Javascript FunctionDescriptors

Fast, foolproof library to describe a Javascript function, including its parameter names and traits.

This library uses acorn to parse the stringified function, then traverses the syntax tree. Because it doesn't rely on Regex, it is much less brittle than other libraries.

const describeFunction = require('function-descriptor')

const fn = (a, b=1) => a + b
const descriptor = describeFunction(fn)

console.log(descriptor.name)  // "fn"
console.log(descriptor.isArrowFunction)  // true
console.log(descriptor.parameters[0].name)  // "a"
console.log(descriptor.parameters[1].name)  // "b"
console.log(descriptor.parameters[1].hasDefault)  // true

Installation

Within a Node.js project, install through npm.

npm install function-descriptor

Or, to use on the web, add:

<!-- Dependency: 'acorn-loose' library. --> 
<script src="https://cdnjs.cloudflare.com/ajax/libs/acorn-loose/8.4.0/acorn-loose.min.js" integrity="sha512-Ffb86Jr5RrEScAcI3I/LQ8Tr/VM6C/I79Icryx8X4cpiBIvdPNKqf2MbXdlpFj61+gCPTWWQkwOw9m/7CHed3A==" crossorigin="anonymous" referrerpolicy="no-referrer"></script>

<!-- This library. -->
<script type="application/javascript" src="https://unpkg.com/function-descriptor/dist/main.umd.js"></script>

Or, poke around the dist folder.

Features

Information you can glean from a function using this library:

• Minimum number of arguments (minArgs)
• Maximum number of arguments (maxArgs)
• Whether it is:
  • An arrow function (isArrowFunction)
  • An async function (isAsyncFunction)
  • A generator function (isGenerator)
  • A class (isClass). In this case, the returned FunctionDescriptor will describe the constructor.
• Parameter traits (parameters)
  • Parameter name (name)
  • Whether there is a default value (hasDefault)
  • If parameters/arguments are not a 1-1 mapping (destructureType). For example:
    • ...args Spread syntax (destructureType = "spread")
    • { arg1, arg2 } Object destructuring (destructureType = "object")
    • [elem1, elem2] Array destructuring (destructureType = "array")

This library supports a number of features including:

• Anonymous functions
• Spread syntax
• Object and array destructuring
• Detecting if a parameter has a default value set
• Class / instance methods
• Arbitrary strings and comments within the signature, such as the following:

function(myarg="Let's try to break the parser: , myotherarg,\",)\n\n[...args]function(){}{") {
  // Definition
}

Complex Example

Here's an arbitrary Javascript function one might write, using some extended JS concepts when defining the parameter definitions.

function f(x, y=1, { operation, printArgs }, ...args) {
  if (printArgs) {
    console.log("Given extra arguments: " + args)
  }
  if (operation == "add") {
    return x + y
  } else {
    return x - y
  }
}
console.log(describeFunction(f))

FunctionDescriptor {
  f: [Function: f],
  name: 'f',
  isAsync: false,
  isClass: false,
  isGenerator: false,
  parameters: [
    ParamDescriptor {
      name: 'x',
      rawName: 'x',
      destructureType: null,
      hasDefault: false
    },
    ParamDescriptor {
      name: 'y',
      rawName: ...,
      destructureType: null,
      hasDefault: true
    },
    ParamDescriptor {
      name: undefined,
      rawName: ...,
      destructureType: 'object',
      hasDefault: false
    },
    ParamDescriptor {
      name: 'args',
      rawName: ...,
      destructureType: 'spread',
      hasDefault: false
    }
  ],
  minArgs: 1,
  maxArgs: Infinity,
  functionString: ...,
  isArrowFunction: false
}

Some things to note:

• Arguments formed by object or array destructuring do not have names.
• When a parameter uses spread syntax, the maximum number of arguments becomes Infinity.
• When there are defaults, the actual values are not detected (it would be possible to parse these values if they were literals, but that's not implemented).