Deciphering intricate Type Script Type declarations

Question

Deciphering intricate Type Script Type declarations

I am seeking clarification on how to utilize the object type for sending headers, aside from HttpHeaders provided in the HTTPClient declaration.

While working with Angular HttpClient, I aim to include headers using an Object. However, I am unsure of how to define an object of type [header: string]: string | string[];. I need assistance understanding this object declaration, as I am encountering a similar issue with HttpParams. My code snippet is as follows:

getLoggedInUser(requestHeaderParam: GetLoggedInUserHeaderRequestParam): Observable<LoggedInUserResponse> {
   return this.http.get<LoggedInUserResponse>(`${environment.apiBaseUrl}/auth/loggedInUser`, { headers: requestHeaderParam }); 
}

The error message displayed in VS Code is as follows:

[ts] Argument of type '{ headers: GetLoggedInUserHeaderRequestParam; }' is not assignable to parameter of type '{ headers?: HttpHeaders | { [header: string]: string | string[]; }; observe?: "body"; params?: Ht...'. Types of property 'headers' are incompatible. Type 'GetLoggedInUserHeaderRequestParam' is not assignable to type 'HttpHeaders | { [header: string]: string | string[]; }'. Type 'GetLoggedInUserHeaderRequestParam' is not assignable to type '{ [header: string]: string | string[]; }'. Index signature is missing in type 'GetLoggedInUserHeaderRequestParam'.

The Request Param type is defined as below:

export interface GetLoggedInUserHeaderRequestParam {
  uid: string;
  PSID?: string;
}

The HttpClient Declaration is outlined as below:

HttpClient.get(url: string, options: {
   headers?: HttpHeaders | {
    [header: string]: string | string[];
   };
   observe?: "body";
   params?: HttpParams | {
    [param: string]: string | string[];
   };
   reportProgress?: boolean;
   responseType: "arraybuffer";
   withCredentials?: boolean;
}): Observable<ArrayBuffer>

Your guidance is greatly appreciated!

Note: My query pertains to utilizing the Object directly, specifically in relation to its declaration type { [header: string]: string | string[]; } within the HttpClient context.

angular typescript httpclient

Answer 1

Answer №1

There is a discrepancy in the signature of

GetLoggedInUserHeaderRequestParam

and Direct Headers object

For instance

  export interface GetLoggedInUserHeaderRequestParam {
      uid: string;
      PSID?: string;
    }

The interface only allows an object with uid and optionally PSID, whereas the Headers direct object

   {
      [header: string]: string | string[];
   }

States that it can accept an object with any number of keys of type string and values as string or an array of strings.

The key difference is that your interface enforces Typescript to require an object with exact key names, while the header object can accommodate any number of keys with string values like

{
  "uid" : "1234",
  "PSID" : "1223",
  "name" : "test",
  ..... 
}

You can resolve this issue by defining the interface as

interface GetLoggedInUserHeaderRequestParam {
    [name: string] : string | string[];

}

And then calling the HTTP method as

let header: GetLoggedInUserHeaderRequestParam  = {
  "uid" : "1234",
  "PSID" : "1234"
}

getLoggedInUser(header);

Answer 2

There is a discrepancy in the signature of

GetLoggedInUserHeaderRequestParam

and Direct Headers object

For instance

  export interface GetLoggedInUserHeaderRequestParam {
      uid: string;
      PSID?: string;
    }

The interface only allows an object with uid and optionally PSID, whereas the Headers direct object

   {
      [header: string]: string | string[];
   }

States that it can accept an object with any number of keys of type string and values as string or an array of strings.

The key difference is that your interface enforces Typescript to require an object with exact key names, while the header object can accommodate any number of keys with string values like

{
  "uid" : "1234",
  "PSID" : "1223",
  "name" : "test",
  ..... 
}

You can resolve this issue by defining the interface as

interface GetLoggedInUserHeaderRequestParam {
    [name: string] : string | string[];

}

And then calling the HTTP method as

let header: GetLoggedInUserHeaderRequestParam  = {
  "uid" : "1234",
  "PSID" : "1234"
}

getLoggedInUser(header);

Answer 3

Answer №2

it might look something like this:

HTTP headers are in the 'headers' variable, while the data you provide goes into the request body.

getLoggedInUser(requestHeaderParam: GetLoggedInUserHeaderRequestParam): Observable<LoggedInUserResponse> {

     let headers = new HttpHeaders();
     headers = headers.set('Content-Type', 'application/json');

   return this.http.get<LoggedInUserResponse>(`${environment.apiBaseUrl}/auth/loggedInUser`, JSON.stringify(requestHeaderParam),
    { headers: headers }); 
}

for parameters:

    import {HttpParams} from "@angular/common/http";

getLoggedInUser(requestHeaderParam: GetLoggedInUserHeaderRequestParam): Observable<LoggedInUserResponse> {

    const params = new HttpParams()
        .set('uid', requestHeaderParam.uid)
        .set('PSID', requestHeaderParam.PSID);

     return this.http.get<LoggedInUserResponse>(`${environment.apiBaseUrl}/auth/loggedInUser`, 
        {params:params}); 
}

Answer 4

it might look something like this:

HTTP headers are in the 'headers' variable, while the data you provide goes into the request body.

getLoggedInUser(requestHeaderParam: GetLoggedInUserHeaderRequestParam): Observable<LoggedInUserResponse> {

     let headers = new HttpHeaders();
     headers = headers.set('Content-Type', 'application/json');

   return this.http.get<LoggedInUserResponse>(`${environment.apiBaseUrl}/auth/loggedInUser`, JSON.stringify(requestHeaderParam),
    { headers: headers }); 
}

for parameters:

    import {HttpParams} from "@angular/common/http";

getLoggedInUser(requestHeaderParam: GetLoggedInUserHeaderRequestParam): Observable<LoggedInUserResponse> {

    const params = new HttpParams()
        .set('uid', requestHeaderParam.uid)
        .set('PSID', requestHeaderParam.PSID);

     return this.http.get<LoggedInUserResponse>(`${environment.apiBaseUrl}/auth/loggedInUser`, 
        {params:params}); 
}

Answer 5

Answer №3

When attempting to pass an object that follows the structure of

GetLoggedInUserHeaderRequestParam

as either headers or params to HttpClient.get's options, TypeScript intervenes due to safety concerns.

The issue lies in declaring the parameter

requestHeaderParam: GetLoggedInUserHeaderRequestParam

. This declaration specifies that requestHeaderParam:

must include a uid field with type string.
may contain a PSID field with type string.

However, this does not restrict additional fields from being present. An object could meet the interface requirements and still have extra fields, some of which may not be of type string. To illustrate this point, consider the following excerpted code:

interface GetLoggedInUserHeaderRequestParam {
    uid: string;
    PSID?: string;
}

function getLoggedInUser(requestHeaderParam: GetLoggedInUserHeaderRequestParam): void {
}

const params = {
    uid: "1",
    rogue: getLoggedInUser, // Passing a function!
};

// The compiler accepts this even though params has an extra field.
getLoggedInUser(params);

// However, this call does not compile.
getLoggedInUser({
    uid: "1",
    rogue: getLoggedInUser,  // Passing a function!
})

I provided two instances of calling getLoggedInUser; the first is successful while the second results in an error. This discrepancy might lead TypeScript users to believe that undefined interface fields are prohibited, but this is not always the case. When applying an interface to an object literal, TypeScript rejects non-defined fields, although this rule only applies to object literals. (A type assertion can override this behavior.) The initial call to getLoggedInUser showcases that objects can fulfill an interface and still feature additional fields.

So why is this problematic? The

{ [header: string]: string | string[] }

definition for headers indicates that HttpClient.get requires an object with string keys and values that are strings or arrays of strings exclusively. There is no room for other types. As shown earlier, there is no guarantee that requestHeaderParam adheres to these specifications, potentially introducing incompatible fields. Consequently, TypeScript raises an error. The same reasoning applies if utilizing params.

The appropriate solution depends on your code's context. A basic approach involves implementing a type assertion:

this.http.get(..., { headers: requestHeaderParam as unknown as Record<string, string>})

An intermediary step through unknown (or any) is necessary due to the mismatched types. Note that solely relying on a type assertion provides no safeguard against passing non-string or non-string[] values. Without validation, issues or undefined behavior may result when invoking HttpClient.get.

Alternatively, you can conduct the type assertion after confirming the absence of extra fields within the object.

Another viable option involves transforming requestHeaderParam into an instance of a specific class that enforces permissible fields and offers a method returning a plain JS object that aligns with headers' criteria.

Answer 6

When attempting to pass an object that follows the structure of

GetLoggedInUserHeaderRequestParam

as either headers or params to HttpClient.get's options, TypeScript intervenes due to safety concerns.

The issue lies in declaring the parameter

requestHeaderParam: GetLoggedInUserHeaderRequestParam

. This declaration specifies that requestHeaderParam:

must include a uid field with type string.
may contain a PSID field with type string.

However, this does not restrict additional fields from being present. An object could meet the interface requirements and still have extra fields, some of which may not be of type string. To illustrate this point, consider the following excerpted code:

interface GetLoggedInUserHeaderRequestParam {
    uid: string;
    PSID?: string;
}

function getLoggedInUser(requestHeaderParam: GetLoggedInUserHeaderRequestParam): void {
}

const params = {
    uid: "1",
    rogue: getLoggedInUser, // Passing a function!
};

// The compiler accepts this even though params has an extra field.
getLoggedInUser(params);

// However, this call does not compile.
getLoggedInUser({
    uid: "1",
    rogue: getLoggedInUser,  // Passing a function!
})

I provided two instances of calling getLoggedInUser; the first is successful while the second results in an error. This discrepancy might lead TypeScript users to believe that undefined interface fields are prohibited, but this is not always the case. When applying an interface to an object literal, TypeScript rejects non-defined fields, although this rule only applies to object literals. (A type assertion can override this behavior.) The initial call to getLoggedInUser showcases that objects can fulfill an interface and still feature additional fields.

So why is this problematic? The

{ [header: string]: string | string[] }

definition for headers indicates that HttpClient.get requires an object with string keys and values that are strings or arrays of strings exclusively. There is no room for other types. As shown earlier, there is no guarantee that requestHeaderParam adheres to these specifications, potentially introducing incompatible fields. Consequently, TypeScript raises an error. The same reasoning applies if utilizing params.

The appropriate solution depends on your code's context. A basic approach involves implementing a type assertion:

this.http.get(..., { headers: requestHeaderParam as unknown as Record<string, string>})

An intermediary step through unknown (or any) is necessary due to the mismatched types. Note that solely relying on a type assertion provides no safeguard against passing non-string or non-string[] values. Without validation, issues or undefined behavior may result when invoking HttpClient.get.

Alternatively, you can conduct the type assertion after confirming the absence of extra fields within the object.

Another viable option involves transforming requestHeaderParam into an instance of a specific class that enforces permissible fields and offers a method returning a plain JS object that aligns with headers' criteria.

Deciphering intricate Type Script Type declarations

Answer №1

Answer №2

Answer №3

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