An Alternative to the Omit Utility Type

TypeScript provides sevral utility types. We can efficiently construct new types with them. Omit<Type, Keys> is one of them to remove specific properties from object types.

The basic usage is like this.

type NewType = Omit<OriginalType, "propertyToRemove">;

Pitfall

There is an pitfall of the use of Omit<Type, Keys>. Suppose you want to remove the prop1 property from the Props interface bellow.

interface Props {
  prop1: string;
  prop2: number;
  prop3: boolean;
  [key: string]: any;
}

You might think that Omit<Type, Keys> can achieve that like this.

type PropsWithoutProp1 = Omit<Props, "prop1">;

However, the PropsWithoutProp1 type is interpreted as following...

type PropsWithoutProp1 = {
  [x: string]: any;
  [x: number]: any;
};

The prop2 and the prop3 properties are gone! This is not what we intended to do!

Let's dive into Omit<Type, Keys> itself.

type Omit<T, K extends keyof any> = Pick<T, Exclude<keyof T, K>>;

Exclude<keyof Props, "prop1"> points to string | number because the Prop type has an string index sigunature, for more details. Thus the string overlaps "prop2" | "prop3".

Solution

This is one of the solutions to preserve the prop2 and the prop3 properties by re-mapping keys.

type PropsWithoutProp1 = {
  [K in keyof Props as K extends "prop1" ? never : K]: Props[K];
};

The resulting type is what we expected!

type PropsWithoutProp1 = {
  [x: string]: any;
  prop2: number;
  prop3: boolean;
};

The K in keyof Props part iterates over "prop1" | "prop2" | "prop3" | string, and the as K extends "prop1" ? never : K part creates the resulting keys removing the prop1 by setting the never in the true branch of the conditional type.