Typescript: No index signature with a parameter of type 'string' was found on type '{ "A": string; }

Question

I have some vanilla javascript code that takes a string input, splits the string into characters, and then matches those characters to a key on an object.

DNATranscriber = {
    "G":"C",
    "C": "G",
    "T": "A",
    "A": "U"
}
function toRna(sequence){
    const sequenceArray = [...sequence];
    const transcriptionArray = sequenceArray.map(character =>{
        return this.DNATranscriber[character];
    });

    return transcriptionArray.join("");
}

console.log(toRna("ACGTGGTCTTAA")); //Returns UGCACCAGAAUU

This works as expected. I'd now like to convert this to typescript.

class Transcriptor {
    DNATranscriber = {
       G:"C",
       C: "G",
       T: "A",
       A: "U"
    }
    toRna(sequence: string) {
        const sequenceArray = [...sequence];
        const transcriptionArray = sequenceArray.map(character =>{
            return this.DNATranscriber[character];
        });
    }
}

export default Transcriptor

But I'm getting the following error.

Element implicitly has an 'any' type because expression of type 'string' >can't be used to index type '{ "A": string; }'. No index signature with a parameter of type 'string' was found on type >'{ "A": string; }'.ts(7053)

I thought that the issue was that I needed my object key to be a string. But converting them to strings didn't work.

DNATranscriber = {
       "G":"C",
       "C": "G",
       "T": "A",
       "A": "U"
    }

I'm quite confused by this. It says that no index signature with a type of string exists on my object. But I'm sure that it does. What am I doing wrong?

Edit - I solved this by giving the DNATranscriber object a type of any.

DNATranscriber: any = {
    "G":"C",
    "C":"G",
    "T":"A",
    "A":"U"
}

Answer 1

Also, you can do this:

(this.DNATranscriber as any)[character];

Edit.

It's HIGHLY recommended that you cast the object with the proper type instead of any. Casting an object as any only help you to avoid type errors when compiling typescript but it doesn't help you to keep your code type-safe.

E.g.

interface DNA {
    G: "C",
    C: "G",
    T: "A",
    A: "U"
}

And then you cast it like this:

(this.DNATranscriber as DNA)[character];