if function does not exist write function - javascript

Use a function expression, not a function declaration.

if(typeof myfunction != 'function'){
   window.myfunction = function(){};
}

(I'm using window since your last paragraph suggests you want a global function)

You should use strict comparison operator !==

if(typeof myFunction !== 'function'){
    window.myFunction = function(){}; // for a global function or
    NAMESPACE.myFunction = function(){}; // for a function in NAMESPACE 
}

Also try to keep js functions inside namespaces, this way you avoid collisions with other js libraries in the future.