A TypeScript interface creating a type with optional keys of various types while enforcing strict null checks

Question

A TypeScript interface creating a type with optional keys of various types while enforcing strict null checks

I am attempting to devise an interface in typescript that resembles the following:

type MoveSpeed = "min" | "road" | "full";

interface Interval {
  min?: number,
  max?: number
}

interface CreepPlan {
  [partName: string] : Interval;
  move?:    MoveSpeed;
}

Unfortunately, this syntax is considered invalid by the compiler. It generates the error message

Property 'move' of type '"min" | "road" | "full" | undefined' is not assignable to string index type 'Interval'.

.

The compiler option strictNullChecks:true is being utilized, leading to the inclusion of undefined implicitly for the move? key.

However, a valid alternative syntax would be:

type MoveSpeed = "min" | "road" | "full";

interface Interval {
  min?: number,
  max?: number
}

interface CreepPlan {
  [partName: string] : Interval;
  move:    MoveSpeed;            // 'move' becomes mandatory
}

My intention is to convey the concept that "CreepPlan is comprised of string:Interval pairs, with the exception of the optional 'move' key which represents a string:MoveSpeed pair." Is there a way to express this idea effectively in TypeScript?

typescript

Answer 1

Answer №1

It was mentioned that all attributes on CreepPlan are of the Interval type as shown below:

interface CreepPlan {
  [partName: string] : Interval;
}

This means that any property accessible by a string index in CreepPlan must be an Interval, which also includes move because foo['move'] is equivalent to foo.move.

However, when examining the Interval interface, there is no mandatory structure specified:

interface Interval {
  min?: number,
  max?: number
}

Although this contract does not enforce any required fields, it necessitates an object. It can accept an empty object, but it mandates an object that potentially has both min and max properties.

In contrast, your MoveSpeed type permits undefined:

Property 'move' of type '"min" | "road" | "full" | undefined' is not assignable to string index type 'Interval'.

Thus, while Interval requires an object without obligatory properties (which can easily be met by a string), it does not allow for undefined, which is allowed by MoveSpeed.

Ultimately, your types are distinct, but this may not be apparent until you dive into them thoroughly.

Answer 2

It was mentioned that all attributes on CreepPlan are of the Interval type as shown below:

interface CreepPlan {
  [partName: string] : Interval;
}

This means that any property accessible by a string index in CreepPlan must be an Interval, which also includes move because foo['move'] is equivalent to foo.move.

However, when examining the Interval interface, there is no mandatory structure specified:

interface Interval {
  min?: number,
  max?: number
}

Although this contract does not enforce any required fields, it necessitates an object. It can accept an empty object, but it mandates an object that potentially has both min and max properties.

In contrast, your MoveSpeed type permits undefined:

Property 'move' of type '"min" | "road" | "full" | undefined' is not assignable to string index type 'Interval'.

Thus, while Interval requires an object without obligatory properties (which can easily be met by a string), it does not allow for undefined, which is allowed by MoveSpeed.

Ultimately, your types are distinct, but this may not be apparent until you dive into them thoroughly.

Answer 3

Answer №2

According to the documentation on indexable types in TypeScript, all return types of properties should be assignable to the return type of the index if you use indexable properties. This is because accessing properties using bracket notation (foo['bar']) is equivalent to dot notation (foo.bar).

So why does this concept work?

interface CreepPlan {
    [partName: string]: Interval;
    move: MoveSpeed;
}

This works because any value of type MoveSpeed can also be considered a valid Interval. Essentially, MoveSpeed is an interval with both min and max set to undefined.

Why does the following not work?

interface CreepPlan {
     [partName: string]: Interval;
     move?: MoveSpeed;
 }

The issue here is that move? can be of type MoveSpeed or undefined. When it holds the value undefined, it cannot be assigned to Interval, leading to a compilation error.

How can this be resolved? By allowing undefined as a valid value in the indexer:

interface CreepPlan {
    [partName: string]: Interval | undefined;
    move?: MoveSpeed;
}

Answer 4

According to the documentation on indexable types in TypeScript, all return types of properties should be assignable to the return type of the index if you use indexable properties. This is because accessing properties using bracket notation (foo['bar']) is equivalent to dot notation (foo.bar).

So why does this concept work?

interface CreepPlan {
    [partName: string]: Interval;
    move: MoveSpeed;
}

This works because any value of type MoveSpeed can also be considered a valid Interval. Essentially, MoveSpeed is an interval with both min and max set to undefined.

Why does the following not work?

interface CreepPlan {
     [partName: string]: Interval;
     move?: MoveSpeed;
 }

The issue here is that move? can be of type MoveSpeed or undefined. When it holds the value undefined, it cannot be assigned to Interval, leading to a compilation error.

How can this be resolved? By allowing undefined as a valid value in the indexer:

interface CreepPlan {
    [partName: string]: Interval | undefined;
    move?: MoveSpeed;
}

A TypeScript interface creating a type with optional keys of various types while enforcing strict null checks

Answer №1

Answer №2

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