Type definition in object literal in TypeScript

Question

In TypeScript classes it s possible to declare types for properties  for example   class className     property  string       How do declare the type of a property in an object literal   I ve tried the following code but it doesn t compile   var obj       property  string       I m getting the following error      The name  string  does not exist in the current scope   Am I doing something wrong or is this a bug

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If you re trying to add typings to a destructured object literal  for example in arguments to a function  the syntax is   function foo   bar  baz      bar  boolean  baz  string                  foo   bar  true  baz   lorem ipsum

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In your code  var obj       myProp  string      You are actually creating a object literal and assigning the variable string to the property myProp  Although very bad practice this would actually be valid TS code  don t use this    var string    A string    var obj       property  string      However  what you want is that the object literal is typed  This can be achieved in various ways  Interface  interface myObj       property  string     var obj  myObj     property   quot My string quot      Type alias  type myObjType         property  string     var obj  myObjType     property   quot My string quot      Object type literal  var obj    property  string        property   quot Mystring quot

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Use     const Per       name   HAMZA     age  20    coords        tele   09       lan   190         setAge age  Number   void       this age   age         getAge    Number       return age         const   age  name      age  Number  name  String     Per  const     coords    tele  lan        coords    tele  String  lan  String       Per   console log Per getAge

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You could use predefined utility type Record lt Keys  Type gt    const obj  Record lt string  string gt        property   quot value quot       It allows to specify keys for your object literal  type Keys    quot prop1 quot     quot prop2 quot   const obj  Record lt Keys  string gt        prop1   quot Hello quot     prop2   quot Aloha quot     something   quot anything quot     TS Error  Type    prop1  string  prop2  string  something  string     is not assignable to type  Record lt Keys  string gt                                 Object literal may only specify known properties  and  something  does not exist in type  Record lt Keys  string gt        And a type for the property value  type Keys    quot prop1 quot     quot prop2 quot  type Value    quot Hello quot     quot Aloha quot   const obj1  Record lt Keys  Value gt        prop1   quot Hello quot     prop2   quot Hey quot      TS Error  Type   quot Hey quot   is not assignable to type  Value

User · Answer

You re pretty close  you just need to replace the   with a    You can use an object type literal  see spec section 3 5 3  or an interface  Using an object type literal is close to what you have   var obj    property  string        property   foo       But you can also use an interface  interface MyObjLayout       property  string     var obj  MyObjLayout     property   foo

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Update 2019-05-15  Improved Code Pattern as Alternative   After many years of using const and benefiting from more functional code  I would recommend against using the below in most cases   When building objects  forcing the type system into a specific type instead of letting it infer types is often an indication that something is wrong    Instead I would recommend using const variables as much as possible and then compose the object as the final step    const id   GetId    const hasStarted   true      const hasFinished   false      return  hasStarted  hasFinished  id      This will properly type everything without any need for explicit typing   There is no need to retype the field names  This leads to the cleanest code from my experience  This allows the compiler to provide more state verification  for example  if you return in multiple locations  the compiler will ensure the same type of object is always returned - which encourages you to declare the whole return value at each position - giving a perfectly clear intention of that value     Addition 2020-02-26  If you do actually need a type that you can be lazily initialized  Mark it is a nullable union type  null or Type   The type system will prevent you from using it without first ensuring it has a value   In tsconfig json  make sure you enable strict null checks     strictNullChecks   true  Then use this pattern and allow the type system to protect you from accidental null undefined access     const state         instance  null as null   ApiService         OR        instance  undefined as undefined   ApiService       const useApi        gt           If I try to use it here  the type system requires a safe way to access it         Simple lazy-initialization      const api   state  instance     state instance   new ApiService         api fun            Also here are some ways to only access it if it has value          The  right  way  Typescript 3 7 required     state instance  fun            Or the old way  If you are stuck before Typescript 3 7     state instance  amp  amp  state instance fun            Or the long winded way because the above just feels weird     if  state instance    state instance fun              Or the I came from C and can t check for nulls like they are booleans way     if  state instance    null    state instance fun              Or the I came from C and can t check for nulls like they are booleans         AND I was told to always use triple     in javascript even with null checks way     if  state instance     null  amp  amp  state instance     undefined    state instance fun          class ApiService       fun                Do something useful here             Do not do the below in 99  of cases   Update 2016-02-10 - To Handle TSX  Thanks  Josh   Use the as operator for TSX   var obj         property  null as string      A longer example   var call         hasStarted  null as boolean      hasFinished  null as boolean      id  null as number       Original Answer  Use the cast operator to make this succinct  by casting null to the desired type    var obj         property   lt string gt  null      A longer example   var call         hasStarted   lt boolean gt  null      hasFinished   lt boolean gt  null      id   lt number gt  null       This is much better than having two parts  one to declare types  the second to declare defaults    var callVerbose        hasStarted  boolean      hasFinished  boolean      id  number            hasStarted  null      hasFinished  null      id  null

User · Answer

I m surprised that no-one s mentioned this but you could just create an interface called ObjectLiteral  that accepts key  value pairs of type string  any   interface ObjectLiteral      key  string   any      Then you d use it  like this   let data  ObjectLiteral       hello   world     goodbye  1                An added bonus is that you can re-use this interface many times as you need  on as many objects you d like   Good luck

User · Answer

If you re trying to write a type annotation  the syntax is  var x    property  string        property   hello      If you re trying to write an object literal  the syntax is  var x     property   hello      Your code is trying to use a type name in a value position

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In TypeScript if we are declaring object then we d use the following syntax    access modifier  variable name        structure of object        For example   private Object   Key1  string  Key2  number

[typescript] Type definition in object literal in TypeScript

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