json2typescript

In Angular applications, everyone consumes JSON API's from an external source. Type checking and object mapping is only possible in TypeScript, but not in the JavaScript runtime. As the API may change at any point, it is important for larger projects to verify the consumed data.

json2typescript is a small package containing a helper class that maps JSON objects to an instance of a TypeScript class. After compiling to JavaScript, the result will still be an instance of this class. One big advantage of this approach is, that you can also use methods of this class.

With json2typescript, only a simple function call is necessary, as demonstrated in this TypeScript snippet:

// Assume that you have a class named User defined at some point
// Assume that you get a JSON string from a webservice
let jsonStr: string =
...
;
let jsonObj: any = JSON.parse(jsonStr);

// Now you can map the json object to the TypeScript object automatically
let jsonConvert: JsonConvert = new JsonConvert();
let user: User = jsonConvert.deserializeObject(jsonObj, User);
console.log(user); // prints User{ ... } in JavaScript runtime, not Object{ ... }

Tip: All serialize() and deserialize() methods may throw an Error in case of failure. Make sure you catch errors in production!

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Changelog

See the changelog in the separate file for bug fixes, new features and breaking changes: Changelog

Warning: If you are reading this document on GitHub, it might be ahead of the published NPM version. Please refer to the ReadMe on NPM if in doubt.

Warning: We earlier suggested to use the @JsonObject(classIdentifier) decorator, but did not enforce it. Since v1.4.0, it is mandatory to use a unique classIdentifier for every class in order to make (de)serialization work properly with class inheritance. In versions above v1.2.0 and below v1.4.0, it is possible to run into issues when not using the decorator.

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Getting started

Requirements

We developed json2typescript for Angular 2+ and Ionic 2+. In this document, we only cover this use case. However, you may use our package for pure TypeScript or even JavaScript applications.

Setup a Test Application

We recommend to use the official angular cli tool in order to set up a new Angular project. Then, all you need to do is type the following into your operating system's terminal:

ng new testApplication
cd testApplication

npm install json2typescript

Our package makes use of TypeScript decorators. If not done already, please activate them in your tsconfig.json under compilerOptions as follows:

{
  "compilerOptions": {
[
  ...
]
"experimentalDecorators": true,
"emitDecoratorMetadata": true,
[...]
}

Tip: We have tried to make the compiler options of json2typescript to be as strict as possible. This enables you to use compiler options such as "strictNullChecks": true or "noImplicitAny": true in your own project.

Now you are ready to use the package.

Mapping example

In order to use the json2typescript package, all you need to do is write decorators and import the package. The following things need to be done if you would like to map JSON to existing classes:

• Classes need to be preceded by @JsonObject(classIdentifier) where the classIdentifier is unique in the whole project
• Properties need to be preceded by @JsonProperty(jsonProperty, conversionOption, convertingMode)
• Properties need to have a default value (or undefined), otherwise the mapper will not work

See below an example, so you can learn from it how json2typescript works best.

Assuming that you have created the testApplication in the step before and installed json2typescript as suggested, create a class in a new file city.ts with the following content:

import { JsonObject, JsonProperty } from "json2typescript";

@JsonObject("City") // Make sure "City" is a unique identifier for this class
export class City {

    // This property has no @JsonProperty. 
    // It will not be mapped.
    id: number = 123;

    // This maps the value of the JSON key "name" to the class property "name".
    // If the JSON value is not of type string (or missing), there will be an exception.
    @JsonProperty("name", String)
    name: string = "";

    // This maps the JSON key "founded" to the private class property "_founded".
    // Note the use of public getter and setter.
    // If the JSON value is not of type number (or missing), there will be an exception.
    @JsonProperty("founded", Number)
    private _founded: number = 0;
    get founded() { return this._founded; }

    set founded(value: number) { this._founded = value; }

    // This maps the JSON key "beautiful" to the class property "beautiful".
    // If the JSON value is not of type boolean (or missing), there will be an exception.
    @JsonProperty("beautiful", Boolean)
    beautiful: boolean = false;

    // This maps the JSON key "data" to the class property "data".
    // We are not sure about the type, so we omit the second parameter.
    // There will be an exception if the JSON value is missing.
    @JsonProperty("data") // is the same as @JsonProperty("data", Any)
    data: any = undefined;

    // This maps the JSON key "keywords" to the class property "keywords".
    // This is an example of a string array. Note our syntax "[String]".
    // In the further examples at the end of this document, you can see how to nest complex arrays.
    @JsonProperty("keywords", [String])
    keywords: string[] = []; // or Array<string>

    printInfo() {
        if (this.beautiful)
            console.log(this.name + " was founded in " + this.founded + " and is really beautiful!");
        else
            console.log(this.name + " was founded in " + this.founded + ".");
    }

}

Now create a file country.ts with the following content:

import { City } from "./city";
import { JsonObject, JsonProperty } from "json2typescript";

@JsonObject("Country") // Make sure "Country" is a unique identifier for this class
export class Country {

    // This maps the value of the JSON key "countryName" to the class property "name".
    // If the JSON value is not of type string (or missing), there will be an exception.
    @JsonProperty("countryName", String)
    name: string = "";

    // This maps the value of the JSON key "cities" to the class property "cities".
    // If the JSON value is not of type array object (or missing), there will be an exception.
    // There will be an exception too if the objects in the array do not match the class "City".
    @JsonProperty("cities", [City])
    cities: City[] = [];

}

Then navigate to the file app.component.ts and add the following code:

import { Component, OnInit } from '@angular/core';
import { JsonConvert, OperationMode, ValueCheckingMode } from "json2typescript"
import { Country } from "./country";

@Component({
    selector: 'app-root',
    templateUrl: './app.component.html',
    styleUrls: ['./app.component.css']
})
export class AppComponent implements OnInit {
    ngOnInit() {
        // Define a JSON object (could come from a HTTP service, parsed with JSON.parse() if necessary)
        const jsonObject: any = {
            "countryName": "Switzerland",
            "cities": [
                {
                    "id": 1,
                    "name": "Basel",
                    "founded": -200,
                    "beautiful": true,
                    "data": 123,
                    "keywords": ["Rhine", "River"]
                },
                {
                    "id": 1,
                    "name": "Zurich",
                    "founded": 0,
                    "beautiful": false,
                    "data": "no",
                    "keywords": ["Limmat", "Lake"]
                }
            ]
        };

        // Choose your settings
        // Check the detailed reference in the chapter "JsonConvert class properties and methods"
        let jsonConvert: JsonConvert = new JsonConvert();
        jsonConvert.operationMode = OperationMode.LOGGING; // print some debug data
        jsonConvert.ignorePrimitiveChecks = false; // don't allow assigning number to string etc.
        jsonConvert.valueCheckingMode = ValueCheckingMode.DISALLOW_NULL; // never allow null

        // Map to the country class
        let country: Country;
        try {
            country = jsonConvert.deserializeObject(jsonObject, Country);
            country.cities[0].printInfo(); // prints: Basel was founded in -200 and is really beautiful!
        } catch (e) {
            console.log((<Error>e));
        }
    }

Play around with the JSON to provocate exceptions when deserializing the object.

Important notes

Avoid circular dependencies on the classes that use json2typescript. Even if you don't have any errors in your IDE, json2typescript will not properly work in this case.

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Detailed reference

Class and property decorators

Decorators should be used whenever you would like to map JSON with TypeScript data. As of now, you must not use more than one decorator per class or property.

Class decorators

The class decorators are used infront of the class declaration and do support one parameter:

@JsonObject("User")
export class User {}

Warning: The class decorator uses the parameter to identify the class. Please use a unique identifier for each class in your project, for example simply the name of the class.

Tip: Make sure you imp