fix: map number key converted to string

[andrea11]

Description

I am trying to use class-transformer with maps, and I having an issue using numbers as keys.
Minimal code-snippet showcasing the problem

I defined the following class:

export class Data {
  @Type(() => String)
  public map: Map<number, string>

  constructor(
    map: Map<number, string> = new Map<number, string>()
  ) {
    this.map = map
  }
}

when I do:

plainToInstance(Data, {map: {123: "test"}}

the key is converted from number to string

Expected behavior

The entry in the map should have 123 as key

Actual behavior

The entry in the map is "123" as key

[diffy0712]

Hello @andrea11,

this seems to be the case. Class-transformer will use the keys from the plain object directly, which means it will always be string or a Symbol.

See Property accessors docs.

Each property name is a string or a Symbol. Any other value, including a number, is coerced to a string. This outputs 'value', since 1 is coerced into '1'.

I know that in a Map the key could be a number as well, but I do not see this as a high priority issue right now.
I am not sure adding the ability to type cast the string to a number would be a good idea.