A function can accept one or more parameters.

const dosomething = () => {
  //do something
}

const dosomethingElse = foo => {
  //do something
}

const dosomethingElseAgain = (foo, bar) => {
  //do something
}

Starting with ES6/ES2015, functions can have default values for the parameters:

const dosomething = (foo = 1, bar = 'hey') => {
  //do something
}

This allows you to call a function without filling all the parameters:

dosomething(3)
dosomething()

ES2018 introduced trailing commas for parameters, a feature that helps reducing bugs due to missing commas when moving around parameters (e.g. moving the last in the middle):

const dosomething = (foo = 1, bar = 'hey',) => {
  //do something
}

dosomething(2, 'ho!')

It is also okay to call your functions with a trailing comma after the last parameter:

dosomething(2, 'ho!',)

You can wrap all your arguments in an array, and use the spread operator when calling the function:

const dosomething = (foo = 1, bar = 'hey') => {
  //do something
}
const args = [2, 'ho!']
dosomething(...args)

With many parameters, remembering the order can be difficult. Using objects, destructuring allows to keep the parameter names:

const dosomething = ({ foo = 1, bar = 'hey' }) => {
  //do something
  console.log(foo) // 2
  console.log(bar) // 'ho!'
}
const args = { foo: 2, bar: 'ho!' }
dosomething(args)

Functions now support default parameters:

const foo = function(index = 0, testing = true) { /* ... */ }
foo()

Default parameter values have been introduced in ES2015, and are widely implemented in modern browsers.

This is a doSomething function which accepts param1.

const doSomething = (param1) => {

}

We can add a default value for param1 if the function is invoked without specifying a parameter:

const doSomething = (param1 = 'test') => {

}

This works for more parameters as well, of course:

const doSomething = (param1 = 'test', param2 = 'test2') => {

}

What if you have an unique object with parameters values in it?

Once upon a time, if we had to pass an object of options to a function, in order to have default values of those options if one of them was not defined, you had to add a little bit of code inside the function:

const colorize = (options) => {
  if (!options) {
    options = {}
  }

  const color = ('color' in options) ? options.color : 'yellow'
  ...
}

With object destructuring you can provide default values, which simplifies the code a lot:

const colorize = ({ color = 'yellow' }) => {
  ...
}

If no object is passed when calling our colorize function, similarly we can assign an empty object by default:

const spin = ({ color = 'yellow' } = {}) => {
  ...
}