Typescript serialize object to json

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Typescript serialize object to json

The collection objects offered by ES6 such as Map and Set are indeed a marvel. One notable downside to them though is that they don't serialise to JSON. For example JSON.stringify(MyMapObject); Results in {} Always ? irrespective of what is in the map collection. Not great and a pain when we want to stash the map object in a cookie or local storage to be used later. It just doesn't do it out of the box. Here I'm going to extend TypeScript to do this so I will never need to think about it again. Serialising the Map object. There are a number of different ways to get the Map object to serialise and deserialise in and out of JSON. This one relies on the contents of the Map object being serialisable. If they are then we can do the following Serialise Map into JSON function toJson(map) { return JSON.stringify(Array.from(map.entries())); } Deserialise JSON into Map function fromJson(jsonStr) { return new Map(JSON.parse(jsonStr)); } This works in JavaScript (ES6) and TypeScript ? which after all is a superset of JavaScript. Serialise the Map Object It feels a bit clunky to have these methods hanging around in some utility library. What I want to do is extend the TypeScript Map class to have these. To do this go to a TypeScript definition file (*.d.ts) and enter the following interface Map { toJson(): string; } This will extend our map object to have a toJson method. In a suitable library file place the implementation and associate with the prototype object of the Map object Map.prototype.toJson = function toJson() { return JSON.stringify(Array.from(this.entries())); } It's not hugely intuitive why we need to extend the prototype object at this point. From the mozilla documention ? All Map instances inherit from Map.prototype. So if we want our new method on an instance of the Map object we need to extend the prototype object ? because Map instances all inherit from this. This is then called thus let jsonString = myMap.toJson(); Which can then be stashed away in local storage or the like. Deserialise a JSON string into a Map object We are going to do similar to implement the deserialisation part. It's not quite the same though. If we are serialising a Map object it makes sense to have the toJson method associated with an instance of a Map object. If we are deserialising into a new Map object we want the method associated with the class- like a static method in C#. The following illustrates the difference let myMap: Map = new Map(); myMap.set("key1", "value1") myMap.set("key2", "value2") myMap.set("key2", "value2") //.. serialise method associated with the instance of the class let jsonString = myMap.toJson(); //.. deserialise method associated with the class let restoredMap: Map = Map.fromJson(jsonString); Again go to a TypeScript definition file (*.d.ts) and enter the following interface MapConstructor { fromJson(jsonStr: string); } Note ? this time I am extending the MapConstructor object rather than the Map object. In TypeScript the MapConstructor object returns a reference to the Map function that created the object. So by extending the MapConstructor Typescript object I am associating the method with the object that created the instance of the map object. I am associating the new method with the class and not the instance. Now associate our implementation directly with the Map object which itself implements MapConstructor Map.jsonToMap = function jsonToMap(jsonStr) { return new Map(JSON.parse(jsonStr)); } Which we call thus let restoredMap: Map = Map.jsonToMap(jsonString); That's it ? the Map object now has toJson and fromJson methods which handle our JSON conversion. The Set object suffers from the same limitations and can be extended in the same way. Useful Links Map and Set objects are part of the ES6 standard. TypeScript offers a strongly typed version of these collections More detail about extending built in types in TypeScript The initial code to serialise and deserialise the Map object came from a stack overflow question i.e. function toJson(map) { return JSON.stringify(Array.from(map.entries())); } function fromJson(jsonStr) { return new Map(JSON.parse(jsonStr)); } but I've forgotten which one ? if I find it I will pop the reference here in the interest of full disclosure and not ripping off someone else's work and claiming it for my own. The quick popularity of JSON can be attributed to the reason that data could be parsed into an usable object very easily using JSON. Parsing data using XML was bit difficult, as the data has to be parsed into the DOM document, making the extraction a bit of a cumbersome exercise Hence, elimination of extra method calls is one of the prime reasons of incredible popularity of JSON with JavaScript webdevelopers Javascript JSON: Objects Earlier, JSON parsers did a bit more than what JavaScript eval() functions could do, i.e parse, interpret and return the data as JavaScript objects and arrays. But now JSON object has two methods : stringify() and parse() stringify() : To serialize JavaScript objects into a JSON string. parse() : To parse JSON into a native JavaScript value. The demo below serializes a JavaScript object into a JSON string by making use of JSON.stringify() and stores the value in jsonString The JSON string can be passed directly into JSON.parse() to create an appropriate JavaScript value. The script first serializes an object and then parses the same string into an appropriate JavaScript Object. This example serializes an object and then parses that string into another object. Give it a TRY! ? Note:An an error is generated if the text passed into JSON.parse() is not a valid JSON. The method JSON.stringify() can accept at the most two arguments, in addition to the object being serialized, these arguments are used to specify alternate ways to serialize a JavaScript Object. The first argument is filter , which can be either a function or an array. The second argument specifies an option for indenting the stringified JSON string. This example serializes only object properties that are listed in the array Give it a TRY! ? Note: Only the properties mentioned in the argument get serialized . The behavior is slightly different when the second argument is a function. The function receives two arguments : the property key and property value Filters apply to all objects contained in the object which is to be serialized, so an array of multiple objects with properties will result in every object including the properties specified for filtering. The example serializes only object properties that are listed in the array Give it a TRY! ? Note: The key is always a string, but it can be an empty string if value isn't part of key-value pair. The third argument of JSON.stringify() is used to control indentation and white spaces. The numerical values of the argument specifies the number of spaces ro indent each level. This example serializes only object properties that are listed in the array Give it a TRY! ? Note: The maximum numeric indentation value is 10, any value larger than 10 sets the value to 10. If the indentation argument is a string , then the string is used as the indentation character for the JSON This example serializes only object properties that are listed in the array Give it a TRY! ? Note:There is a limit of 10 characters, if a longer string is used then it is truncated to 10 characters only. Other than JSON.stringify() , sometimes objects need custom JSON serialization, for that purpose the method toJSON() to return the proper JSON representation. The method toJSON() can be used on any object to return a serialization value. This example serializes only object properties that are listed in the array Give it a TRY! ? Note: The method toJSON() can be used in addtion to the filter method The method JSON.parse() accepts an additon argument, it is called on each key-value pair. The function is also called as reviver function, the other function JSON.stringify() (filter) is called as a replacer The format of both the function looks the same, they take two arguments : key and value and are supposed to return a value. This example serializes only object properties that are listed in the array Give it a TRY! ? Note: If the reviver function returns undefined, then the key gets removed from the result . Thu 09 April 2020 If you're writing a server in JavaScript, you might write an endpoint that converts an object to JSON: app.get('/user', (request, response) => { const user = getCurrentUser(); response.json(user);}); On the client, you might use the fetch API to hit this endpoint and deserialize (parse) the data: const response = await fetch('/user');const user = await response.json(); What's the relationship between the user object in the server and the corresponding user object in the client? And how would you model this in TypeScript? Because the serialization and deserialization ultimately happens via JavaScript's built-in JSON.stringify and JSON.parse functions, we can alternatively ask: what's the return type of this function? function jsonRoundTrip(x: T) { return JSON.parse(JSON.stringify(x));} If you mouse over jsonRoundTrip on the TypeScript playground, you'll see that its inferred return type is any. That's not very satisfying! It's tempting to make the return type T, so that this is like an identity function: function jsonRoundTrip(x: T): T { return JSON.parse(JSON.stringify(x));} But this isn't quite right. First of all, there are many objects which can't be directly represented in JSON. A regular expression, for instance: > JSON.stringify(/foo/)'{}' Second, there are some values that get transformed in the conversion process. For example, undefined in an array becomes null: > arr = [undefined]> jsonRoundTrip(arr)[ null ] With strictNullChecks in TypeScript, null and undefined have distinct types. If an object has a toJSON method, it will get called by JSON.stringify. This is implemented by some of the standard types in JavaScript, notably Date: > d = new Date();> jsonRoundTrip(d)'2020-04-09T01:07:48.835Z' So Dates get converted to strings. Who knew? You can read the full details of how this works on MDN. How to model this in TypeScript? Let's just focus on the behavior around Dates. For a complex mapping like this, we're going to want a conditional type: type Jsonify = T extends Date ? string : T; This is already doing something sensible: type T1 = Jsonify; type T2 = Jsonify; type T3 = Jsonify; We even get support for union types because conditional types distribute over unions: type T = Jsonify; But what about object types? Usually the Dates are buried somehwere in a larger type. So we'll need to make Jsonify recursive. This is possible as of TypeScript 3.7: type Jsonify = T extends Date ? string : T extends object ? { [k in keyof T]: Jsonify; } : T; In the case that we have an object type, we use a mapped type to recursively apply the Jsonify transformation. This is already starting to make some interesting new types! interface Student { id: number; name: string; birthday: Date | null}type T1 = Jsonify;interface Class { valedictorian: Student; salutatorian?: Student;}type T2 = Jsonify; What if there's an array involved? Does that work? interface Class { teacher: string; start: Date; stop: Date; students: Student[];}type T = Jsonify; It does! How was TypeScript able to figure that out? First of all, Arrays are objects, so T extends object is true for any array type. And keyof T[] includes number, since you can index into an array with a number. But it also includes methods like length and toString: type T = keyof Student[]; So it's a bit of a surprise Jsonify produces such a clean type for the array. Perhaps mapped types over arrays are special cased. But regardless, this is great! We can even loosen the definition slightly to handle any object with a toJSON() method (including Dates): type Jsonify = T extends {toJSON(): infer U} ? U : T extends object ? { [k in keyof T]: Jsonify; } : T;function jsonRoundTrip(x: T): Jsonify { return JSON.parse(JSON.stringify(x));}const student: Student = { id: 327, name: 'Bobby', birthday: new Date('2007-10-10')};const studentRT = jsonRoundTrip(student);const objWithToJSON = { x: 5, y: 6, toJSON(){ return this.x + this.y; }};const objRT = jsonRoundTrip(objWithToJSON); Here we've used the infer keyword to infer the return type of the toJSON method of the object. Try the last example out in the playground. It really does return a number! As TypeScript Development lead Ryan Cavanaugh once said, it's remarkable how many problems are solved by conditional types. The types involved in JSON serialization are one of them! If you produce and consume JSON in a TypeScript project, consider using something like Jsonify to safely handle Dates in your objects. JSON logoAfter many hours of development, I finally released the first version of the jackson-js library. As the name implies, jackson-js decorators are heavily inspired by the Java annotations of the famous Java FasterXML/jackson library.You can install it using npm install ---save jackson-js and it can be used on both client (browser) and server (Node.js) side.Why this library? What's the difference between using this library instead of JSON.parse and JSON.stringify ?For simple cases, you don't need this library, of course, you can just use JSON.parse and JSON.stringify to serialize/deserialize JSON.With jackson-js , you can easily manipulate your JavaScript objects/values serialization/deserialization using decorators such as @JsonProperty() , @JsonFormat() , @JsonIgnore() , etc. However, this library uses JSON.parse and JSON.stringify under the hood.Furthermore:it not only deserialize JSON text into a JavaScript object, it also converts it into an instance of the class specified in the context option (similar packages are: class-transformer and TypedJSON); instead, with JSON.parse you will get just a simple plain (literal) JavaScript object (just Object type);it supports more advanced Object concepts such as polymorphism and Object identity;it supports cyclic object serialization/deserialization;it supports serialization/deserialization of other native JavaScript types: Map , Set , BigInt , Typed Arrays (such as Int8Array);This library can be useful in more complex cases, for example when you want to:manipulate JSON in depth;restore a JavaScript type (a similar package is class-transformer);preserve type information (using polymorphic type handling decorators: @JsonTypeInfo, @JsonSubTypes, and @JsonTypeName. A similar package is TypedJSON);hide some properties for certain HTTP endpoints or some other external service;have different JSON response for some external application or manage different JSON data coming from other application (for example you need to communicate with a Spring Boot application that uses different JSON Schema for the same model or with other applications made with Python, PHP, etc...);manage cyclic references;manage other JavaScript native types such as Maps and Sets;etc.Most of the use cases of the Java FasterXML/jackson annotations are similar or equal.In this article, I will present a basic example for each decorator.ObjectMapper, JsonParser and JsonStringifier classesThe main classes that jackson-js offers to serialize and deserialize JavaScript objects are: ObjectMapper , JsonStringifier and JsonParser .ObjectMapperObjectMapper provides functionality for both reading and writing JSON and applies jackson-js decorators. It will use instances of JsonParser and JsonStringifier for implementing actual reading/writing of JSON. It has two methods:stringify(obj: T, context?: JsonStringifierContext): string: a method for serializing a JavaScript object or a value to a JSON string with decorators applied;parse(text: string, context?: JsonParserContext): T: a method for deserializing a JSON string into a JavaScript object/value (of type T, based on the context given) with decorators applied.JsonParserJsonParser provides functionality for writing JSON and applies jackson-js decorators. The main methods are:parse(text: string, context?: JsonParserContext): T : a method for deserializing a JSON string into a JavaScript object/value (of type T, based on the context given) with decorators applied;transform(value: any, context?: JsonParserContext): any : a method for applying jackson-js decorators to a JavaScript object/value parsed. It returns a JavaScript object/value with decorators applied.JsonStringifierJsonStringifier provides functionality for reading JSON and applies jackson-js decorators. The main methods are:stringify(obj: T, context?: JsonStringifierContext): string: a method for serializing a JavaScript object or a value to a JSON string with decorators applied;transform(value: any, context?: JsonStringifierContext): any: a method for applying jackson-js decorators to a JavaScript object/value. It returns a JavaScript object/value with decorators applied and ready to be JSON serialized.DecoratorsBefore we go on, I need to say that the most important decorators are:@JsonProperty() : each class property (or its getter/setter) must be decorated with this decorator, otherwise, deserialization and serialization will not work properly! That's because, for example, given a JavaScript class, there isn't any way or API (such as Reflection API for Java) to get for sure all the class properties; also because, sometimes, compilers such as TypeScript and Babel, can strip class properties after compilation from the class properties declaration;@JsonClassType() : this decorator, instead, is used to define the type of a class property or method parameter. This information is used during serialization and, more important, during deserialization to know about the type of a property/parameter. This is necessary because JavaScript isn't a strongly-typed programming language, so, for example, during deserialization, without the usage of this decorator, there isn't any way to know the specific type of a class property, such as a Date or a custom Class type.Later, they will be explained in more detail.Quick example of @JsonProperty() and @JsonClassType() decorators.@JsonAliasThe @JsonAlias decorator defines one or more alternative names for a property during deserialization.API: JsonAlias -- decorator options JsonAliasOptions.@JsonAnyGetterThe @JsonAnyGetter decorator allows the flexibility of using a Map or an Object Literal field as standard properties.API: JsonAnyGetter -- decorator options JsonAnyGetterOptions.@JsonAnySetter@JsonAnySetter allows us to define a logical "any setter" mutator using a non-static two-argument method to be used as a "fallback" handler for all otherwise unrecognized properties found from JSON content.API: JsonAnySetter -- decorator options JsonAnySetterOptions.@JsonAppend@JsonAppend can be used to add "virtual" properties to be written after regular properties.API: JsonAppend -- decorator options: JsonAppendOptions.@JsonManagedReference and @JsonBackReferenceThe @JsonManagedReference and @JsonBackReference decorators can handle parent/child relationships and work around loops.API: JsonManagedReference -- decorator options JsonManagedReferenceOptions, JsonBackReference -- decorator options JsonBackReferenceOptions.@JsonClassTypeAs said before, the @JsonClassType is used to define the type of a class property or method parameter. A type is defined as an Array of JavaScript classes, such as [Number] for properties of type number or [Array, [Number]] for properties of type Array or [Map, [String, Object]] for properties of type Map.Why an Array of JavaScript classes? Because in this way you can map complex types such as Map using [Map, [String, Object]] or Array using [Array, [Set, [Object]]] .API: JsonClassType -- decorator options JsonClassTypeOptions.@JsonCreatorWe can use the @JsonCreator decorator to define constructors and factory methods as one to use for instantiating new instances of the associated class.It's very helpful when we need to deserialize some JSON that doesn't exactly match the target entity we need to get, also with the help of the @JsonProperty decorator.API: JsonCreator -- decorator options JsonCreatorOptions.@JsonSerialize and @JsonDeserialize@JsonSerialize and @JsonDeserialize are used to indicates the use of a custom serializer/deserializer.API: JsonSerialize -- decorator options JsonSerializeOptions, JsonDeserialize -- decorator options JsonDeserializeOptions.@JsonFilter@JsonFilter can be used to indicate which logical filter is to be used for filtering out properties of type (class) decorated.API: JsonFilter -- decorator options JsonFilterOptions.@JsonFormat@JsonFormat is a general-purpose decorator used for configuring details of how values of properties are to be serialized.API: JsonFormat -- decorator options JsonFormatOptions.@JsonGetter and @JsonSetter@JsonGetter and @JsonSetter are alternatives to more general @JsonProperty decorator to mark a method as a getter/setter method for a logical property.API: JsonGetter -- decorator options: JsonGetterOptions, JsonSetter -- decorator options JsonSetterOptions.@JsonIdentityInfo@JsonIdentityInfo indicates that Object Identity should be used when serializing/deserializing values -- for instance, to deal with infinite recursion type of problems.API: JsonIdentityInfo -- decorator options JsonIdentityInfoOptions.@JsonIdentityReference@JsonIdentityReference can be used for customizing details of a reference to Objects for which "Object Identity" is enabled (see @JsonIdentityInfo). The main use case is that of enforcing the use of Object Id even for the first time an Object is referenced, instead of the first instance being serialized as full Class.API: JsonIdentityReference -- decorator options JsonIdentityReferenceOptions.@JsonIgnore, @JsonIgnoreProperties, and @JsonIgnoreType@JsonIgnore is used to mark a property to be ignored at the field level during serialization and deserialization.API: JsonIgnore -- decorator options JsonIgnoreOptions.@JsonIgnoreProperties can be used as a class-level decorator that marks a property or a list of properties that will be ignored during serialization and deserialization.API: JsonIgnoreProperties -- decorator options JsonIgnorePropertiesOptions.@JsonIgnoreType indicates that all properties of decorated type are to be ignored during serialization and deserialization.API: JsonIgnoreType -- decorator options JsonIgnoreTypeOptions.@JsonInclude@JsonInclude can be used to exclude properties with empty/null/default values.API: JsonInclude -- decorator options JsonIncludeOptions.@JsonInject@JsonInject decorator is used to indicate that value of decorated property will be injected during deserialization.API: JsonInject -- decorator options JsonInjectOptions.@JsonNaming@JsonNaming decorator is used to choose the naming strategies (SNAKE_CASE, UPPER_CAMEL_CASE, LOWER_CAMEL_CASE, LOWER_CASE, KEBAB_CASE, and LOWER_DOT_CASE) for properties in serialization, overriding the default.API: JsonNaming -- decorator options JsonNamingOptions.@JsonProperty@JsonProperty can be used to define a non-static method as a "setter" or "getter" for a logical property or non-static object field to be used (serialized, deserialized) as a logical property.API: JsonProperty -- decorator options JsonPropertyOptions.@JsonPropertyOrder@JsonPropertyOrder can be used to specify the order of properties on serialization.API: JsonPropertyOrder -- decorator options JsonPropertyOrderOptions.@JsonRawValue@JsonRawValue decorator indicates that the decorated method or field should be serialized by including literal String value of the property as is, without quoting of characters. This can be useful for injecting values already serialized in JSON or passing javascript function definitions from server to a javascript client.API: JsonRawValue -- decorator options JsonRawValueOptions.@JsonRootName@JsonRootName decorator is used -- if wrapping is enabled -- to specify the name of the root wrapper to be used.API: JsonRootName -- decorator options JsonRootNameOptions.Polymorphic Type Handling Decorators: @JsonTypeInfo, @JsonSubTypes, and @JsonTypeName@JsonTypeId decorator is used to indicate that the annotated property should be serialized as the type id when including polymorphic type information, rather than as a regular property. That polymorphic metadata is used during deserialization to recreate objects of the same subtypes as they were before serialization, rather than of the declared supertypes.API: JsonTypeId -- decorator options JsonTypeIdOptions.@JsonTypeIdResolver@JsonTypeIdResolver decorator can be used to plug a custom type identifier handler to be used for converting between JavaScript types and type id included in JSON content.API: JsonTypeIdResolver -- decorator options JsonTypeIdResolverOptions.@JsonUnwrapped@JsonUnwrapped defines values that should be unwrapped/flattened when serialized/deserialized.API: JsonUnwrapped -- decorator options JsonUnwrappedOptions.@JsonValue@JsonValue decorator indicates that the value of decorated accessor (either field or "getter" method) is to be used as the single value to serialize for the instance, instead of the usual method of collecting properties of value.API: JsonValue -- decorator options JsonValueOptions.@JsonView@JsonView decorator is used for indicating view(s) that the property that is defined by method or field decorated is part of. If multiple View class identifiers are included, the property will be part of all of them. It is also possible to use this decorator on classes to indicate the default view(s) for properties of the type, unless overridden by per-property decorator.API: JsonView -- decorator options JsonViewOptions.ConclusionIn the next part ("Jackson-js: Examples for client (Angular) and server (Node.js) side (Part 2)"), I will give a simple example using jackson-js with Angular 9 for the client side and two examples for the server side: one using Node.js + Express + SQLite3 (with Sequelize 5) and another one using Node.js + LoopBack 4.

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