How do I override a property in typescript?

Question

This may not be possible due to current language limitations, but I'm using the latest TS (1.8.10) and am running into an issue with the ui-grid typings. The isRowSelectable property on IGridOptions is defined as an optional boolean but the documentation says it's a function (and it is). I am trying to override the boolean property to be a function that returns a boolean.

Normally, I just extend the typing interface and do what I need, but that's not working in this case. Here's what I have:

interface ISelectionGridOptions extends IGridOptions {
  isRowSelectable: (row: IGridRowOf<Profile>) => boolean;
}

Where the relevant field in IGridOptions is:

export interface IGridOptions {
  ...
  isRowSelectable?: boolean
  ...
}

And the error I'm getting is:

(41,11): error TS2430: Interface 'ISelectionGridOptions' incorrectly extends interface 'IGridOptionsOf<any>'.
  Types of property 'isRowSelectable' are incompatible.
    Type '(row: IGridRowOf<Profile>) => boolean' is not assignable to type 'boolean'.

Short of fixing the core typings definitions, is there a way to fix this in my code?

Answer 1

You can use the utility type Omit : https://www.typescriptlang.org/docs/handbook/utility-types.html#omittk

interface ISelectionGridOptions extends Omit<IGridOptions, 'isRowSelectable'> {
  isRowSelectable: (row: IGridRowOf<Profile>) => boolean;
}

Answer 2

If the type definitions are incorrect, you can't use overriding in order to fix them - the type system correctly treats this as an error. In a typed language, it's a good thing that subclasses are prevented from changing type signatures. The problem here is the broken type definition. If it's not practical to fix the def, and the issue occurs in only a few places in your code, you could use a cast to any to disable typechecking until the defs are fixed:

var foo = <boolean> (<any>bar).isRowSelectable(args);
bar.isRowSelectable = <any>foo; 

Just leave a comment explaining what's going on. It is, of course, best to fix the type defs.

Note: It breaks, like the error says, because a function isn't assignable to boolean. The function peg is failing to fit into the boolean hole, not vice versa

Answer 3

Update for TS 3.5+

You can add this utility type to your project:

type Modify<T, R> = Omit<T, keyof R> & R;

And then use it like this:

type ISelectionGridOptions = Modify<IGridOptions, {
  isRowSelectable: (row: IGridRowOf<Profile>) => boolean;
}>

Demo in TS Playground

Further Reading

• Overriding interface property type defined in Typescript d.ts file
• How to override type properties in TypeScript

Answer 4

You can use the Merge utility from the excellent type-fest package to override or overwrite any property on a type, regardless of it's original type:

import type {Modify} from 'type-fest'

type SelectionGridOptions = Modify<GridOptions, {
  isRowSelectable: (row: GridRow<Profile>) => boolean;
}>