Typescript. When properties are enum values, they are treated as read only

On this code:

enum SpeedRanges {
    slow,
    fast,
}

type SpeedHistogram = {
    [key in keyof typeof SpeedRanges]?: number;
};

const sh: SpeedHistogram = {};
sh.slow = 9;

I get this error:

> Cannot assign to 'slow' because it is a read-only property.(2540)

Why is it read only?

This seems to do the trick.

enum SpeedRanges {
	slow,
	fast,
}

type SpeedRangeKey = keyof typeof SpeedRanges;

type SpeedHistogram = {
	[key in SpeedRangeKey]?: number;
};

const sh: SpeedHistogram = {};
sh.slow = 9;

Enums in typescript are read-only which totally makes sense.
There are two possible solutions:

1. Remove the readonly modifier using mapping modifiers:

type SpeedHistogram = {
    -readonly [key in keyof typeof SpeedRanges]?: number;
};

However, enums contain index signature of [x: number]: number:

type SpeedHistogram = {
    [x: number]: number | undefined;
    slow?: number | undefined;
    fast?: number | undefined;
}

With an index signature, you can add any number key with a number value. We can remove the index signature by only taking the string keys of the enum, which can be done either by using the built-in utility type Extract or by interesting the keys with string, which will only take the intersection, which is equivalent to any string key:

type SpeedHistogram = {
  -readonly [Key in (keyof typeof SpeedRanges) & string]?: number;
};

2. Define a separate type for the keys of the SpeedRanges:

type SpeedRangesKeys = keyof typeof SpeedRanges;

type SpeedHistogram = {
  [Key in SpeedRangesKeys]?: number;
};

The reason why the separate type works are because in the OP version, the keyof is used inside of the mapped type which gets the property modifier (readonly) and applies to the property. When defining a separate type there is no context so we just get the union of all keys without any modifiers.

In TypeScript enums are readonly. When you iterate over the keys directly to define another keys, you also inherit the original key properties in the newly defined keys, as the readonly property.

If you want to remove the readonly property, just use -readonly modificator, like so:

enum SpeedRanges {
    slow,
    fast,
}

type SpeedHistogram = {
    -readonly [key in keyof typeof SpeedRanges]?: number;
};

const sh: SpeedHistogram = {};
sh.slow = 9;

You can also use @IgorDanchenko's answer, which is using the original key list to define a string union, which being just a list of strings, they do not have any properties in them.

It's possible that you are trying to use number enum when you want a string enum

enum SpeedRanges {
  slow = "slow",
  fast = "fast",
}
 
type SpeedHistogram = {
  [key in SpeedRanges]?: number;
};

const sh: SpeedHistogram = {};
sh.slow = 9;