How can one fix the error "Type 'undefined' cannot be assigned to type 'XXXX' in Typescript"?

Currently, I am facing an issue with my interface properties while trying to access a service. The error message I encountered is - Type 'undefined' is not assignable in displayfile at this.isOpen. Any assistance in resolving this error would be greatly appreciated.

Please note that altering values in the config file like

"strictNullChecks":false
is not an option for me.

The Interface I'm working with is as follows:

export interface DataFile {
  file?: DataFile[];
  location: string;
}

Here is a snippet of my Typescript class:


  isOpen?: DataFile;

  Scan(aero: string, TeamClass: string) {
    if (TeamClass === 'Hawkins') {
      if (this.redService.name.file && this.redService.name.file.length > 0) {
        this.isOpen = this.redService.name.file.filter((data: DataFile) => data.location == aero[0])[0];
      }
    }
    
    this.displayfile(this.isOpen).then(async () => {  
            //rest of the functionality
        }
      }
    });
  }

The specific error I'm encountering is:

 Argument of type 'DataFile | undefined' is not assignable to parameter of type
'DataFile'.
  Type 'undefined' is not assignable to type 'DataFile'.

Answer №1

When isOpen is set with a question mark, it can have two possible types: one being DataFile (the specified type) and the other being undefined. This is why Typescript is flagging an error.

To address this issue, you have a couple of options to choose from:

  • Check if isOpen is populated with data before proceeding, like so:
if (!isOpen) { /* do something */ }
...continue with the rest of your code
  • Add a ! before isOpen (e.g., this.isOpen!) to explicitly exclude the possibility of it being undefined. However, be cautious as this might lead to errors within your this.displayfile function if isOpen does end up being undefined.

For more information on the question mark operator, you can visit: https://www.geeksforgeeks.org/why-use-question-mark-in-typescript-variable/

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