Splat operators in JavaScript, equivalent to *args and **kwargs in Python?
I found a good solution here: http://readystate4.com/2008/08/17/javascript-argument-unpacking-converting-an-array-into-a-list-of-arguments/
Basically, use function.apply(obj, [args])
instead of function.call
. apply takes an array as the 2nd arg and 'splats' it for you.
The closest idiom for *args
would be
function func (a, b /*, *args*/) {
var star_args = Array.prototype.slice.call (arguments, func.length);
/* now star_args[0] is the first undeclared argument */
}
taking advantage of the fact that Function.length
is the number of arguments given in the function definition.
You could package this up in a little helper routine like
function get_star_args (func, args) {
return Array.prototype.slice.call (args, func.length);
}
and then do
function func (a, b /*, *args*/) {
var star_args = get_star_args (func, arguments);
/* now star_args[0] is the first undeclared argument */
}
If you're in the mood for syntactic sugar, write a function which transforms one function into another one which is called with required and optional arguments, and passes the required arguments along, with any additional optional arguments as an array in final position:
function argsify(fn){
return function(){
var args_in = Array.prototype.slice.call (arguments); //args called with
var required = args_in.slice (0,fn.length-1); //take first n
var optional = args_in.slice (fn.length-1); //take remaining optional
var args_out = required; //args to call with
args_out.push (optional); //with optionals as array
return fn.apply (0, args_out);
};
}
Use this as follows:
// original function
function myfunc (a, b, star_args) {
console.log (a, b, star_args[0]); // will display 1, 2, 3
}
// argsify it
var argsified_myfunc = argsify (myfunc);
// call argsified function
argsified_myfunc (1, 2, 3);
Then again, you could just skip all this mumbo jumbo if you are willing to ask the caller to pass the optional arguments as an array to start with:
myfunc (1, 2, [3]);
There is really no analogous solution for **kwargs
, since JS has no keyword arguments. Instead, just ask the caller to pass the optional arguments in as an object:
function myfunc (a, b, starstar_kwargs) {
console.log (a, b, starstar_kwargs.x);
}
myfunc (1, 2, {x:3});
ES6 Update
For completeness, let me add that ES6 solves this problem with the rest parameter feature. See Javascript - '...' meaning
The nearest equivalent is the arguments
pseudo-array.
ES6 added a spread operator to JavaScript.
function choose(choice, ...availableChoices) {
return availableChoices[choice];
}
choose(2, "one", "two", "three", "four");
// returns "three"