A small patch to Function.prototype that helps make functions and classes a bit more self-documenting

Function.signature.js

// Patch Function.prototype to provide the signature in the REPL
Object.defineProperty(
  Function.prototype,
  'signature',
  {
    get() {
      let source = this.toString()

      // For classes we need to extract the signature of the 
      // constructor function within; so use some regular 
      // expressions in order to make that happen
      if (source.startsWith('class')) {
        let parts = /constructor\((.*?)\)\s*\{/.exec(source)
        return `class ${this.name}(${parts[1]})`
      }

      // Otherwise, grab everything up to the first opening 
      // curly bracket and call it a day.
      return source.slice(0, this.toString().indexOf('{')).trim()
    }
  }
)

Subsequent usage might look like this

Person.signature // returns 'class Person(name)'
Array.signature // returns 'function Array()'
String.signature // returns 'function String()'