Default function parameter ordering

Solution 1:

fill = ({container, liquid} = {}) ->
     container ?= "mug"
     liquid ?= "coffee"

     "Filling the #{container} with #{liquid}..."

alert fill(liquid: "juice", container: "glass")
alert fill()

fill = (quantity="500 mL", {container, liquid} = {}) ->
     container ?= "mug"
     liquid ?= "coffee"

     "Filling the #{container} with #{quantity} of #{liquid}..."

alert fill("1L", liquid: "juice", container: "glass")
alert fill()
alert fill "1L"
alert fill "1L", liquid: "water"

Solution 2:

Amir and Jeremy already have this. As they point out, container="mug" in a function's argument list is really just shorthand for container ?= "mug" in the function body.

Let me just add that when calling functions,

fill(liquid="juice")

means the same thing as in JavaScript: First, assign the value "juice" to the liquid variable; then pass liquid along to fill. CoffeeScript doesn't do anything special here, and liquid has the same scope in that situation as it would outside of the function call.

By the way, I've suggested that the default argument syntax should be made more powerful by allowing arguments to be skipped (e.g. (first, middle ?= null, last) -> would assign values to first and last if only two arguments were passed), and that the ?= syntax should be used rather than =. You might want to express support for that proposal here: issue 1091.

Solution 3:

Currently, there's no way to call with named arguments. It would require knowing the arguments (names, positions, and/or default values) at the calling site, which is not always feasible in javascript/coffeescript.

Instead if you have many arguments and you want to name them and have default values, you could do something like this:

fill = (opts = {}) ->
    opts.container ?= "mug"
    opts.liquid ?= "coffee"
    "Filling the #{opts.container} with #{opts.liquid}..."

alert fill
    liquid:"juice"
    container:"cup"

alert fill
    liquid:"juice"

alert fill()