快速技巧：深入剖析jQuery - Grep

// jQuery source for the grep method 
grep: function( elems, callback, inv ) {
		var ret = [];

		// Go through the array, only saving the items
		// that pass the validator function
		for ( var i = 0, length = elems.length; i < length; i++ ) {
			if ( !inv !== !callback( elems[ i ], i ) ) {
				ret.push( elems[ i ] );
			}
		}

		return ret;
	}