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

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# 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!!

```JavaScript
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.

```TypeScript
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

```bash
npm install @bitr/castable
```

and set tsconfig ``emitDecoratorMetadata``.
```json
{
    "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
4. Add `@cast` decorator to nested type
5. Do same to all nested types

# Example
This library is extensively used in [R2](https://github.com/bitrinjani/r2), e.g. [type.ts](https://github.com/bitrinjani/r2/blob/master/src/Quoine/types.ts).

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