Javascript未知数量的参数

Question

in my project, I register different functions (having different number of arguments) as listeners to a number of events. When the event takes place, I need to fire the associated function. I receive the parameters to be passed to listener method in the form of an array whereas the listener function expect each separate argument. So, I am doing it like this but I do not like the approach and would like to know if there is an elegant way of doing it,

function callListenerWithArgs(func, args){
        switch(args.length){
            case 1:
                func(args[0]);
                break;
            case 2:
                func(args[0], args[1]);
                break;
            case 3:
                func(args[0], args[1], args[2]);
                break;
            case 4:
                func(args[0], args[1], args[2], args[3]);
                break;
            default:
                func();
        }
    }

Answer 1

Use .apply

func.apply(null, args)

If you need to bind to a specific scope, you can pass another argument in to use as this inside the function:

func.apply(scope, args);

Also, a nuance of JavaScript is that you can call functions with undefined values. So making a small tweak to your existing code will work in 95% of all cases (this isn't suggested as a solution, just pointing it out):

// will handle any number of args up to 7
function callListenerWithArgs(func, args){
    func(args[0], args[1], args[2], args[3], args[4], args[5], args[6]);
}

If your func is defined as:

function foo(a, b, c){
}

you get a , b , c passed in, along with some more undefined values that get ignored. As I said above, this works in 95% of cases. It doesn't work if you ever check arguments.length in the called function since it will always be the same, regardless of the number of parameters the function defines.

Answer 2

Try this using function.apply

function callListenerWithArgs(func, args){
    func.apply(window, args);
}

Answer 3

functionName.apply(thisScope, arguments) would be more elegant. The arguments argument must be an array.

You can build the Array like:

var args = [];

switch (arguments.length - 1) {
    case 0:
        break;
    case 1:
        args.push(arguments[1]);
        break;
    case 2:
        args.push(arguments[1], arguments[2]);
        break;
    case 3:
        args.push(arguments[1], arguments[2], arguments[3]);
        break;
    default:
        args = Array.prototype.slice.call(arguments, 1);
}

or if the array is already built, just pass it as the second argument to .apply

Answer 4

func.apply(this, args);

看到这里 。

Answer 5

You can get the number of arguments of a function:

var f = function ( ) { console.log(arguments.length); }
f( 2, 3 )  // > 2

That lets you implement your func function directly in place.

function func() {
  var nbArgs = arguments.length;
  // ...
}