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Block access to certain variables within function

How can I make a function which was originally defined elsewhere in my code be denied or limited access to certain objects when it gets assigned to an object?
Short Example
var func = function() {
console.log(window.screenX); // <-- this should work
document.createElement("div"); // <-- this should fail (ideally in
// some graceful way)
};
var makeObj = function(pFunc) {
var window = {
"screenX": window.screenX
};
var document = null;
var rObj = {};
rObj.func = pFunc;
return rObj;
};
var obj = makeObj(func);
obj.func(); // <-- want it to have no access to document and
// limited access to window
func is defined by privileged users as a sort of plugin but I want to limit what they can do such as not creating new elements but still be able to access certain variables and functions they will need (such as window.screenX)

I'm sure that there are tons of possible answers, my answer make use of the Function-constructor so that window and document are defined locally within the scope of an also locally defined function. This is very important because you do not want to overwrite both objects globally.
But lets come to this answer. Within makeObj all allowed objects in func are defined with the first and second param of a new Function-Constructor.
The third param is used to create an "anonymous" function that contains certain assignments to those allowed objects. These assignments were created locally by using serializeFakeObject.
In other words, the first and second param with new Function are going to be the params (header or signature) while the third param is going to be the body of the „anonymous“ function.
Please note that this "anonymous" function is created dynamically and looks like this afterwards:
function anonymous(window,document) {
// overwrites allowed objects
window = {"screenX":-8};
document = {};
document.createElement = function (){ throw Error("document.createElement is not allowed"); };
// this function must take assignments above
// no chance to use the globally defined objects window or document
return function (){
console.log(window);
console.log(document.createElement());
}();
}
This anonymous function is dynamically created within makeObj and after that executed by calling obj.func() at the end.
Lets put all together:
var func = function (){
console.log(window); // has just got screenX
console.log(document.createElement()); // throws an error "document.createElement is not allowed"
};
var makeObj = function(pFunc) {
var serializeFakeObject = function(name,obj){
var serializedObject = "";
// when its a property of type function then omit JSON.stringify otherwise it gets undefined
if(obj.__proto__.constructor === Function)serializedObject = obj;
else serializedObject = JSON.stringify(obj);
return name + ' = ' + serializedObject + ';';
};
var fakeWindow = serializeFakeObject("window", { "screenX": window.screenX });
var fakeDocument = serializeFakeObject("document", {});
// create property createElement of document that should fail in a graceful way
var fakeDocumentProperty = serializeFakeObject("document.createElement", function (){ throw Error("document.createElement is not allowed"); });
// if you like you can add more properties here....
var concatFakeObjects = fakeWindow+fakeDocument+fakeDocumentProperty;
var rObj = {};
// the line below creates the "anonymous" function described above
rObj.func = new Function('window', 'document', concatFakeObjects +'return '+pFunc+'();');
return rObj;
};
var obj = makeObj(func);
console.log(obj.func());

Why I cannot access prototype function (using closures)?

I am trying to create function findStrings which should share it's variables with the function init, stringSplit and stringSearch.
String.prototype.findStrings = (function () {
var action = 'split';
var params = ',';
var init = function (action = 'split', params = ',')
{ this.stringSplit(); }
var stringSplit = function (action)
{ return this.init(); }
var stringSearch = function (action)
{return this.init(); }
return init;
})();
What I need.
When I call test.findStrings('split'); it should 1) initiate variables by calling init 2) then call stringSplit so this should be string test.
When I call var result = test.findStrings( 'search', { params ... } ); 1) initiate variables, process arguments from object params 2) then call stringsSearch and this should be test.
var test = '#a > :matches(.b, #c)';
test.findStrings('split', "," );
var result = test.findStrings( 'search', { params ... } );
Error which I got is saying that this does not have stringSplit (because this is Window) and I need to this to be test.
How to access the test string within the functions and make it possible to access the variables and arguments within them?
Actually, I could remove the function searchStrings to simplify it.
Solution:
String.prototype.findStrings = (function () {
var text = this;
...
String.prototype.init = function (action = 'split', params = ',') {
text.stringSplit();
}
}
There were two errors.

I need to this to be test.
When init() is first called, this does equal test. The problem is that stringSplit and stringSearch are just variables, and have nothing to do with the current context.
Either way, the code is pretty difficult to understand. If you really want to override String, then why don't you do something like:
String.prototype.findStrings = function(action = 'split', params = ',') {
// do logic here (not entirely sure what you want to accomplish)
return (action === 'split') ? this.stringSplit() : this.stringSearch();
}
// Add other functions also to string prototype
String.prototype.stringSplit = function() { ... }
String.prototype.stringSearch = function() { ... }
Personally though, I don't like adding methods to types like String for various reasons. Why not adopt a more functional approach, and create functions that you simply pass strings to, and get an appropriate result?
For example:
function findStrings(str, params){
// do logic and return result
}

First of all, you have a circular reference in your code: the init function calls stringSplit which calls init.
But to reach to that problem you should remove the this keyword from your prototype function. this will get the value of the object that is calling your function, in your case the window object, but your functions are created in a closure so you can't access them using the window object.

Javascript, getting away from 'this' and using local scope

In a typical js class, all calls to member functions must be preceded by this. I was looking at a technique that would let me create a library of inter-dependent STATIC functions and relying on closure/scope to make things a bit easier.
Example:
var Singleton={
//main entry point
// call with fn name, args...
call:function(){
var args=[];
if (arguments.length==0) {
return;
}
// get the fn name
var fn=arguments[0];
var x;
// make args array
for (x=1;x<arguments.length;x++) {
args[args.length]=arguments[x];
}
// I want to get rid of this part
// See below for what I wish
// Here I have access to fns below due to hoisting in js
// so I put them in a map...
var fns={
test:test
// etc, more like this I do not want to type/maintain
}
// ... all so I can do this
// get my function.
var fun=fns[fn];
// instead of that, I would like to "override whitespace" and
// say something like:
// var fun=['fn_name'];
// so I can index into local scope and get a fn
//
// log error if not found
if (typeof fun=='undefined') {
loge('Singleton: function not found:'+fn);
return;
}
// ok, run the function
return fun.apply(window,args);
// the test fn accesses test2() without dot notation
function test(a){
// Note: here in test fn it can access test2()
// without using this.test2() syntax
// as you would in normal objects
var s=test2();
alert(s+' test:'+a);
};
function test2(){
return 'test2';
};
}
}
I was hoping someone more familiar with advances in javascript might have advice on how to emulate an "implied but unnecessary this", it always struck me as strange that this defaults to window, and wouldn't it be nice if this could be pointed to an anonymous object with the local scope attached.
I would love to say ['localObject'] to get something in scope.
Edit:
After seeing some of the responses, I will restate this in the form of a challenge:
What I am looking for is a syntax cheat, a way to, as #Varuna put it, "1. Access static methods without using this variable i.e. they will remain global to one another. 2. Do not want to maintain a local array for static methods and want to achieve with in the local scope itself."
Put differently, I need to have the declared functions Register themselves, but I don't want to state the function name more than once. I suppose #Varuna has a solution using eval to access the local scope.
The following approach wont work:
var o={};
o['fn']=function fn(){};
o['fn2']=function fn2(){};
...because you have to state the fn name twice, but closure is preserved.
And this:
var a=[
function fn(){}
,function fn2(){}
];
Register(a);
Will not work because, AFAIK, you lose closure, ie. fn2 cannot see fn. Which also makes the following declarative style a "this nightmare":
window.MINE={
fn:function fn(){
//this?
// want to say fn2(), not this.fn2(), nor MINE.fn2()
}
,fn2:function fn2(){
//this?
}
,deeper:{
//more
}
};
But something like this might work, if you created a weird property that does the registration on assignment:
var registar=new Registar();
registar.reg=function fn(){};
registar.reg=function fn2(){};
//then access
var fn=registar.getFn(n);
// or
var fn=registar._[n];
The above relies on js properties and having access to fn.name, which is not available in all cases AFAIK.

If I understand correctly, you want to create objects that:
have static members
... which can be accessed without using the this notation
The easiest solution (assuming I've properly understood your query), would be to simply use a closure to store your stratic fields, access them directly by name, then explicitly add them as object members.
Consider:
var myConstructor = (function(){
var foo = 'someStaticField';
var bar = function(){
alert('A static method returns ' + foo);
};
return function(){
return {
foo : foo,
bar : bar
};
};
})();
var myInstance = new myConstructor();

As per my understanding, you want to:
1. Access static methods without using this variable i.e. they will remain global to one another.
2. Do not want to maintain a local array for static methods and want to achieve with in the local scope itself.
You can check whether a method exist using eval.Check Here
Only drawback is that this will be using eval method.
Code will be:
var Singleton = {
//main entry point
// call with fn name, args...
call: function () {
var args = [];
if (arguments.length == 0) {
return;
}
// get the fn name
var fn = arguments[0];
var x;
// make args array
for (x = 1; x < arguments.length; x++) {
args[args.length] = arguments[x];
}
//check whether function exist in local scope and not in global scope
if (typeof eval(fn) !== 'undefined' && typeof window[fn] === 'undefined') {
// ok, run the function
return eval(fn).apply(window, args);
}
else{
// log error if not found
loge('Singleton: function not found:' + fn);
return;
}
// the test fn accesses test2() without dot notation
function test(a) {
// Note: here in test fn it can access test2()
// without using this.test2() syntax
// as you would in normal objects
var s = test2();
alert(s + ' test:' + a);
};
function test2() {
return 'test2';
};
}
}

How about declaring functions that can access each other in separate closure, and exporting them to main method by binding your call method to an object containing the functions? Something like previous post (modified slightly):
var Singleton = {
call: (function() {
// here 'call' is bound to object containig your test functions
// this: {test, test2}
if (0 == arguments.length) return;
// log error if not found
if ('function' != typeof this[arguments[0]]) {
console.warn('Singleton: function not found:' + arguments[0]);
return;
}
// '...index into local scope and get function
// ie. get the function by it's name
return this[arguments[0]].
apply(window, Array.prototype.slice.call(arguments, 1));
// --- or:
// you can explicitly introduce function names to current scope,
// by `eval`-ing them here (not very much preferred way in JavaScript world):
for (var fname in this)
if (this.hasOwnProperty(fname))
eval('var ' + fname + ' = ' + this[fname]);
// and you can reference them directly by using their names
var fn = eval(arguments[0]);
return fn.apply(window, Array.prototype.slice.call(arguments, 1));
}).bind(
(function() {
var _exports = {};
function test (a) {
var s = test2();
alert(s + ' test: ' + a);
}
function test2 () {
return 'test2';
}
_exports['test'] = test;
_exports['test2'] = test2;
return _exports;
})()
)};
Singleton.call('test', 'foo and stuff');
//
previous post:
You are talking about Function#bind functionality that enables 'customizing' function's context. .bind() your call method to required 'local context' like this:
var Singleton = {
//main entry point
// call with fn name, args...
call: (function() {
// here `this` (context) is object bound to `call` method
// not `global` object, which is default for 'unbound' functions
var locals = this; // {fns, shift, loge, isfunc}
var fn;
var fun;
var x;
if (arguments.length == 0)
return;
// get the fn name
fn = locals.shift(arguments);
// '...index into local scope and get a fn'
fun = locals.fns[fn];
// log error if not found
if (!locals.isfunc(fun)) {
locals.loge('Singleton: function not found:' + fn);
return;
}
// ok, run the function
return fun.apply(window, arguments);
// lock `call`'s context to provided object
// and use `this` to reference it inside `call`
}).bind({
fns: (function(_) {
// and you can '...create a library of inter-dependent STATIC functions'
// in this closure and invoke them in `call` method above
_.test = function (a) {
var s = _.test2();
alert(s + ' test: ' + a);
};
_.test2 = function() {
return 'test2';
};
return _;
})({}),
// and create couple of helper methods as well...
isfunc: (function(_getclass) {
_getclass.func = _getclass(_getclass);
return ('function' !== typeof(/foo/)) ?
function(node) {
return 'function' == typeof node;
} :
function(node) {
return _getclass.func === _getclass(node);
};
})(Function.prototype.call.bind(Object.prototype.toString)),
loge: console.warn,
shift: Function.prototype.call.bind(Array.prototype.shift)
}),
};
Singleton.call('test', 'foo and stuff');
// eof

Here's one 'in your face answer', because I really don't like what I see here.
I don't see why you need this kind of construct, you already have that as part of language core.
1. dynamic lookup
you are doing it in a rather 'unprecedented' kind of way,
hashes already do that for you, and it's lightning fast to do a hash search.
If you are eval()-ing random strings to do simple name lookup you really have to
step aside from a keybord for a while... (no offense please)
2. closures
you are saying about 'using closures' which you actualy don't use.
your call function redeclares test functions each time it gets called,
and looks the ('fresh version') functions in it's own variable scope table,
instead of lookig them up in parent scope chains (aka. closures)
outside it's lexical location
3. nfe vs. nfd
ie. named function expressions vs. named function declarations
...you cannot assign a function to a local var and have it retain closure.
It is a feature, you might not be aware of how it works (it tripped me up as well).
check this article out for clarification
4. exceptions
Singleton: function name not found... x4!
Just go ahead and call a function,
interpreter will throw for you anyway if it cannot find/execute
5. eval (aka. ^^)
Singleton.call.ctx.fun = eval(Singleton.call.ctx.fn);
eval takes any string here(#!), and gladly executes ones like:
'for(;;);', or 'while(1);'... forever.
You probably don't want to have any code running unless it was your stuff.
6. arguments handling
It is considered best practice out there to use single (Object) options parameter
to 'fine tune' any significant piece of bundled functionality,
instead of trying to figure that out by type checking provided argument list
Here's, in couple of simple lines, what I (and as I can see #Jimmy Breck-McKye) suggest you should do:
var Singleton.call = (function () {
var funcmap = {
'f_1': function () {},
// etc.
'f_N': function () {},
};
return function (options) {
// options members:
// context, (Object) context, (defaults to global if none is given)
// func, (String) function_name,
// args, (Array) arguments to pass into a function
// this line does everything your 100+ lines long snippet was trying to:
// look's up parent scope for a function, tries to run it
// passing provided data, throws if it gets stuck.
return funcmap[options.func].apply(options.context, options.args);
};
})();
//

Answering my own question here.
The core of the issue is that you cannot assign a function to a local var and have it retain closure.
Consider that when writing a function with global and window scope, this is not necessary to call another function with identical scope. Such is not the case with member functions.
Another way of saying this is that there is no space where your cursor can sit and as you declare a function it automatically gets attached to the current this.
function fn(){}// if we are in global scope, then window.fn becomes defined
// but if we are inside, say, a constructor, simple declaration will not attach
// it to this, but fn is available in scope.
Any assignment on function declaration BREAKS part of the expected closure:
var IdentifierAvailableToClosure=function Unavailable(){}
But assignment after declaration works:
function NowAvailable(){}
var SynonymAvailableToo=NowAvailable;
This is what I meant by not wanting to repeat the name twice to get the mechanism to work.
This fact made me abandon other methods and rely on eval as suggested. Here is a first draft:
// This object is an encapsulation mechanism for a group of
// inter-dependent, static-ish, functions that can call each other
// without a this pointer prefix.
// Calls take the form of:
// Singleton.call(functionName:String [,arg1]...)
// or
// Singleton.call(contextObject:Object, functionName:String [,arg1]...)
// If a context is not provided, window is used.
//
// This type of mechanism is useful when you have defined a group
// of functions in the window/global scope and they are not ready
// to be formalized into a set of classes, or you have no intention
// of doing that
//
// To illustrate the issue, consider that a function
// which is defined in window/global scope
// does not have to use the this pointer to call a function of
// identical scope -- yet in a class member function, the this pointer
// MUST be used
// Therefore, trying to package such functions requires injecting
// the this pointer into function bodies where calls to associater
// functions are made
//
// Usage is primarily for development where one has control over
// global namespace pollution and the mechanism is useful in
// refactoring prior to formalization of methods into classes
var Singleton={
// Main call point
call:function(){
// Bail with error if no args
if (arguments.length==0) {
throw('Singleton: need at least 1 arg');
}
// As all functions in the local scope library below
// have access to the local scope via closure, we want to reduce
// pollution here, so lets attach locals to this call
// function instead of declaring locals
//
// Prepare to call anon fn
Singleton.call.args=arguments;
// Make ctx have args, context object, and function name
Singleton.call.ctx=(function (){// return args,ctx,name
// out
var args=[];
//locals
var x, fn;
// collapse identifier
var a=Singleton.call.args;
// closure object avail to functions, default to window
that=window;
// first real function argument
var arg_start=1;
// first arg must be function name or object
if (typeof a[0]=='string') {// use window ctx
fn=a[0];
// if first arg is object, second is name
}else if (typeof a[0]=='object') {
// assign given context
that=a[0];
// check second arg for string, function name
if (typeof a[1]!='string') {
var err='Singleton: second argument needs to be a fn name'
+' when first arg is a context object';
throw(err)
return;
}
// ok, have a name
fn=a[1];
// args follow
arg_start=2;
}else{
// improper arg types
var err='Singleton: first argument needs to be a string or object';
throw(err)
}
// build args array for function
for (x=arg_start;x<a.length;x++) {
args[args.length]=a[x];
}
// return context
return {
args: args
,that:that
,fn:fn
};
})();
// using function library present in local scope, try to find specified function
try{
Singleton.call.ctx.fun=eval(Singleton.call.ctx.fn);
}catch (e){
console.error('Singleton: function name not found:' + Singleton.call.ctx.fn);
throw('Singleton: function name not found:' + Singleton.call.ctx.fn);
}
// it must be a function
if (typeof Singleton.call.ctx.fun !== 'function') {
console.error('Singleton: function name not found:' + Singleton.call.ctx.fn);
throw('Singleton: function name not found:' + Singleton.call.ctx.fn);
}
// library functions use that instead of this
// that is visible to them due to closure
var that=Singleton.call.ctx.that;
// Do the call!
return Singleton.call.ctx.fun.apply(that, Singleton.call.ctx.args);
//
// cool library of functions below,
// functions see each other through closure and not through this.fn
function test(s){
alert(test2()+' test:'+s);
}
function info_props(){
console.info(this_props());
}
function test2(){
return 'test2';
}
function this_props(){
var s='';
for (var i in that) {
s+=' '+i;
}
return s;
};
}
};

How to cache/precalculate something(without global variable)?

What I want to do is to sending data between two handlers.
element.onmousedown = function() {
data = precalculate();
}
element.onmouseup = function() {
dosomething(data);
}
if the data is a global variable it works. People says global variable is evil. But I don't know how to do without it.
or I misunderstood "global variable"?

Just scope the variable if you don't want/need it to be global:
(function() {
var data;
element.onmousedown = function() {
data = precalculate();
}
element.onmouseup = function() {
dosomething(data);
}
})();
EDIT: To clarify, the only way to create a new variable scope in javascript is in a function.
Any variable declared with var inside a function is inaccessible to the outer scope.
In the code above, I created an IIFE (immediately invoked function expression), which is simply a function that is invoked as soon as it is created, and I placed your data variable (along with the handler assignments) inside of it.
Because the handlers were created in a scope that has access to the data variable, they retain their access to that variable.
To give another example:
var a = "a"; // global variable
(function() {
var b = "b"; // new variable in this scope
(function() {
var c = "c"; // new variable in this scope
// in this function, you have access to 'a', 'b' and 'c'
})();
// in this function you have access to 'a' and 'b' variables, but not 'c'
})();
// globally, you have access to the 'a' variable, but not 'b' or 'c'

In this case a global variable would make sense. Another possibility is to attach the value to the DOM element:
element.onmousedown = function() {
// 'this' should point to the element being mouse downed
this.data = precalculate();
};
element.onmouseup = function() {
// 'this' should point to the element being mouse upped
var data = this.data;
dosomething(data);
};

You misunderstood "global variable is evil".
In fact, what really happened is that someone wanted to be "part of the crowd", and so told you a sweeping generalisation, when in fact they should have said "only use global variables where appropriate".
Well, they are appropriate here, my friend.

JQuery makes this possible by using the .data() function:
http://api.jquery.com/jQuery.data/

You can get away with using global variables as long as you keep them to a minimum, for example you can place stuff in a single global namespace:
App = {};
element.onmousedown = function() {
App.data = "hello";
}
element.onmouseup = function() {
console.log(App.data);
}

Another more general solution is to create functions that cache their results using a technique that is called memoization. There's plenty of stuff if you search.
The following code does exactly that :
Function.prototype.memoize = function() {
var fn = this;
this.memory = {};
return function() {
var args = Array.prototype.slice.call(arguments);
return fn.memory[args] ? fn.memory[args] : fn.memory[args] = fn.apply(this, arguments);
};
};
e.g. You have an expensive function called exfunc..
newFunc = exfunc.memoize();
The above statement creates a new function called newFunc that caches the result of the original function so that the first time the actual code is being executed and all subsequent calls are retrieved from a local cache.
This mostly works with functions whose return value does not depend on global state.
More info :
http://osteele.com/archives/2006/04/javascript-memoization

Dynamically attach a function to grant access to private variables

I'm trying to assign a callback dynamically to an Object of mine, I can't seem to figure out a way to do this while granting this function access to private variables. I've listed the relavant code below with comments where I ran into walls.
Object Factory
function makeObj ( o ) {
function F() {}
F.prototype = o;
return new F();
}
Module
var MODULE = (function(){
var myMod = {},
privateVar = "I'm private";
return myMod;
})();
Various Attempts
myMod.someDynamicFunc = function someDynamicFunc(){
//privateVar === undefined;
alert( privateVar );
}
myMod.someDynamicFunc();
myMod.prototype.someDynamicFunc = function someDynamicFunc(){
//ERROR: Cannot set property 'someDynamicFunc' of undefined
alert(privateVar);
}
myMod.someDynamicFunc();
In this attempt I tried making a setter in the module object... to no avail.
var MODULE = (function(){
var myMod = {},
privateVar = "I'm private";
myMod.setDynamicFunction = function ( func ){
if(func !== undefined && typeof func === "function"){
//Uncaught TypeError:
// Cannot read property 'dynamicFunction' of undefined
myMod.prototype.dynamicFunction = func;
//also tried myMod.dynamicFunction = func;
}
}
return myMod;
})();
var myModule = makeObject( MODULE );
myModule.setDynamicFunction(function(){
alert(privateVar);
});
myModule.dynamicFunction();
Am I just using JavaScript wrong? I'd really like to be able to assign callbacks after the object is initiated. Is this possible?

You can't access the private variable via a callback function set dynamically (since it can't be a closure if it's attached later), but you can set up a system by which you would be able to access the variable:
var MODULE = (function(){
var myMod = {},
privateVar = "I'm private";
myMod.callback = function(fn) {fn(privateVar);};
return myMod;
})();
var someDynamicFunc = function(param) {alert(param);};
myMod.callback(someDynamicFunc);
Of course, this makes it not really private, since anyone could do this. I don't see how it would be possible at all for you to have a "private" variable that you access via dynamically attached functions, without allowing anyone else's dynamically attached functions to have the same privilege (thus making it not really private).

I guess you did not really understand exactly how closures work.
Closures mean that scopes always have access to the outer scope they were defined in.
function Counter(start) {
var count = start;
return {
increment: function() { // has access to the outer scope
count++;
},
get: function() {
return count;
}
}
}
var foo = new Counter(4);
foo.increment();
foo.get(); // 5
The above example returns two closures, both the function increment as well as get keep a reference to the count variable defined in the constructor.
One cannot access count from the outside, the only way to interact with it is via the two "closured" functions.
Remember, closures work by keeping a reference to their outer scopes, so the following does not work:
var foo = new Counter(4);
foo.hack = function() { // is not getting defined in the same scope that the original count was
count = 1337;
};
This will not change the variable count that's inside of Counter since foo.hack was not defined in that scope, instead, it will create or override the global variable count.