splat.js

splat.js

// see http://blog.aijoona.com/2012/05/16/splat-operator-en-javascript/
function splat(fn) {
	var _ = function(target) {
		if(typeof target == 'undefined') {
			return _.byIndex(-1);
		}
	
		if(typeof target == 'number') {
			return _.byIndex(target);
		}
		
		return _.byName(target);
	};
	
	_.rest = function(index) {
		return fn.length - index;
	};
	
	// from Prototype.js (https://github.com/sstephenson/prototype/blob/master/src/prototype/lang/function.js#L38)
	_.argumentNames = function() {
		var names = fn.toString()
			.match(/^[\s\(]*function[^(]*\(([^)]*)\)/)[1]
			.replace(/\/\/.*?[\r\n]|\/\*(?:.|[\r\n])*?\*\//g, '')
			.replace(/\s+/g, '').split(',');
			
		return names.length == 1 && !names[0] ? [] : names;	
	};
	
	_.byIndex = function(index) {	
		return function __splatted() {
			var args, splat, elements;
					
			args = [].slice.call(arguments);
		
			if(index < 0) {
				index = fn.length + index;
			}
			
			to = args.length - fn.length + index + 1;
			
			splat = args.slice(0, index);
			splat = splat.concat([args.slice(index, to)]);
			splat = splat.concat(args.slice(to));
				
			return fn.apply(this, splat);
		}
	};
	
	_.byName = function(name) {
		return _.byIndex(_.argumentNames().indexOf(name));
	};
	
	return _;
};


// Sugar
Function.prototype.splat = function(target) {
	return splat(this)(target);
};

// Examples
(function(winner, runners, me) {
	console.log(winner, ', ', runners, ', ', me);
}).splat(0)(1, 2, 3, 4, 5);

(function(winner, runners, me) {
	console.log(winner, ', ', runners, ', ', me);
}).splat('winner')(1, 2, 3, 4, 5);

(function(winner, runners, me) {
	console.log(winner, ', ', runners, ', ', me);
}).splat(-2)(1, 2, 3, 4, 5);

tests.js

var f0 = function() {
	return [];
};

var f1 = function(a) {
	return [a];
};

var f2 = function(a, b) {
	return [a, b];
};

var f3 = function(a, b, c) {
	return [a, b, c];
};

var f4 = function(a, b, c, d) {
	return [a, b, c, d];
};

var f5 = function(a, b, c, d, e) {
	return [a, b, c, d, e];
};

var f6 = function(a, b, c, d, e, f) {
	return [a, b, c, d, e, f];
};

test('default (index argument: -1)', function() {
	deepEqual(
		f0.splat()(1, 2, 3),
		[]
	);
	deepEqual(
		f1.splat()(1, 2, 3),
		[[1, 2, 3]]
	);
	deepEqual(
		f2.splat()(1, 2, 3),
		[1, [2, 3]]
	);
	deepEqual(
		f3.splat()(1, 2, 3),
		[1, 2, [3]]
	);
	deepEqual(
		f4.splat()(1, 2, 3),
		[1, 2, 3, []]
	);
});	

test('index argument: 0', function() {
	deepEqual(
		f0.splat(0)(1, 2, 3),
		[]
	);
	deepEqual(
		f1.splat(0)(1, 2, 3),
		[[1, 2, 3]]
	);
	deepEqual(
		f2.splat(0)(1, 2, 3),
		[[1, 2], 3]
	);
	deepEqual(
		f3.splat(0)(1, 2, 3),
		[[1], 2, 3]
	);
	deepEqual(
		f4.splat(0)(1, 2, 3),
		[[], 1, 2, 3]
	);
});	

test('index argument: a', function() {
	raises(
		function() {
			return f0.splat('a')(1, 2, 3);
		}
	);
	deepEqual(
		f1.splat('a')(1, 2, 3),
		[[1, 2, 3]]
	);
	deepEqual(
		f2.splat('a')(1, 2, 3),
		[[1, 2], 3]
	);
	deepEqual(
		f3.splat('a')(1, 2, 3),
		[[1], 2, 3]
	);
	deepEqual(
		f4.splat('a')(1, 2, 3),
		[[], 1, 2, 3]
	);
});	

test('named argument: c', function() {
	raises(
		function() {
			f0.splat('c')(1, 2, 3);
		}
	);
	raises(
		function() {
			f1.splat('c')(1, 2, 3);
		}
	);
	raises(
		function() {
			f2.splat('c')(1, 2, 3);
		}
	);
	deepEqual(
		f3.splat('c')(1, 2, 3),
		[1, 2, [3]]
	);
	deepEqual(
		f4.splat('c')(1, 2, 3),
		[1, 2, [], 3]
	);
});	
	
test('index argument: -1', function() {
	deepEqual(
		f0.splat(-1)(1, 2, 3),
		[]
	);
	deepEqual(
		f1.splat(-1)(1, 2, 3),
		[[1, 2, 3]]
	);
	deepEqual(
		f2.splat(-1)(1, 2, 3),
		[1, [2, 3]]
	);
	deepEqual(
		f3.splat(-1)(1, 2, 3),
		[1, 2, [3]]
	);
	deepEqual(
		f4.splat(-1)(1, 2, 3),
		[1, 2, 3, []]
	);
});		
	
test('index argument: -2', function() {
	deepEqual(
		f0.splat(-2)(1, 2, 3),
		[]
	);
	deepEqual(
		f1.splat(-2)(1, 2, 3),
		[1]
	);
	deepEqual(
		f2.splat(-2)(1, 2, 3),
		[[1, 2], 3]
	);
	deepEqual(
		f3.splat(-2)(1, 2, 3),
		[1, [2], 3]
	);
	deepEqual(
		f4.splat(-2)(1, 2, 3),
		[1, 2, [], 3]
	);
});