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

You have two options with simple and idiomatic Typescript:

  1. Use index type
DNATranscriber: { [char: string]: string } = {
  G: "C",
  C: "G",
  T: "A",
  A: "U",
};

This is the index signature the error message is talking about. Reference

  1. Type each property:
DNATranscriber: { G: string; C: string; T: string; A: string } = {
  G: "C",
  C: "G",
  T: "A",
  A: "U",
};