TypeScript generic array type predicate

Question

I would like to have a generic type predicate that allows me to check the type of an object's property.

To illustrate this, I want to achieve the following:

const a: { [key in string]: string | string[]} = {
  arrayProp: ["this", "is", "an", "array"],
  otherProp: "this is a string"
};
Object.keys(a).forEach((prop: keyof typeof a) => {
  if (isArrayProperty(a, prop)) {
    // `a[prop]` is correctly detected as array
    a[prop].push("another value")
  }
})

I was expecting something like this to work

function isArrayProperty<T, P extends keyof T>(
  obj: T,
  prop: P,
): T[P] is Array<any> {
  return Array.isArray(obj[prop]);
}

However TypeScript seems to have problems with the generics and is statement in the return type.

Additional notes

I know I could just pass the value to a function like Array.isArray(a["arrayProp"]) to get it work. However, I want to go even one step further where I pass in a constructor and a property to see whether the property of an object is an array type:

type ClassConstr<T> = new (...props) => T

function isArrayProperty<T, P extends keyof T>(
  obj: ClassConstr<T>,
  prop: P,
): T[P] is Array<any> {
  return // some magic happening here;
}

class A {
  someProp: any;
}

const a = new A()
a = ["some", "array"];

if (isArrayProperty(A, "someProp") {
  // `a.someProp` is correctly detected as array
  a.someProp.push("new value");
}

The background is, that I have a separate schema definition for my classes that is only available at runtime. These schema definitions then decide, whether a property is an array, a string, a date, ... Therefore, I would like to have a function that allows me to still achieve type-safety in the components where I use these classes.

captain-yossarian from Ukraine · Accepted Answer · 2023-01-24 14:48:49Z

1

Consider this:

const a: { [key in string]: string | string[] } = {
  arrayProp: ["this", "is", "an", "array"],
  otherProp: "this is a string"
};

function isArrayProperty<T, P extends keyof T>(
  obj: T,
  prop: P,
): obj is T & Record<P, Array<any>> {
  return Array.isArray(obj[prop]);
}

Object.keys(a).forEach((prop: keyof typeof a) => {
  if (isArrayProperty(a, prop)) {
    a[prop].push("another value")
  }
})

Playground

You need to assure TS that obj in isArrayProperty is a record with prop P and value Array<any>

answered Jan 24, 2023 at 14:48

captain-yossarian from Ukraine

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