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: object = 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()anddeserialize()methods may throw anErrorin case of failure. Make sure you catch errors in production!
See the changelog in the seperate file for bug fixes, new features and breaking changes: Changelog
Tip: At version 1.2.0 we introduced one more soft breaking change.
nullvalues will now be ignored when you serialize or deserialize optional values. This means that you are now able to useValueCheckingMode.DISALLOW_NULLeven when your API returns null values in the JSON. In that case, you should mark them optional.
Tip: At version 1.1.0 we introduced some soft breaking change.
undefinedis an invalid value now for serializing or deserializing. If you are working with TypeScript, you might receive some type issues after the update to v1.1.0. This is due to the addition of generics; they now help the compiler to detect invalid return types in the serialize and deserialize methods.
Tip: Starting from version 1.0.6, we recommend to use unique class identifiers in the
@JsonObjectdecorator. Read below how to use the decorators properly.
We developed json2typescript for Angular 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.
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 json2typescriptOur 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
json2typescriptto be as strict as possible. This enables you to use compiler options such as"strictNullChecks": trueor"noImplicitAny": truein your own project.
Now you are ready to use the package.
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 preceeded by
@JsonObject(classIdentifier) - Properties need to be preceeded by
@JsonProperty(jsonProperty, conversionOption, isOptional) - 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")
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 = undefined;
// 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 = undefined;
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 = undefined;
// 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[] = undefined; // 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")
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 = undefined;
// 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[] = undefined;
}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: object = {
"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.
Avoid circular depencencies 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.
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.
The class decorators are used infront of the class declaration and do support one parameter:
@JsonObject("User")
export class User {}Warning: Minification removes the actual names of certain classes. This can in certain cases cause the deserializer to not work as same class names will overwrite the property descriptors. It is advised to explicitly ID your objects as above.
Tip: Make sure you import
JsonObjectfromjson2typescript.
The first parameter of @JsonObject is meant to be a unique class identifier, usually just the class name.
In many applications, developers deploy minified code which also minifies class names.
Adding a class identifier is highly recommended because it will prevent collision of class names.
Property decorators are a vital part for type checking. It is important that the type in the decorator matches the TypeScript type.
@JsonObject("User")
export class User {
@JsonProperty("jsonPropertyName", String, false)
name: string = undefined;
}Important note: You must assign any (valid) value or undefined to your property at
initialization, otherwise our mapper does not work and will simply ignore the property.
Tip: Make sure you import
JsonObjectandJsonPropertyfromjson2typescript.
The first parameter of @JsonProperty is the JSON object property name.
It happens that the property names given by the server are very ugly.
Here you can map any json property name to the User property name.
In our case, json["jsonPropertyName"] gets mapped to user.name.
The second parameter of @JsonProperty describes what happens when doing the mapping between JSON and TypeScript objects.
This parameter is optional; the default value is Any (which means no type check is done when the mapping happens).
If you would like that json2typescript performs an automatic type
check according to given TypeScript types, you can pass a type you
expect. Follow the following guide when doing that:
- Make sure you pass the class name and not an instance of the class.
- In case of primitive types, you have to use the upper case names.
- In case of
anytype, import fromjson2typescriptthe classAny.
See the following cheat sheet for reference:
| Expected type | TypeScript type |
|---|---|
| String | string |
| Number | number |
| Boolean | boolean |
| User | User |
| Any | any |
| [String] | string[] |
| [Number] | number[] |
| [Boolean] | boolean[] |
| [User] | User[] |
| [Any] | any[] |
At first, our array notation on the left looks odd. But this notation allows you to define even nested arrays. See the examples at the end of this document for more info about nesting arrays.
More advanced users may need to use custom converters. If you want
json2typescript to use your custom converter, you need to follow these
steps:
- Write a converter class that implements
JsonCustomConvert<T>where<T>is the type resulting in the TypeScript class. - Make sure you add the
@JsonConverterdecorator to this class (see next chapter how). - Pass your converter class as second param in the
@JsonPropertydecorator
Assume you would like to transform 1893-11-15 (string from JSON) to a TypeScript
Date instance, then you would write a class DateConverter (see next chapter how)
and pass it as second param in @JsonProperty as below:
@JsonObject("User")
export class User {
@JsonProperty("birthdate", DateConverter)
birthdate: Date = undefined;
}The third parameter of @JsonProperty determines whether the jsonProperty has to be present in the json.
This parameter is optional; the default value is false.
By default, JsonConvert throws an exception if a decorated class property cannot be found in the given JSON when deserializing.
If you set the third parameter to true, there is no exception when it is missing.
The type is still checked as soon the property is present again.
The same applies for the case when you try to serialize a TypeScript object to a JSON object. If the property is not defined in the class and optional, it will not be added to the JSON object.
Tip: Some API's return null instead of omitting optional values. If you flag a property as optional,
json2typescriptwill ignore null values and keep the default value of the property instead. This fact is particularly helpful if your project uses TypeScriptstrictNullChecksor/and disallowsnullvalues through thevalueCheckingModeproperty injson2typescript.
- Make sure you define the expected type as accurate as possible, even if you expect primitive types.
- By default, casting primitives into other primitives is not allowed. Check the public properties below in this document to change this behaviour.
- By default, primitives are not allowed to be null. Check the public properties below in this document to change this.
- If you don't know the type, you may use
Anyas expected type. You may also omit the second parameter of@JsonProperty.
- You can allow arrays by using the bracket notation combined with the types as seen above. For example:
[String]for a string array - Mixing arrays is allowed. For example:
[String, Number]for an array where the first entry is a string, the second is a number. - If the real array is longer than indicated here, the last type will be forced to the rest of the array entries (above:
Number). - This means,
[String, Number]is equivalent to[String, Number, Number]and so on. - Nesting arrays and objects are allowed. For example:
[[String, Number], User]. - This is equivalent to
[[String, Number, Number], User, User]and so on. - As further example,
[[String]]is equal to[[String],[String]]which is equal to[[String,String], [String,String]]and so on. - If an array has less elements as given in the expected type, no exception is thrown.
- For example, if we define
[String]or the equivalent[String, String]no exception is thrown - even if the JSON gives us an empty array.
Tip: See the examples at the end of this document for advanced examples for nesting arrays.
In some cases, you may need to make custom conversion between JSON objects and TypeScript objects. You can define custom converters like this:
@JsonConverter
class DateConverter implements JsonCustomConvert<Date> {
serialize(date: Date): any {
return date.getFullYear() + "-" + (date.getMonth() + 1) + "-" + date.getDate();
}
deserialize(date: any): Date {
return new Date(date);
}
}Tip: Make sure that you import
JsonConverterfromjson2typescript. Also don't forget to use the same time between the brackets<>, as theserialize()param anddeserialize()return value.
Assume that in your JSON you have a date in a standardized format, such as 2017-07-19 10:00:00. You could use the custom converter class above to make sure it is stored as a real TypeScript Date in your class. For your property, you simply have use the @JsonProperty decorator as follows:
@JsonObject("User")
export class User {
@JsonProperty("date", DateConverter)
date: Date = undefined;
}With this approach, you will achieve that your property date is going to be a real instance of Date.
(number) JsonConvert.operationMode
Determines how the JsonConvert class instance should operate.
You may assign three different values:
OperationMode.DISABLE: json2typescript will be disabled, no type checking or mapping is doneOperationMode.ENABLE: json2typescript is enabled, but only errors are loggedOperationMode.LOGGING: json2typescript is enabled and detailed information is logged
The default value is OperationMode.ENABLE. It will only print errors to the console and is suited for production.
In some cases, you might consider disabling json2typescript in production by setting the OperationMode.DISABLE flag.
This only works in case you only use plain objects without functionality and no mapping.
However, disabling json2typescript might give you a performance advantage in heavy projects.
In case you have issues to find bugs, you can enable additional logging by setting the OperationMode.LOGGING flag.
Please note that every serializing and deserializing is heavily logged to the console and will make your application slower.
Never use this flag in production.
Tip: Make sure you import the
ENUMOperationModewhen assigning a value to this property.
(number) JsonConvert.valueCheckingMode
Determines which types are allowed to be null. You may assign three different values:
ValueCheckingMode.ALLOW_NULL: All given values can be nullValueCheckingMode.ALLOW_OBJECT_NULL: Objects can be null, but primitive types cannot be nullValueCheckingMode.DISALLOW_NULL: No null values are tolerated
The default is ValueCheckingMode.ALLOW_OBJECT_NULL.
Tip: Make sure you import the
ENUMValueCheckingModewhen assigning a value to this property.
Tip: The TypeScript documentation suggests to avoid null values. Compile your TypeScript code with
strictNullChecks=trueand set thevalueCheckingModeto disallow null values. If your API returnsnullin some cases, simply mark these properties as optional in the correspondingJsonPropertydecorator to avoid errors on runtime.
(bool) JsonConvert.ignorePrimitiveChecks
Determines whether primitive types should be checked. If true, it will be allowed to assign primitive to other primitive types.
The default is false.
(number) JsonConvert.propertyMatchingRule
Determines the rule of how JSON properties shall be matched with class properties during deserialization. You may assign the following two values:
PropertyMatchingRule.CASE_STRICT: JSON properties need to match exactly the names in the decoratorsPropertyMatchingRule.CASE_INSENSITIVE: JSON properties need to match names in the decorators, but names they are not case sensitive
The default is PropertyMatchingRule.CASE_STRICT.
json2typescript allows you to map JSON objects (or arrays) to TypeScript objects (or arrays) and vice versa.
(any) serialize(data: T | T[])
Tries to serialize a TypeScript object or array of objects to JSON.
Tip: The return value is not a string. In case you need a string as result, use
JSON.stringify()after calling the serialize method.
(T | T[]) deserialize(json: any, classReference: { new(): T | T[] })
Tries to deserialize given JSON to a TypeScript object or array of objects.
Tip: The param
jsonmust not be a string, but anobjector anarray. UseJSON.parse()before applying the deserialize method in case you have a json string.
The methods serialize() and deserialize() will automatically detect the dimension of your param (either object or array).
In case you would like to force json2typescript to use a specific way, you can use the following methods instead:
(any) serializeObject(instance: T)(any[]) serializeArray(instanceArray: T[])(T) deserializeObject(jsonObject: any, classReference: { new(): T })(T[]) deserializeArray(jsonArray: any[], classReference: { new(): T })
In case you don't have enough complex examples yet, you may find some more in this section.
It is heavily discouraged to use nested arrays and use different types in a JSON api. If you need them anyway, here is how you have to define the types:
In the following example, jsonKeyOfWeirdKeywords is a key in the JSON object defined like this:
{
"jsonKeyOfWeirdKeywords": [
["Hello", "World"],
["Bye", "Earth"]
]
}As we have an array of array of strings, you can define the expected type like this:
@JsonObject("User")
export class User {
@JsonProperty("jsonKeyOfWeirdKeywords", [[String, String], [String, String]])
keywords: any = undefined;
}Tip: In our syntax,
[[String, String], [String, String]]is equivalent to[[String], [String]]and[[String]]. That is because the last type in an array will be automatically repeated as much as needed.
In the following example, JSONKeyOfWeirdKeywords is a key in the JSON object defined like this:
{
"jsonKeyOfWeirdKeywords": [
["FC", "Basel"],
1893
]
}You can define the expected type in your class like this:
@JsonObject("User")
export class User {
@JsonProperty("jsonKeyOfWeirdKeywords", [[String, String], Number])
keywords: any = undefined;
}Tip: In our syntax,
[[String, String], Number]is equivalent to[[String], Number].
This NPM package was originally created by Andreas Aeschlimann, scientific researcher of the Digital Humanities Lab (DHlab) of University of Basel in 2016.
The Digital Humanities Lab makes use of json2typescript in several Angular web applications, for example in the new website of http://www.salsah.org. You may find more information about us on http://dhlab.unibas.ch
Thanks for the input and pull requests by:
- @gempain for fixing several issues and adding karma tests.
- @mrwogu for fixing the
noImplicitAnyissue. - @bblommers for the support of multiple decorators.
- @archfz for fixing the constructor.name issue