Can you help me figure out how to create a data type that represents changes made to a function parameter?

Question

Can you help me figure out how to create a data type that represents changes made to a function parameter?

Let's consider a scenario in which there is a JavaScript function as follows:

//index.js
function foo(obj) {
  obj['bar'] = 'biz';
}
module.exports.foo = foo;

The challenge here is to create a TypeScript definition for this particular function.

An attempt has been made using the is operator:

//index.d.ts
export function foo<T>(obj: T): T is T & { bar: string }

However, it seems that the type system is unable to recognize bar as a valid key within whatever object is passed to foo.

How can this issue be resolved?

typescript typescript-typings

Answer 1

Answer №1

A type guard is a crucial feature that influences the type of a parameter and requires a boolean return value. An appropriate implementation might look like this:

function checkType<T>(obj: T): obj is T & { prop: string }{
    obj['prop'] = 'value';
    return true;
}

let object = { prop: "example" }
if(checkType(object)) {
    object.prop //valid
    object.foo //valid
}

It's important to note that there are no assertion modes for type guards. They must explicitly return a boolean and be utilized within an if statement in order to affect the type of a variable.

Answer 2

A type guard is a crucial feature that influences the type of a parameter and requires a boolean return value. An appropriate implementation might look like this:

function checkType<T>(obj: T): obj is T & { prop: string }{
    obj['prop'] = 'value';
    return true;
}

let object = { prop: "example" }
if(checkType(object)) {
    object.prop //valid
    object.foo //valid
}

It's important to note that there are no assertion modes for type guards. They must explicitly return a boolean and be utilized within an if statement in order to affect the type of a variable.

Answer 3

Answer №2

After receiving guidance from @TitianCernicova-Dragomir, I implemented the solution in the following way:

function checkProperty<T>(obj: T): obj is T & { property: string } {
    obj['property'] = 'value';
    return true;
}

const object = { value: "example" }
if(!checkProperty(object)) throw 'This scenario will never occur';

object.property //valid
object.value //valid

Answer 4

After receiving guidance from @TitianCernicova-Dragomir, I implemented the solution in the following way:

function checkProperty<T>(obj: T): obj is T & { property: string } {
    obj['property'] = 'value';
    return true;
}

const object = { value: "example" }
if(!checkProperty(object)) throw 'This scenario will never occur';

object.property //valid
object.value //valid

Can you help me figure out how to create a data type that represents changes made to a function parameter?

Answer №1

Answer №2

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