Argument of type '...' is not assignable to parameter of type '...' TS 2345

Question

Given the following:

interface MyInterface {
  type: string;
}

let arr: object[] = [ {type: 'asdf'}, {type: 'qwerty'}]

// Alphabetical sort
arr.sort((a: MyInterface, b: MyInterface) => {
      if (a.type < b.type) return -1;
      if (a.type > b.type) return 1;
      return 0;
    });

Can someone help decipher the TS error:

// TypeScript Error
[ts]
Argument of type '(a: MyInterface, b: MyInterface) => 0 | 1 | -1' is not assignable to parameter of type '(a: object, b: object) => number'.
  Types of parameters 'a' and 'a' are incompatible.
    Type '{}' is missing the following properties from type 'MyInterface': type [2345]

Answer 1

Here is a simplified example to reproduce the error:

interface MyInterface {
  type: string;
}
let arr:object[] = []
// Error: "object" is not compatible with MyInterface 
arr.sort((a: MyInterface, b: MyInterface) => {});

The reason its an error is because object cannot be assigned to something that is of type MyInterface:

interface MyInterface {
  type: string;
}
declare let foo: object;
declare let bar: MyInterface;
// ERROR: object not assignable to MyInterface
bar = foo; 

And the reason this is an error is because object is synonymous with {}. {} does not have the type property and therefore incompatible with MyInterface.

Fix

Perhaps you meant to use any (instead of object). any is compatible with everything.

Better fix

Use the exact type i.e. MyInterface

interface MyInterface {
  type: string;
}
let arr:MyInterface[] = []; // Add correct annotation 🌹
arr.sort((a: MyInterface, b: MyInterface) => {});