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Type-safe JSON.parse() with joi

As developers, we often interact with APIs and databases that return a response in JSON format. To parse that JSON response we have to reach for the JSON.parse() method.
By default, JSON.parse() will return the object with type any. This means that we can simply declare the type of our object inline, like this:

interface MyObject {
    key1: string;
    key2?: number;
    key3: boolean;

const jsonString = api.get()

const myObject: MyObject = JSON.parse(jsonString)
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This would be sufficient if we were absolutely sure that jsonString is assignable to type MyObject. However, if for any reason the API sends us a different response, Typescript will not prevent us from misusing that response and potentially getting runtime errors.

To address this issue we can use a runtime data validator such as joi:

import { ObjectSchema, ValidationError } from 'joi';

export const typedParse = <TSchema>(
    jsonString: string,
    schema: ObjectSchema<TSchema>
): TSchema => {
    const {
    }: { value: TSchema; error?: ValidationError; warning?: unknown } =
    // handle error and warning cases as needed
    if (error) throw error;
    if (warning) console.log(warning);
    return value;
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Now we can import and use this reusable function anywhere we use JSON.parse() by providing the JSON string and the corresponding joi schema:

import Joi from 'joi'
import { typedParse } from './util/typedParse.ts'

interface MyObject {
    key1: string;
    key2?: number;
    key3: boolean;

const jsonString = api.get()

const schema = Joi.object<MyObject>()
        key1: Joi.string().required(),
        key2: Joi.number().integer(),
        key3: Joi.boolean()

const myObject = typedParse(jsonString, schema)
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This allows us to take advantage of all the type safety and autocompletion that Typescript offers when the API request matches what we expect, and at the same time prevents unexpected behavior when the API returns a different response.

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11 Tips That Make You a Better Typescript Programmer


1 Think in {Set}

Type is an everyday concept to programmers, but it’s surprisingly difficult to define it succinctly. I find it helpful to use Set as a conceptual model instead.

#2 Understand declared type and narrowed type

One extremely powerful typescript feature is automatic type narrowing based on control flow. This means a variable has two types associated with it at any specific point of code location: a declaration type and a narrowed type.

#3 Use discriminated union instead of optional fields


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