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Type-safely parsing JSON to a TypeScript Interface

codeprototype profile image Kevin Le ・2 min read

Let's say we're working with a REST API that returns a JSON string representing a user. The JSON API agrees to a contract that it returns a string firstName, a string lastName and a number accountBalance. We model the user as an Interface in TypeScript as follow:

interface User {
    firstName: string;
    lastName: string;
    accountBalance: number;
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The following code is sufficient if we always have the happy path:

const json = '{"firstName": "Kevin", "lastName": "Le", "accountBalance": 100}'
const user: User = JSON.parse(json)
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But things can go wrong on the API side. The json string returned could be:

const json  '{"firstName": "Kevin", "lastName": "Le", "accountBalance": "100"}'
const user: User = JSON.parse(json)
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The code does not blow up because accountBalance is now being treated as an any instead of a number. But this "forgiving" behavior leads to other problem downstream:

const balanceAfterInterest = user.accountBalance + user.accountBalance * 0.05
console.log(balanceAfterInterest) //1005
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The balanceAfterInterest is only supposed to be 105. A better approach is to catch this problem early and handle it immediately and appropriately.

Head over to

Paste the string {"firstName": "Kevin", "lastName": "Le", "accountBalance": 100} to the left pane. Type User as the Name, and select JSON as the Source type.

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In the box on the right, select TypeScript as the Language and make sure Verify JSON.parse results at runtime is turned-on. Quicktype will generate the resulting code with instruction on how to use it in the middle.

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Now the following code can be much safer:

import { Convert, User } from "./user";

const json =
  '{"firstName": "Kevin", "lastName": "Le", "accountBalance": "100"}';

try {
  const user = Convert.toUser(json);
} catch (e) {
  console.log("Handle error", e);
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The exception is now caught and handled early:

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The generated code by quicktype for user.js is:

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