep your code clean.

Here is a general example:

for (const key in thing) {
  doThings(key as keyof typeof thing)
}

And here's an example relevant to your query:

for (const key in param) {
  query += `&${key}=${encodeURIComponent(param[key as keyof typeof param])}`
}

Answer 5

Answer №3

Attempt this:

for(let index=0;index<parameters.length; index++) {
  let keyValue = parameters[index];
  searchQuery += `&${keyValue}=${encodeURIComponent((parameters as any)[keyValue])}`;
}

Answer 6

Attempt this:

for(let index=0;index<parameters.length; index++) {
  let keyValue = parameters[index];
  searchQuery += `&${keyValue}=${encodeURIComponent((parameters as any)[keyValue])}`;
}

Specify the object key type when using a `for-in` loop

1. Redefining the interface ShortUrlParam

2. Treating param as any

Answer №1

Answer №2

Answer №3

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