Possible Function (pfn)

Wraps what might be a function, with fallback behavior in case it’s not. Perfect for use in functions that accept optional callback arguments.

Installation

Requires Node.js 6.0.0 or above.

npm i pfn

API

There are two ways you can import the module: require('pfn') or require('pfn/strict'). Each exposes a single function with the same signature. The difference is that strict mode will throw an error if fn is anything other than a function, null, or undefined. Normal mode will silently defer to or if fn is of an unexpected type.

Parameters

1. fn (any): The value that may or may not be a function.
2. Optional: or (any): The function that will be called, or the value that will be returned, if fn is not a function. Defaults to a passthrough function (a => a).

Return Value

Always returns a Function.

• fn, if fn is a Function
• or, if or is a Function
• A Function that returns or, if neither fn nor or is a function

Examples

pfn wraps a value that may or may not be a function. If the underlying value is not a function, then pfn will execute one of the following fallback behaviors.

Passthrough Fallback

If the value turns out to not be a function, pfn will, by default, pass through whatever is given as the first argument. This is useful for optional filters.

const pfn = require('pfn')

function sayHello (name, filter) {
  filter = pfn(filter)
  return filter('Hello, ' + name)
}

// No filter is provided, so the hello message is returned without change:
sayHello('world') // 'Hello, world'

// A filter is provided which changes the hello message:
sayHello('world', m => m + '!!') // 'Hello, world!!'

Return-Value Fallback

If the value turns out to not be a function, pfn can be configured to return a value of your choosing.

const pfn = require('pfn')

function sayHello (nameCallback) {
  nameCallback = pfn(nameCallback, 'world')
  return 'Hello, ' + nameCallback()
}

sayHello() // 'Hello, world'
sayHello(() => 'Dolly') // 'Hello, Dolly'

Self Fallback

If you provide the possible function as its own fallback, then you can accept either a value or a function as an argument for your code. For example, the sayHello function in the following example can accept either a string or a function.

Don’t use the module’s strict mode (require('pfn/strict')) if you want self-fallback behavior, because strict mode will throw an error if the first parameter is anything other than a function, null, or undefined.

const pfn = require('pfn')

function sayHello (name) {
  name = pfn(name, name)
  return 'Hello, ' + name()
}

sayHello('world') // 'Hello, world'
sayHello(() => 'world') // 'Hello, world'

Custom Fallback

If the wrapped value turns out to not be a function, the wrapping function can execute a custom fallback function instead:

const pfn = require('pfn')
const mightBeAFunction = null
const callback = pfn(mightBeAFunction, (...args) => args.length)
callback('arg 1', 'arg 2') // 2

Related

This module is part of the fn family of modules.

• efn: Extracted Function
• ffn: Filtering Function
• jfn: Joined Function
• mfn: Memoized Function
• ofn: Overloaded Function
• qfn: Qualified Function
• vfn: Variadic Function
• wfn: Wrapper Function
• xfn: Extended Function
• 3fn: Three-Way Comparison Function