About d ts in TypeScript

Question

I m feeling curious about   d ts because I m a newbie in TypeScript  And I was told by someone that this kind of file is something like  head file  in C   but for JS only  But I cannot convert a pure JS file to   d ts file unless I forcely change the   js to   ts  So I have three files  a JS file  a TS file and a   d ts file      What s the relationship between them  How can I use the   d ts file  Does it mean I can delete the   ts file permanently  If so  how can the   d ts file know which JS file is mapping to itself      It would be very nice if someone can give me an example

User · Accepted Answer

The  d ts  file is used to provide typescript type information about an API that s written in JavaScript  The idea is that you re using something like jQuery or underscore  an existing javascript library  You want to consume those from your typescript code   Rather than rewriting jquery or underscore or whatever in typescript  you can instead write the d ts file  which contains only the type annotations  Then from your typescript code you get the typescript benefits of static type checking while still using a pure JS library

User · Answer

I could not comment and thus am adding this as an answer  We had some pain trying to map existing types to a javascript library   To map a  d ts file to its javascript file you need to give the  d ts file the same name as the javascript file  keep them in the same folder  and point the code that needs it to the  d ts file   eg  test js and test d ts are in the testdir  folder  then you import it like this in a react component   import   as Test from    testdir test     The  d ts file was exported as a namespace like this   export as namespace Test   export interface TestInterface1   export class TestClass1

User · Answer

This answer assumes you have some JavaScript that you don t want to convert to TypeScript  but you want to benefit from type checking with minimal changes to your  js  A  d ts file is very much like a C or C   header file  Its purpose is to define an interface  Here is an example   mashString d ts      Makes a string harder to read     declare function mashString          The string to obscure        str  string   string  export   mashString    mashString js      ts-check      type  import    mashString       module exports    str    gt      str  reverse   join        main js      ts-check const mashString   require    mashString    console log mashString  12345       The relationship here is  mashString d ts defines an interface  mashString js implements the interface and main js uses the interface   To get the type checking to work you add     ts-check  to your  js files  But this only checks that main js uses the interface correctly  To also ensure that mashString js implements it correctly we add      type  import    mashString       before the export   You can create your initial  d ts files using tsc -allowJs main js -d then edit them as required manually to improve the type checking and documentation   In most cases the implementation and interface have the same name  here mashString  But you can have alternative implementations  For example we could rename mashString js to reverse js and have an alternative encryptString js

User · Answer

d stands for Declaration Files      When a TypeScript script gets compiled there is an option to generate   a declaration file  with the extension  d ts  that functions as an   interface to the components in the compiled JavaScript  In the process   the compiler strips away all function and method bodies and preserves   only the signatures of the types that are exported  The resulting   declaration file can then be used to describe the exported virtual   TypeScript types of a JavaScript library or module when a third-party   developer consumes it from TypeScript       The concept of declaration files is analogous to the concept of header   file found in C C      declare module arithmetics       add left  number  right  number   number      subtract left  number  right  number   number      multiply left  number  right  number   number      divide left  number  right  number   number         Type declaration files can be written by hand for existing JavaScript   libraries  as has been done for jQuery and Node js       Large collections of declaration files for popular JavaScript   libraries are hosted on GitHub in DefinitelyTyped and the Typings   Registry  A command-line utility called typings is provided to help   search and install declaration files from the repositories

User · Answer

Worked example for a specific case   Let s say you have my-module that you re sharing via npm    You install it with npm install my-module  You use it thus   import   as lol from  my-module    const a   lol  abc    def      The module s logic is all in index js   module exports   function firstString  secondString          your code    return result     To add typings  create a file index d ts   declare module  my-module      export default function anyName arg1  string  arg2  string   MyResponse     interface MyResponse     something  number    anything  number

User · Answer

Like  takeshin said  d stands for declaration file for typescript   ts    Few points to be clarified before proceeding to answer this post -   Typescript is syntactic superset of javascript  Typescript doesn t run on its own  it needs to be transpiled into javascript  typescript to javascript conversion    Type definition  and  Type checking  are major add-on functionalities that typescript provides over javascript   check difference between type script and javascript    If you are thinking if typescript is just syntactic superset  what benefits does it offer - https   basarat gitbooks io typescript docs why-typescript html the-typescript-type-system  To Answer this post -   As we discussed  typescript is superset of javascript and needs to be transpiled into javascript  So if a library or third party code is written in typescript  it eventually gets converted to javascript which can be used by javascript project but vice versa does not hold true   For ex -   If you install javascript library -  npm install --save mylib   and try importing it in typescript code -   import   from  mylib     you will get error       Cannot find module  mylib      As mentioned by  Chris   many libraries like underscore  Jquery are already written in javascript  Rather than re-writing those libraries for typescript projects  an alternate solution was needed   In order to do this  you can provide type declaration file in javascript library named as   d ts  like in above case mylib d ts  Declaration file only provides type declarations of functions and variables defined in respective javascript file   Now when you try -   import   from  mylib     mylib d ts gets imported which acts as an interface between javascript library code and typescript project

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