Castable TypeScript Library

Castable sanitizes dirty external data by casting all properties at run time to the types specified at compile time.

Why do you need this library?

A lot of web services return number type fields with double-quotes in JSON format. If you convert them by JSON.stringify, the double-quoted numbers will become string type!!

const serverResponse = `{
　　"name": "Milk", 
　　"price": "200", 
　　"tax": "10", 
}`;
const product = JSON.parse(serverResponse);
const sum = product.price + product.tax;
console.log(`sum: ${sum}`); // "200" + "10" = "20010"⛔️

TypeScript type annotation can help it? No, TypeScript cannot check such run-time type mismatch. You will get the exactly same result even type annotation is perfect.

That's why I've made this library. Castable can convert those types at run time. All fields will be converted to the annotated types.

import { cast, Castable } from 'castable';

class Product extends Castable { 
  @cast name: string;
  @cast price: number;
  @cast tax: number;
}

const serverResponse = `{"name": "Milk", "price": "200", "tax": "10"}`;
const product = new Product(JSON.parse(serverResponse));
const sum = product.price + product.tax;
console.log(`sum: ${sum}`); // 200 + 10 = 210👍

Castable internally applies Number("200") for price field and Number("10") for tax field in order to cast them to the correct type, recognizing those are actually number type, not string.

Supported types:

• number: "100" will be converted to real number 100.
• boolean: string "true", "false" in JSON will be real boolean true, false.
• Date: any string representaion supported by Date constructor, like "Thu Dec 21 2017 18:38:58 GMT+0900 (Tokyo Standard Time)"
• Nested type
• Array<T>
• Multi-dementional Array<T>

Install

npm install @bitr/castable

and set tsconfig emitDecoratorMetadata.

{
    "compilerOptions": {
      "experimentalDecorators": true,
      "emitDecoratorMetadata": true
    }
}

Usage

1. Extend Castable
2. Add @cast decorator to primitive type field (string, number, boolean)
3. Add @cast(Date) decorator to Date type field
4. Add @cast @element(T) to Array<T> type field
5. Add @cast decorator to nested type
6. Do same to all nested types

Example

This library is extensively used in R2, e.g. type.ts.