How to parse JSON string in Typescript

Typescript is (a superset of) javascript, so you just use JSON.parse as you would in javascript:

let obj = JSON.parse(jsonString);

Only that in typescript you can have a type to the resulting object:

interface MyObj {
    myString: string;
    myNumber: number;
}

let obj: MyObj = JSON.parse('{ "myString": "string", "myNumber": 4 }');
console.log(obj.myString);
console.log(obj.myNumber);

(code in playground)

If you want your JSON to have a validated Typescript type, you will need to do that validation work yourself. This is nothing new. In plain Javascript, you would need to do the same.

Validation

I like to express my validation logic as a set of "transforms". I define a Descriptor as a map of transforms:

type Descriptor<T> = {
  [P in keyof T]: (v: any) => T[P];
};

Then I can make a function that will apply these transforms to arbitrary input:

function pick<T>(v: any, d: Descriptor<T>): T {
  const ret: any = {};
  for (let key in d) {
    try {
      const val = d[key](v[key]);
      if (typeof val !== "undefined") {
        ret[key] = val;
      }
    } catch (err) {
      const msg = err instanceof Error ? err.message : String(err);
      throw new Error(`could not pick ${key}: ${msg}`);
    }
  }
  return ret;
}

Now, not only am I validating my JSON input, but I am building up a Typescript type as I go. The above generic types ensure that the result infers the types from your "transforms".

In case the transform throws an error (which is how you would implement validation), I like to wrap it with another error showing which key caused the error.

Usage

In your example, I would use this as follows:

const value = pick(JSON.parse('{"name": "Bob", "error": false}'), {
  name: String,
  error: Boolean,
});

Now value will be typed, since String and Boolean are both "transformers" in the sense they take input and return a typed output.

Furthermore, the value will actually be that type. In other words, if name were actually 123, it will be transformed to "123" so that you have a valid string. This is because we used String at runtime, a built-in function that accepts arbitrary input and returns a string.

You can see this working here. Try the following things to convince yourself:

• Hover over the const value definition to see that the pop-over shows the correct type.
• Try changing "Bob" to 123 and re-run the sample. In your console, you will see that the name has been properly converted to the string "123".

Type-safe JSON.parse

You can continue to use JSON.parse, as TypeScript is a superset of JavaScript:

This means you can take any working JavaScript code and put it in a TypeScript file without worrying about exactly how it is written.

There is a problem left: JSON.parse returns any, which undermines type safety (don't use any).

Here are three solutions for stronger types, ordered by ascending complexity:

1. User-defined type guards

Playground

// For example, you expect to parse a given value with `MyType` shape
type MyType = { name: string; description: string; }

// Validate this value with a custom type guard (extend to your needs)
function isMyType(o: any): o is MyType {
  return "name" in o && "description" in o
}

const json = '{ "name": "Foo", "description": "Bar" }';
const parsed = JSON.parse(json);
if (isMyType(parsed)) {
  // do something with now correctly typed object
  parsed.description
} else { 
// error handling; invalid JSON format 
}

isMyType is called a type guard. Its advantage is, that you get a fully typed object inside truthy if branch.

2. Generic JSON.parse wrapper

Playground

Create a generic wrapper around JSON.parse, which takes one type guard as input and returns the parsed, typed value or error result:

const safeJsonParse = <T>(guard: (o: any) => o is T) => 
  (text: string): ParseResult<T> => {
    const parsed = JSON.parse(text)
    return guard(parsed) ? { parsed, hasError: false } : { hasError: true }
  }

type ParseResult<T> =
  | { parsed: T; hasError: false; error?: undefined }
  | { parsed?: undefined; hasError: true; error?: unknown }

Usage example:

const json = '{ "name": "Foo", "description": "Bar" }';
const result = safeJsonParse(isMyType)(json) // result: ParseResult<MyType>
if (result.hasError) {
  console.log("error :/")  // further error handling here
} else {
  console.log(result.parsed.description) // result.parsed now has type `MyType`
}

safeJsonParse might be extended to fail fast or try/catch JSON.parse errors.

3. External libraries

Writing type guard functions manually becomes cumbersome, if you need to validate many different values. There are libraries to assist with this task - examples (no comprehensive list):

• io-ts: has fp-ts peer dependency, uses functional programming style
• zod: strives to be more procedural / object-oriented than io-ts
• typescript-is: TS transformer for compiler API, additional wrapper like ttypescript needed
• typescript-json-schema/ajv: Create JSON schema from types and validate it with ajv

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More infos

• Runtime type checking #1573
• Interface type check with Typescript
• TypeScript: validating external data