Generate TypeScript type definitions based on function invocations and embed them as JSDoc comments in your JavaScript code.

npm i -D derive-type

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')(x, y) // <- 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, insert the JSDoc snippet and remove the call to derive-type:

// main.js
/** @type { import("/var/folders/ls/xxxx/T/derive-type-gen/KC9Vc2Vycy9kMDY1MDIzL3Byb2plY3RzL0Rldk9uRHV0eS9kZXJpdmUtdHlwZS10ZXN0L3Rlc3QuanM6NDoyNSk=").GEN } Generated */
function myFunction(x, y) {
  // now you know that `x` and `y` are of type `number`
  console.log(x, y)
}
myFunction(1, 2)

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

  npx derive-type npm test

• The generated types are based on the merged input of all function invocations.
• The function definition must begin in a new line.
• You can also install derive-type globally, then you must use the path of your global package, e.g. require("/usr/local/lib/node_modules/derive-type/").
• The generated types are located in os.tmpdir() (changeable via DERIVE_TYPE_FOLDER).
• The generated types are deleted after 5 days (changeable via DERIVE_TYPE_MAX_AGE_DAYS).
• Debugging output can be enabled by setting DERIVE_TYPE_DEBUG to true.
• You can find some example type generations in the test file.