Calculate inferred types from a typescript file

inferred.ts

import * as ts from 'typescript'
import * as util from 'util'

const program = ts.createProgram(['src/foo.ts'], {
  target: ts.ScriptTarget.ES5,
  module: ts.ModuleKind.CommonJS
})
const typeChecker = program.getTypeChecker()

const findFunctions = (parent: ts.Node) => {
  parent.forEachChild(node => {
    if (ts.isArrowFunction(node)) {
      const type = typeChecker.getTypeAtLocation(node)
      for (const signature of type.getCallSignatures()) {
        const returnType = signature.getReturnType()
        console.log('return', typeChecker.typeToString(returnType))
        if (returnType.flags === ts.TypeFlags.Number) {
          console.log('returns a number')
        } else if (returnType.flags === ts.TypeFlags.Object) {
          console.log('returns an object')
          returnType.getProperties().forEach(property => {
            console.log(
              'property name',
              property.getName(),
              typeChecker.typeToString(
                typeChecker.getTypeOfSymbolAtLocation(property, property.valueDeclaration!)
              )
            )
          })
        }
      }
    }
    findFunctions(node)
  })
}

for (const sourceFile of program.getSourceFiles()) {
  if (!sourceFile.isDeclarationFile) {
    findFunctions(sourceFile)
  }
}