Derive Type

A simple function to generate types based on function invocations.

Installation

npm i -D derive-type

Simple Example

Original source code:

// main.js

function myFunction(x, y) {
  console.log(x, y)
}

myFunction(1, 2)

Add this function call at the top of your function body:

// main.js

function myFunction(x, y) {
  require('derive-type')(...arguments) // <- ADD THIS
  console.log(x, y)
}

myFunction(1, 2)

Then invoke the binary with the code to trigger the function invocations:

npx derive-type node main.js

This will generate the TypeScript definition and insert the JSDoc snippet:

// main.js

/** @type { import("/var/folders/ls/xxxx/T/derive-type-gen/KC9Vc2Vycy9kMDY1MDIzL3Byb2plY3RzL0Rldk9uRHV0eS9kZXJpdmUtdHlwZS10ZXN0L3Rlc3QuanM6NDoyNSk=").GEN } Generated */
function myFunction(x, y) {
  require('derive-type')(...arguments) // <- ADD THIS
  // now you know that `x` and `y` are of type `number`
  console.log(x, y)
}

myFunction(1, 2)

Notes

• The function invocations don't need to be in the same file, usally they are based on tests:

  npx derive-type npm test

• The generated types are based on the merged input of all function invocations.
• The generated types are located in os.tmpdir(), this can be changed through the environment variable DERIVE_TYPE_FOLDER.
• Debugging output can be enabled by setting the environment variable DERIVE_TYPE_DEBUG to true.

Introduction Video