Convert a type name to the actual type

Question

Suppose we have the following enum

enum PrimayValType {
  "string",
  "boolean",
  "integer",
  "float"
}

Now i want to write a function that inputs a Buffer and a parameter of type PrimaryValType and converts the buffer based on the PrimaryValType. How to write such a function in Typescript?

function f(b: Buffer, t: PrimayValType): ??? {
    // converts b to a literal based on t
}

const b = Buffer.from('test', 'utf-8');
f(b, PrimayValType.string) // should be of type string

Answer 1

You can write the return type of your function as a mapped type. First, define PrimaryValType as a mapping from the string literals to the actual types:

type PrimaryValType = {
    "string": string,
    "boolean": boolean,
    "integer": number,
    "float": number,
}

Then given a string of type K extends keyof PrimaryValType, we can map it to the correct return type using the mapped type PrimaryValType[K].

To parse the input as the right type, you can switch on the type-string:

function parse<K extends keyof PrimaryValType>(s: string, t: K): PrimaryValType[K] {
    switch (t) {
        case "integer": return parseInt(s) as PrimaryValType[K];
        case "float":   return parseFloat(s) as PrimaryValType[K];
        case "boolean": return (s === "true") as PrimaryValType[K];
        case "string":  s as PrimaryValType[K];

        default: throw new Error("Illegal t: " + t);
    }
}

The type assertions are needed because Typescript can't tell that when t === 'integer' then K can't be 'string', for example. The code can be neatened by storing the parser functions in an object, as in @kmos.w's answer:

const parsers: { [K in keyof PrimaryValType]: (s: string) => PrimaryValType[K] } = {
    "integer": parseInt,
    "float":   parseFloat,
    "boolean": s => s === "true",
    "string":  s => s,
};

function parse<K extends keyof PrimaryValType>(s: string, t: K): PrimaryValType[K] {
    return parsers[t](s) as PrimaryValType[K];
}

The type assertion is still needed, for the same reason.

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