redeclare javascript function

I hope that somebody can help me. I want to redeclare js function by extension. For example, there is the basic js function on website: ``` function foo(){ ..something here.. } ``` i want to redeclare it by own function with the same name. how it will be easiest to do? edit 1. i'll try to explain better. there is a native code in website: ``` Notifier = { debug: false, init: function (options) { curNotifier = extend({ q_events: [], q_shown: [], q_closed: [], q_max: 3, q_idle_max: 5, done_events: {}, addQueues: curNotifier.addQueues || {}, recvClbks: curNotifier.recvClbks || {}, error_timeout: 1, sound: new Sound('mp3/bb1'), sound_im: new Sound('mp3/bb2') }, options); if (!this.initFrameTransport() && !this.initFlashTransport(options)) { return false; } this.initIdleMan(); if (!(curNotifier.cont = ge('notifiers_wrap'))) { bodyNode.insertBefore(curNotifier.cont = ce('div', {id: 'notifiers_wrap', className: 'fixed'}), ge('page_wrap')); } }, destroy: function () { Notifier.hideAllEvents(); curNotifier.idle_manager.stop(); curNotifier = {}; re('notifiers_wrap'); re('queue_transport_wrap'); }, reinit: function () { ajax.post('notifier.php?act=a_get_params', {}, { onDone: function (options) { if (options) { curNotifier.error_timeout = 1; this.init(options); } else { curNotifier.error_timeout = curNotifier.error_timeout || 1; setTimeout(this.reinit.bind(this), curNotifier.error_timeout * 1000); if (curNotifier.error_timeout < 256) { curNotifier.error_timeout *= 2; } } }.bind(this), onFail: function () { curNotifier.error_timeout = curNotifier.error_timeout || 1; setTimeout(this.reinit.bind(this), curNotifier.error_timeout * 1000); if (curNotifier.error_timeout < 256) { curNotifier.error_timeout *= 2; } return true; }.bind(this) }); } } ``` and function Sound ``` function Sound(filename) { var audioObjSupport = false, audioTagSupport = false, self = this, ext; if (!filename) throw 'Undefined filename'; try { var audioObj = ce('audio'); audioObjSupport = !!(audioObj.canPlayType); if (('no' != audioObj.canPlayType('audio/mpeg')) && ('' != audioObj.canPlayType('audio/mpeg'))) ext = '.mp3?1'; else if (('no' != audioObj.canPlayType('audio/ogg; codecs="vorbis"')) && ('' != audioObj.canPlayType('audio/ogg; codecs="vorbis"'))) ext = '.ogg?1'; else audioObjSupport = false; } catch (e) {} // audioObjSupport = false; if (audioObjSupport) { audioObj.src = filename + ext; var ended = false; audioObj.addEventListener('ended', function(){ended = true;}, true); audioObj.load(); this.playSound = function() { if (ended) { audioObj.load(); } audioObj.play(); ended = false; }; this.pauseSound = function() { audioObj.pause(); }; } else { cur.__sound_guid = cur.__sound_guid || 0; var wrap = ge('flash_sounds_wrap') || utilsNode.appendChild(ce('span', {id: 'flash_sounds_wrap'})), guid = 'flash_sound_' + (cur.__sound_guid++); var opts = { url: '/swf/audio_lite.swf?4', id: guid } var params = { swliveconnect: 'true', allowscriptaccess: 'always', wmode: 'opaque' } if (renderFlash(wrap, opts, params, {})) { var swfObj = browser.msie ? window[guid] : document[guid], inited = false, checkLoadInt = setInterval(function () { if (swfObj && swfObj.paused) { try { swfObj.setVolume(1); swfObj.loadAudio(filename + ext); swfObj.pauseAudio(); } catch (e) {debugLog(e);} } inited = true; clearInterval(checkLoadInt); }, 300); self.playSound = function() { if (!inited) return; swfObj.playAudio(0); }; self.pauseSound = function() { if (!inited) return; swfObj.pauseAudio(); }; } } } Sound.prototype = { play: function() { try {this.playSound();} catch(e){} }, pause: function() { try {this.pauseSound();} catch(e){} } }; ``` when i try to add injection with redeclaration function Sound it doesn't work. if i create my own function, for example, xSound and сall it this way: ``` cur.sound = new xSound('mp3/bb1'); ``` it's working.