Overloads a function. Makes parameters optional.

  • ofn

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Overload a Function. Make parameters optional.
True overloading is not a feature of the JavaScript language, but the ofn module does give you one key feature of overloading: the ability to omit arguments at the beginning or in the middle of a function call.


Requires Node.js 6.0.0 or above.
npm i ofn


The module exports a single function.


  1. argPrecedence (Array): An array of unique integers ranging from 0 (inclusive) to fn.length (exclusive)
  2. fn (Function): The function to be overloaded

Return Value

A wrapper function that overloads the fn function based on the number of arguments with which it is called.


JavaScript already lets you make parameters optional if they’re at the end:
function example (a, b, c = 'default') {}

But let’s say you want both a and b to be optional, with default values of 1 and 2 respectively, and you want b to be the one omitted if there are only two arguments. Normally you’d see this implemented like so:
function example (a, b, c) {
  if (arguments.length === 1) [a, b, c] = [1, 2, a]
  if (arguments.length === 2) [a, b, c] = [a, 2, b]
  // ...

Thankfully we have array destructuring in ES6 — things were a lot messier before that! But the ofn module makes things even easier and cleaner. With the ofn module, the above example would look like this:
const ofn = require('ofn')
const example = ofn([2, 0, 1], (a = 1, b = 2, c) => {
  // ...

See the array passed as the first argument to ofn? That tells ofn the precedence of the parameters. The number 2 comes first. That means if example() is called with only 1 argument, it’ll be given to the parameter with a (zero-based) index of 2, which is c. Calling example('three') will result in argument values of 1, 2, and three. The a and b arguments have nothing passed to them, so they keep their default values of 1 and 2.
The number 0 is the second in the list. That means if there are two arguments, the arguments will be passed as the 0th and 2nd parameters, a and c, in that order. So calling example('one', 'three') means the function will receive argument values of one, 2, and three respectively.
And finally, we have number 1 third. If there are three arguments, then the parameter with an index of 1 (which is b) will get included. example('one', 'two', three') will result in one for a, two for b, and three for c.

Related Modules

For more projects like this, check out the xfn family of modules.