How to parse JSON string in Typescript

Question

Is there a way to parse strings as JSON in Typescript  Example  In JS  we can use JSON parse    Is there a similar function in Typescript   I have a JSON object string as follows     name    Bob    error   false

User · Accepted Answer

Typescript is  a superset of  javascript  so you just use JSON parse as you would in javascript   let obj   JSON parse jsonString     Only that in typescript you can have a type to the resulting object   interface MyObj       myString  string      myNumber  number     let obj  MyObj   JSON parse     myString    string    myNumber   4      console log obj myString   console log obj myNumber      code in playground

User · Answer

TS has a JavaScript runtime Typescript has a JavaScript runtime because it gets compiled to JS  This means JS objects which are built in as part of the language such as JSON  Object  and Math are also available in TS  Therefore we can just use the JSON parse method to parse the JSON string  Example  const JSONStr      quot name quot    quot Bob quot    quot error quot   false       The JSON object is part of the runtime const parsedObj   JSON parse JSONStr    console log parsedObj       LOG           quot name quot    quot Bob quot         quot error quot   false           The Object object is also part of the runtime so we can use it in TS const objKeys   Object keys parsedObj    console log objKeys       LOG     quot name quot    quot error quot     The only thing now is that parsedObj is type any which is generally a bad practice in TS  We can type the object if we are using type guards  Here is an example  const JSONStr      quot name quot    quot Bob quot    quot error quot   false   const parsedObj   JSON parse JSONStr    interface nameErr     name  string    error  boolean     function isNameErr arg  any   arg is nameErr     if  typeof arg name      string   amp  amp  typeof arg error      boolean         return true      else       return false         if  isNameErr parsedObj          Within this if statement parsedObj is type nameErr    parsedObj

User · Answer

Yes it s a little tricky in TypeScript but you can do the below example like this  x000D   x000D  let decodeData     JSON parse    jsonResponse     x000D   x000D   x000D

User · Answer

You can additionally use libraries that perform type validation of your json  such as Sparkson  They allow you to define a TypeScript class  to which you d like to parse your response  in your case it could be   import   Field   from  sparkson   class Response      constructor         Field  name   public name  string         Field  error   public error  boolean             The library will validate if the required fields are present in the JSON payload and if their types are correct  It can also do a bunch of validations and conversions

User · Answer

If you want your JSON to have a validated Typescript type  you will need to do that validation work yourself  This is nothing new  In plain Javascript  you would need to do the same   Validation  I like to express my validation logic as a set of  transforms   I define a Descriptor as a map of transforms   type Descriptor lt T gt         P in keyof T    v  any    gt  T P        Then I can make a function that will apply these transforms to arbitrary input   function pick lt T gt  v  any  d  Descriptor lt T gt    T     const ret  any         for  let key in d        try         const val   d key  v key          if  typeof val      undefined             ret key    val                catch  err          const msg   err instanceof Error   err message   String err         throw new Error  could not pick   key     msg                 return ret      Now  not only am I validating my JSON input  but I am building up a Typescript type as I go  The above generic types ensure that the result infers the types from your  transforms    In case the transform throws an error  which is how you would implement validation   I like to wrap it with another error showing which key caused the error   Usage  In your example  I would use this as follows   const value   pick JSON parse    name    Bob    error   false         name  String    error  Boolean        Now value will be typed  since String and Boolean are both  transformers  in the sense they take input and return a typed output   Furthermore  the value will actually be that type  In other words  if name were actually 123  it will be transformed to  123  so that you have a valid string  This is because we used String at runtime  a built-in function that accepts arbitrary input and returns a string   You can see this working here  Try the following things to convince yourself    Hover over the const value definition to see that the pop-over shows the correct type  Try changing  Bob  to 123 and re-run the sample  In your console  you will see that the name has been properly converted to the string  123

User · Answer

There is a great library for it ts-json-object  In your case you would need to run the following code   import  JSONObject  required  from  ts-json-object   class Response extends JSONObject        required     name  string        required     error  boolean     let resp   new Response   name    Bob    error   false      This library will validate the json before parsing

User · Answer

JSON parse is available in TypeScript  so you can just use it     JSON parse    name    Bob    error   false       Returns a value of type  any    However  you will often want to parse a JSON object while making sure it matches a certain type  rather than dealing with a value of type any  In that case  you can define a function such as the following    function parse json lt TargetType extends Object gt     json  string    type definitions     Key in keyof TargetType    raw value  any    gt  TargetType Key       TargetType     const raw   JSON parse json      const result  any         for  const key in type definitions  result key    type definitions key  raw key      return result      This function takes a JSON string and an object containing individual functions that load each field of the object you are creating  You can use it like so   const value   parse json       name    Bob    error   false        name  String  error  Boolean

User · Answer

Type-safe JSON parse You can continue to use JSON parse  as TS is a JS superset  There is still a problem left  JSON parse returns any  which undermines type safety  Here are two options for stronger types  1  User-defined type guards  playground  Custom type guards are the simplest solution and often sufficient for external data validation     For example  you expect to parse a given value with  MyType  shape type MyType     name  string  description  string        Validate this value with a custom type guard function isMyType o  any   o is MyType     return  quot name quot  in o  amp  amp   quot description quot  in o    A JSON parse wrapper can then take a type guard as input and return the parsed  typed value  const safeJsonParse    lt T gt  guard   o  any    gt  o is T    gt   text  string   ParseResult lt T gt    gt      const parsed   JSON parse text    return guard parsed      parsed  hasError  false       hasError  true      type ParseResult lt T gt          parsed  T  hasError  false  error   undefined         parsed   undefined  hasError  true  error   unknown    Usage example  const json       quot name quot    quot Foo quot    quot description quot    quot Bar quot      const result   safeJsonParse isMyType  json     result  ParseResult lt MyType gt  if  result hasError      console log  quot error    quot       further error handling here   else     console log result parsed description     result parsed now has type  MyType     safeJsonParse might be extended to fail fast or try catch JSON parse errors  2  External libraries Writing type guard functions manually becomes cumbersome  if you need to validate many different values  There are libraries to assist with this task - examples  no comprehensive list    io-ts  rel  popular  3 2k stars currently   fp-ts peer dependency  functional programming style zod  quite new  repo  2020-03-07   strives to be more procedural object-oriented than io-ts typescript-is  TS transformer for compiler API  additional wrapper like ttypescript needed typescript-json-schema ajv  Create JSON schema from types and validate it with ajv  More infos  Runtime type checking  1573 Interface type check with Typescript TypeScript  validating external data

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