Implementing strict type enforcement for the `toString` method

Is there a way to prevent the compiler from overloading toString? I've tried using the never type, but it still allows implicit assignments and only raises an error when something is done with the variable. It's inconvenient to remember to explicitly declare toString calls as strings.

type ArrayToStringMethod = {
  (this: { join(a: string): string, length: number }): string
  (this: any): never // if this overload isn't here typescript uses 
                     // Object.toString automatically.
}

type ArrayDontMutate<t> = {
    toString: ArrayToStringMethod
    readonly [index: number]: t
    readonly length: number
} &
Pick<
  Array<t>, 
  'find'   |
  'map'    |
//'join'   | deliberately removed to make toString fail
  'some'   |
  'slice'  |
  'concat' |
  'reduce'>

let a:ArrayDontMutate<string> =['a','b','c'] as any
let b = a.toString() //should fail no join method. b is the never type

Answer №1

Is there a way to prevent the compiler from overloading toString?

In the type definitions file lib.d.ts, there are native interfaces that dictate how the type checker functions. One of these interfaces is the Object interface, which includes the toString method.

If you wish to restrict the use of toString, you can utilize the --noLib flag and modify a duplicate of lib.d.ts without including this method.

For further information:

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