You can't assign the type "any [] | undefined" to the type "any []"

Ever since I updated my npm and all modules today, I've run into a new problem. Everything was working perfectly fine before the update.

The error message reads: "the type 'any [] | undefined' cannot be assigned to the type 'any []'"

Here is the code snippet:

this.myservice.myfunction().toPromise().then((data) => {
  this.variable = data;
}).catch((error) => {
  console.log("Promise rejected with " + JSON.stringify(error));
});

In the service file, the function looks like this:

myfunction(): Observable<any[]> {
  return this.http.post<any[]>(`${this.baseUrl}/GetDataFromBackend`, {}).pipe(
    map((res) => res),
    catchError(this.handleError);
  );
}

Does anyone have any insights on why this error started popping up after the update? How can I go about fixing it? I seem to encounter this issue across multiple components and backend connections...

Answer №1

This is a common typescript error that can be resolved in multiple ways.

1: To handle undefined values, you can do the following:

    this.myservice.myfunction().toPromise().then((data)=>{
       if(data !== undefined && data !== null) {
          this.variable = data;
      }
    }).catch((error)=>{
      console.log("Promise rejected with " + JSON.stringify(error));
    });

2: If you are certain that a value will never be undefined, you can use the '!' operator like so:

    this.myservice.myfunction().toPromise().then((data)=>{
       
          this.variable = data!;
      
    }).catch((error)=>{
      console.log("Promise rejected with " + JSON.stringify(error));
    });

3: Another approach is to specify the variable type when declaring it:

let variable: any [] | undefined = [];

There are other potential solutions for this issue, but these 3 methods are commonly utilized based on personal experience.

It is important to maintain a strict and strongly typed TypeScript codebase without compromising essential features like "strict" mode.

The goal should be to enhance code quality by embracing strong typing, rather than diluting the integrity of the TypeScript language.

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