TypeScript - migrate JavaScript function

Question

I have a JavaScript helper function which allows to remove a property of an object :

removeProp(obj, prop) {
  const { [prop]: _, ...result } = obj;
  return result;
}

TypeScript version :

removeProp(obj: Object, prop: string): Object {
  // tslint:disable-next-line: no-shadowed-variable
  const { [prop]: _, ...result } = obj;
  return result;
}

1- I have a lint error : type 'Object' has no index signature corresponding to type 'string' tslint in [prop].

2- How i can initialize result variable with the right type if obj is a generic (takes all properties of the generic type - the removed property) ?

Titian Cernicova-Dragomir · Accepted Answer · 2019-07-19 09:54:31Z

2

You need to make the function generic, with a generic parameter (lets call in T) to represent the type of the object and a second parameter (lets call it K) that should be a keyof T. For the return type you can use Omit<T, K> which will remove a key from a given type (although strictly speaking you can leave the return type blank and it will be correctly inferred)

function removeProp<T, K extends keyof T>(obj: T, prop: K): Omit<T, K> {
  const { [prop]: _, ...result } = obj;
  return result;
}

let o = removeProp({ a: 1, b: 2}, "b")

A version that works on 3.1 can't use spread (as spread with generic type parameters is not supported in that version, that was allowed in 3.2) and has to define Omit as that did not exist in 3.1 either (added in 3.5)


type Omit<T, K extends PropertyKey> = Pick<T, Exclude<keyof T, K>>
function removeProp<T, K extends keyof T>(obj: T, prop: K): Omit<T, K> {
  const result = Object.assign({}, obj);
  delete result[prop]
  return result;
}

let o = removeProp({ a: 1, b: 2}, "b");
console.log(o);

edited Jul 19, 2019 at 9:54

answered Jul 19, 2019 at 9:31

Titian Cernicova-Dragomir

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5 Comments

Mouad Ennaciri Over a year ago

I have 2 lint errors Cannot find name 'Omit' and Rest types may only be created from object types . Being I am on TypeScript 3.1 I understand the Omit error but not the second error

Titian Cernicova-Dragomir Over a year ago

@MouadEnnaciri both of those errors happen because you are in 3.1 the latest version is 3.5 and the code works with that. I can provide a backward compatible version but it can't use spread to remove the property. Correctly typing spread on generic types happened in 3.2

Mouad Ennaciri Over a year ago

I understand, that it is the version compatible the v3.1?

Titian Cernicova-Dragomir Over a year ago

@MouadEnnaciri i added a version for 3.1. All ts releases are mostly compatible, but they all also have a number of breaking changes, upgrading might yield new errors as the type system becomes stricter in newer versions. Although if it compiles I expect it to work as it did before.

Mouad Ennaciri Over a year ago

I would like to avoid the delete operator but I mark the question as resolved being the limitation caused by my TS version. Thanks

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