Question 118859 in Json Question posted in
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Typescript: import and access JSON with arbitrary keys

I’m trying to import a JSON file with arbitrary keys (e. g. device names) but can’t figure out how to access the imported data with variable (unknown) keys at runtime in TypeScript.

Example JSON file "test.json":

{
    "a": {
        "b": "1"
    }
}

Example TS code:

import t from "./test.json";

let k = "a"          // <- variable key

console.log(t["a"]); // <- ok
console.log(t[k]);   // <- error

The TypeScript error:
Element implicitly has an ‘any’ type because expression of type ‘string’ can’t be used to index type ‘{ a: { b: string; }; }’.
No index signature with a parameter of type ‘string’ was found on type ‘{ a: { b: string; }; }’.

I’ve already read about TypeScript Records and Index Signatures, but how would I import or "cast" the imported JSON data so that it can be accessed using variable keys? The data will (must) be verified at runtime.

Tags:

2

Answers


  1. 0

    This should work. Playground

      const json = { "a": { "b": "1" }, "c": {"d": "1"} }; // <-- simulate import

  type Unsafe = { [parentKey: string]: {[childKey: string]: any} } ;
  // key not tpyed
  let k = "a"

  const t: Unsafe = json;
  console.log(t["a"]); 
  console.log(t[k]);   // <- ok but could fail on runtime

  // safe approch
  type Safe =Record<'a'| 'c', {[key: string]: string}>
  // typed key
  let saveKey: keyof Safe = "a"          // <- variable key
  
  const safe: Safe  = json;
  console.log(safe['a'])
  console.log(safe[saveKey])
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  2. 0

    This might be working

    const t = {
    "a": {
        "b": "1"
    }
}
type JsonKeys = keyof typeof t;
let k: JsonKeys = "a"; // <- variable key

console.log(t["a"]); // <- ok
console.log(t[k]);   // <- ok
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