Develop Reference

Sandboxing a function to make it safe? [duplicate]

Suppose I have a variables in the global scope.
Suppose I wish to define a function which I can guarantee will not have access to this variable, is there a way to wrap the function, or call the function, that will ensure this?
In fact, I need any prescribed function to have well defined access to variables, and that access to be defined prior to, and separate from that function definition.
Motivation:
I'm considering the possibility of user submitted functions. I should be able to trust that the function is some variety of "safe" and therefore be happy publishing them on my own site.

Run the code in an iframe hosted on a different Origin. This is the only way to guarantee that untrusted code is sandboxed and prevented from accessing globals or your page's DOM.

Using embedded Web Workers could allow to run safe functions. Something like this allows a user to enter javascript, run it and get the result without having access to your global context.
globalVariable = "I'm global";
document.getElementById('submit').onclick = function() {
createWorker();
}
function createWorker() {
// The text in the textarea is the function you want to run
var fnText = document.getElementById('fnText').value;
// You wrap the function to add a postMessage
// with the function result
var workerTemplate = "\
function userDefined(){" + fnText +
"}\
postMessage(userDefined());\
onmessage = function(e){console.log(e);\
}"
// web workers are normally js files, but using blobs
// you can create them with strings.
var blob = new Blob([workerTemplate], {
type: "text/javascript"
});
var wk = new Worker(window.URL.createObjectURL(blob));
wk.onmessage = function(e) {
// you listen for the return.
console.log('Function result:', e.data);
}
}
<div>Enter a javascript function and click submit</div>
<textarea id="fnText"></textarea>
<button id="submit">
Run the function
</button>
You can try these for example by pasting it in the textarea:
return "I'm a safe function";
You can see that it's safe:
return globalVariable;
You can even have more complex scripts, something like this:
var a = 4, b = 5;
function insideFn(){
// here c is global, but only in the worker context
c = a + b;
}
insideFn();
return c;
See info about webworkers here, especially embedded web workers:
https://developer.mozilla.org/en-US/docs/Web/API/Web_Workers_API/Using_web_workers#Embedded_workers

A little late, but maybe it will help you a bit
function RestrictFunction(params) {
params = ( params == undefined ? {} : params );
var scope = ( params.scope == undefined ? window : params.scope );
var data = ( params.data == undefined ? {} : params.data );
var script = ( params.script == undefined ? '' : params.script );
if (typeof params.script == 'function') {
script = params.script.toString();
script = script.substring(script.indexOf("{") + 1, script.lastIndexOf("}"));
}
// example: override native functions that on the white list
var setTimeout = function(_function,_interval) {
// this is important to prevent the user using `this` in the function and access the DOM
var interval = scope.setTimeout( function() {
RestrictFunction({
scope:scope,
data:data,
script:_function
});
} , _interval );
// Auto clear long user intervals
scope.setTimeout( function() {
scope.clearTimeout(interval);
} , 60*1000 );
return interval;
}
// example: create custom functions
var trace = function(str) {
scope.console.log(str);
}
return (function() {
// remove functions, objects and variables from scope
var queue = [];
var WhiteList = [
"Blob","Boolean","Date","String","Number","Object","Array","Text","Function",
"unescape","escape","encodeURI","encodeURIComponent","parseFloat","parseInt",
"isNaN","isFinite","undefined","NaN",
"JSON","Math","RegExp",
"clearTimeout","setTimeout"
];
var properties = Object.getOwnPropertyNames(scope);
for (var k = 0; k<properties.length; k++ ) {
if (WhiteList.indexOf(properties[k])!=-1) continue;
queue.push("var "+properties[k]+" = undefined;");
}
for (var k in scope) {
if (WhiteList.indexOf(k)!=-1) continue;
queue.push("var "+k+" = undefined;");
}
queue.push("var WhiteList = undefined;");
queue.push("var params = undefined;") ;
queue.push("var scope = undefined;") ;
queue.push("var data = undefined;") ;
queue.push("var k = undefined;");
queue.push("var properties = undefined;");
queue.push("var queue = undefined;");
queue.push("var script = undefined;");
queue.push(script);
try {
return eval( '(function(){'+ queue.join("\n") +'}).apply(data);' );
} catch(err) { }
}).apply(data);
}
Example of use
// dummy to test if we can access the DOM
var dummy = function() {
this.notify = function(msg) {
console.log( msg );
};
}
var result = RestrictFunction({
// Custom data to pass to the user script , Accessible via `this`
data:{
prop1: 'hello world',
prop2: ["hello","world"],
prop3: new dummy()
},
// User custom script as string or function
script:function() {
trace( this );
this.msg = "hello world";
this.prop3.notify(this.msg);
setTimeout( function() {
trace(this);
} , 10 );
trace( data );
trace( params );
trace( scope );
trace( window );
trace( XMLHttpRequest );
trace( eval );
return "done!"; // not required to return value...
},
});
console.log( "result:" , result );

You can't restrict the scope of a Function using the "call" or "apply" methods, but you can use a simple trick using "eval" and scoping to essentially hide any specific global variables from the function to be called.
The reason for this is because the function has access to the "global" variables that are declared at the scope that the function itself what declared. So, by copying the code for the method and injecting it in eval, you can essentially change the global scope of the function you are looking to call. The end result is essentially being able to somewhat sandbox a piece of javascript code.
Here's a full code example:
<html>
<head>
<title>This is the page title.</title>
<script>
function displayTitle()
{
alert(document.title);
}
function callMethod(method)
{
var code = "" +
// replace global "window" in the scope of the eval
"var window = {};" +
// replace global "document" in the scope of the eval
"var document = {}; " +
"(" +
// inject the Function you want to call into the eval
method.toString() +
// call the injected method
")();" +
"";
eval(code);
}
callMethod(displayTitle);
</script>
</head>
<body></body>
</html>
The code that gets eval'd looks like this:
var window = {};
var document = {};
(function displayTitle()
{
alert(document.title);
})();

You can use WebWorkers to isolate your code:
Create a completely separate and parallel execution environment (i.e. a separate thread or process or equivalent construct), and run the rest of these steps asynchronously in that context.
Here is a simple example:
someGlobal = 5;
//As a worker normally take another JavaScript file to execute we convert the function in an URL: http://stackoverflow.com/a/16799132/2576706
function getScriptPath(foo) {
return window.URL.createObjectURL(new Blob([foo], {
type: 'text/javascript'
}));
}
function protectCode(code) {
var worker = new Worker(getScriptPath(code));
}
protectCode('console.log(someGlobal)'); // prints 10
protectCode('console.log(this.someGlobal)');
protectCode('console.log(eval("someGlobal"))');
protectCode('console.log(window.someGlobal)');
This code will return:
Uncaught ReferenceError: someGlobal is not defined
undefined
Uncaught ReferenceError: someGlobal is not defined and
Uncaught ReferenceError: window is not defined
so you code is now safe.

EDIT: This answer does not hide the window.something variables. But it has a clean way to run user-defined code. I am trying to find a way to mask the window variables
You can use the javascript function Function.prototype.bind() to bind the user submitted function to a custom scope variable of your choosing, in this custom scope you can choose which variables to share with the user defined function, and which to hide. For the user defined functions, the code will be able to access the variables you shared using this.variableName. Here is an example to elaborate on the idea:
// A couple of global variable that we will use to test the idea
var sharedGlobal = "I am shared";
var notSharedGlobal = "But I will not be shared";
function submit() {
// Another two function scoped variables that we will also use to test
var sharedFuncScope = "I am in function scope and shared";
var notSharedFuncScope = "I am in function scope but I am not shared";
// The custom scope object, in here you can choose which variables to share with the custom function
var funcScope = {
sharedGlobal: sharedGlobal,
sharedFuncScope: sharedFuncScope
};
// Read the custom function body
var customFnText = document.getElementById("customfn").value;
// create a new function object using the Function constructor, and bind it to our custom-made scope object
var func = new Function(customFnText).bind(funcScope);
// execute the function, and print the output to the page.
document.getElementById("output").innerHTML = JSON.stringify(func());
}
// sample test function body, this will test which of the shared variables does the custom function has access to.
/*
return {
sharedGlobal : this.sharedGlobal || null,
sharedFuncScope : this.sharedFuncScope || null,
notSharedGlobal : this.notSharedGlobal || null,
notSharedFuncScope : this.notSharedFuncScope || null
};
*/
<script type="text/javascript" src="app.js"></script>
<h1>Add your custom body here</h1>
<textarea id="customfn"></textarea>
<br>
<button onclick="submit()">Submit</button>
<br>
<div id="output"></div>
The example does the following:
Accept a function body from the user
When the user clicks submit, the example creates a new function object from the custom body using the Function constructor. In the example we create a custom function with no parameters, but params can be added easily as the first input of the Function constructor
The function is executed, and its output is printed on the screen.
A sample function body is included in comments, that tests which of the variables does the custom function has access to.

Create a local variable with the same name.
If you have a global variable like this:
var globalvar;
In your function:
function noGlobal(); {
var globalvar;
}
If the function refers to globalvar, it will refers to the local one.

In my knowledge, in Javascript, any variable declared outside of a function belongs to the global scope, and is therefore accessible from anywhere in your code.
Each function has its own scope, and any variable declared within that function is only accessible from that function and any nested functions. Local scope in JavaScript is only created by functions, which is also called function scope.
Putting a function inside another function could be one possibility where you could achieve reduced scope ( ie nested scope)

if you are talking about a function that is exposed to you by loading a third party script, you are pretty much out of luck. that's because the scope for the function is defined in the source file it's defined in. sure, you can bind it to something else, but in most cases, that's going to make the function useless if it needs to call other functions or touch any data inside it's own source file - changing it's scope is only really feasible if you can predict what it needs to be able to access, and have access to that yourself - in the case of a third party script that touches data defined inside a closure, object or function that's not in your scope, you can't emulate what might need.
if you have access to the source file then it's pretty simple - look at the source file, see if it attempts to access the variable, and edit the code so it can't.
but assuming you have the function loaded, and it doesn't need to interact with anything other than "window", and for academic reasons you want to do this, here is one way - make a sandbox for it to play in. here's a simple shim wrapper that excludes certain items by name
function suspectCode() {
console.log (window.document.querySelector("#myspan").innerText);
console.log('verbotten_data:',typeof verbotten_data==='string'?verbotten_data:'<BANNED>');
console.log('secret_data:',typeof secret_data==='string'?secret_data:'<BANNED>'); // undefined === we can't
console.log('window.secret_data:',typeof window.secret_data==='string'?window.secret_data:'<BANNED>');
secret_data = 'i changed the secret data !';
console.log('secret_data:',typeof secret_data==='string'?secret_data:'<BANNED>'); // undefined === we can't
console.log('window.secret_data:',typeof window.secret_data==='string'?window.secret_data:'<BANNED>');
}
var verbotten_data = 'a special secret';
window.secret_data = 'special secret.data';
console.log("first call the function directly");
suspectCode() ;
console.log("now run it in a sandbox, which banns 'verbotten_data' and 'secret_data'");
runFunctionInSandbox (suspectCode,[
'verbotten_data','secret_data',
// we can't touch items tied to stack overflows' domain anyway so don't clone it
'sessionStorage','localStorage','caches',
// we don't want the suspect code to beable to run any other suspect code using this method.
'runFunctionInSandbox','runSanitizedFunctionInSandbox','executeSandboxedScript','shim',
]);
function shim(obj,forbidden) {
const valid=Object.keys(obj).filter(function(key){return forbidden.indexOf(key)<0;});
var shimmed = {};
valid.forEach(function(key){
try {
shimmed[key]=obj[key];
} catch(e){
console.log("skipping:",key);
}
});
return shimmed;
}
function fnSrc (fn){
const src = fn.toString();
return src.substring(src.indexOf('{')+1,src.lastIndexOf('}')-1);
}
function fnArgs (fn){
let src = fn.toString();
src = src.substr(src.indexOf('('));
src = src.substr(0,src.indexOf(')')-1);
src = src.substr(1,src.length-2);
return src.split(',');
}
function runSanitizedFunctionInSandbox(fn,forbidden) {
const playground = shim(window,forbidden);
playground.window = playground;
let sandboxed_code = fn.bind(playground,playground.window);
sandboxed_code();
}
function runFunctionInSandbox(fn,forbidden) {
const src = fnSrc(fn);
const args = fnArgs(fn);
executeSandboxedScript(src,args,forbidden);
}
function executeSandboxedScript(sourceCode,arg_names,forbidden) {
var script = document.createElement("script");
script.onload = script.onerror = function(){ this.remove(); };
script.src = "data:text/plain;base64," + btoa(
[
'runSanitizedFunctionInSandbox(function(',
arg_names,
['window'].concat(forbidden),
'){ ',
sourceCode,
'},'+JSON.stringify(forbidden)+')'
].join('\n')
);
document.body.appendChild(script);
}
<span id="myspan">Page Access IS OK<span>
or a slightly more involved version that allows arguments to be passed to the function
function suspectCode(argument1,argument2) {
console.log (window.document.querySelector("#myspan").innerText);
console.log(argument1,argument2);
console.log('verbotten_data:',typeof verbotten_data==='string'?verbotten_data:'<BANNED>');
console.log('secret_data:',typeof secret_data==='string'?secret_data:'<BANNED>'); // undefined === we can't
console.log('window.secret_data:',typeof window.secret_data==='string'?window.secret_data:'<BANNED>');
secret_data = 'i changed the secret data !';
console.log('secret_data:',typeof secret_data==='string'?secret_data:'<BANNED>'); // undefined === we can't
console.log('window.secret_data:',typeof window.secret_data==='string'?window.secret_data:'<BANNED>');
}
var verbotten_data = 'a special secret';
window.secret_data = 'special secret.data';
console.log("first call the function directly");
suspectCode('hello','world') ;
console.log("now run it in a sandbox, which banns 'verbotten_data' and 'secret_data'");
runFunctionInSandbox (suspectCode,['hello','sandboxed-world'],
[
'verbotten_data','secret_data',
// we can't touch items tied to stack overflows' domain anyway so don't clone it
'sessionStorage','localStorage','caches',
// we don't want the suspect code to beable to run any other suspect code using this method.
'runFunctionInSandbox','runSanitizedFunctionInSandbox','executeSandboxedScript','shim',
]);
function shim(obj,forbidden) {
const valid=Object.keys(obj).filter(function(key){return forbidden.indexOf(key)<0;});
var shimmed = {};
valid.forEach(function(key){
try {
shimmed[key]=obj[key];
} catch(e){
console.log("skipping:",key);
}
});
return shimmed;
}
function fnSrc (fn){
const src = fn.toString();
return src.substring(src.indexOf('{')+1,src.lastIndexOf('}')-1);
}
function fnArgs (fn){
let src = fn.toString();
src = src.substr(src.indexOf('('));
src = src.substr(0,src.indexOf(')'));
src = src.substr(1,src.length);
return src.split(',');
}
function runSanitizedFunctionInSandbox(fn,args,forbidden) {
const playground = shim(window,forbidden);
playground.window = playground;
let sandboxed_code = fn.bind(playground,playground.window);
sandboxed_code.apply(this,new Array(forbidden.length).concat(args));
}
function runFunctionInSandbox(fn,args,forbidden) {
const src = fnSrc(fn);
const arg_names = fnArgs(fn);
executeSandboxedScript(src,args,arg_names,forbidden);
}
function executeSandboxedScript(sourceCode,args,arg_names,forbidden) {
var script = document.createElement("script");
script.onload = script.onerror = function(){ this.remove(); };
let id = "exec"+Math.floor(Math.random()*Number.MAX_SAFE_INTEGER).toString();
window.execArgs=window.execArgs||{};
window.execArgs[id]=args;
let script_src = [
'runSanitizedFunctionInSandbox(function(',
['window'].concat(forbidden),
(arg_names.length===0?'':','+arg_names.join(","))+'){',
sourceCode,
'},',
'window.execArgs["'+id+'"],',
JSON.stringify(forbidden)+');',
'delete window.execArgs["'+id+'"];'
].join('\n');
let script_b64 = btoa(script_src);
script.src = "data:text/plain;base64," +script_b64;
document.body.appendChild(script);
}
<span id="myspan">hello computer...</span>

I verified #josh3736's answer but he didn't leave an example
Here's one to verify it works
parent.html
<h1>parent</h1>
<script>
abc = 'parent';
function foo() {
console.log('parent foo: abc = ', abc);
}
</script>
<iframe></iframe>
<script>
const iframe = document.querySelector('iframe');
iframe.addEventListener('load', function() {
console.log('-calling from parent-');
iframe.contentWindow.foo();
});
iframe.src = 'child.html';
</script>
child.html
<h1>
child
</h1>
<script>
abc = 'child';
function foo() {
console.log('child foo: abc = ', abc);
}
console.log('-calling from child-');
parent.foo();
</script>
When run it prints
-calling from child-
parent foo: abc = parent
-calling from parent-
child foo: abc = child
Both child and parent have a variable abc and a function foo.
When the child calls into the parent's foo that function in the parent sees the parent's global variables and when the parent calls the child's foo that function sees the child's global variables.
This also works for eval.
parent.html
<h1>parent</h1>
<iframe></iframe>
<script>
const iframe = document.querySelector('iframe');
iframe.addEventListener('load', function() {
console.log('-call from parent-');
const fn = iframe.contentWindow.makeFn(`(
function() {
return abc;
}
)`);
console.log('from fn:', fn());
});
iframe.src = 'child.html';
</script>
child.html
<h1>
child
</h1>
<script>
abc = 'child';
function makeFn(s) {
return eval(s);
}
</script>
When run it prints
-call from parent-
from fn: child
showing that it saw the child's abc variable not the parent's
note: if you create iframes programmatically they seem to have to be added to the DOM or else they won't load. So for example
function loadIFrame(src) {
return new Promise((resolve) => {
const iframe = document.createElement('iframe');
iframe.addEventListener('load', resolve);
iframe.src = src;
iframe.style.display = 'none';
document.body.appendChild(iframe); // iframes don't load if not in the document?!?!
});
}
Of course in the child above we saw that the child can reach into the parent so this code is NOT SANDBOXED. You'd probably have to add some stuff to hide the various ways to access the parent if you want make sure the child can't get back but at least as a start you can apparently use this technique to give code a different global scope.
Also note that of course the iframes must be in the same domain as the parent.

Here's another answer. This one's based on how chained scopes work in javascript. It also uses Function(), which produces faster code than eval.
/** This takes a string 'expr', e.g. 'Math.max(x,1)' and returns a function (x,y)=>Math.max(x,1).
* It protects against malicious input strings by making it so that, for the function,
* (1) no names are in scope other than the parameters 'x' and 'y' and a whitelist
* of other names like 'Math' and 'Array'; (2) also 'this' binds to the empty object {}.
* I don't think there's any way for malicious strings to have any effect.
* Warning: if you add something into the global scope after calling make_fn but
* before executing the function you get back, it won't protect that new thing.
*/
function make_fn(expr) {
const whitelist = ['Math', 'Array']; // Warning: not 'Function'
let scope = {};
for (let obj = this; obj; obj = Object.getPrototypeOf(obj)) {
Object.getOwnPropertyNames(obj).forEach(name => scope[name] = undefined);
}
whitelist.forEach(name => scope[name] = this[name]);
const fn = Function("scope","x","y","with (scope) return "+expr).bind({});
return (x,y) => fn(scope,x,y);
}
This is how it behaves: https://jsfiddle.net/rkq5otme/
make_fn("x+y")(3,5) ==> 8
make_fn("Math.max(x,y)")(3,5) ==> 5
make_fn("this")(3,5) ==> {}
make_fn("alert('oops')") ==> TypeError: alert is not a function
make_fn("trace((function(){return this}()))")(3,5) ==> ReferenceError: trace is not defined
Explanation. Consider a simpler version
Function("x", "return Math.max(x, window, this);")
This creates a function with the specified body which has two chained scopes: (1) the function scope which binds x, (2) the global scope. Let's spell out how the symbols Math, x, window and this are all resolved.
x is bound to the property in the function scope
window is bound to the property in the next chained scope, global, i.e. window['window'].
We don't want this! To prevent it, we create our own scope object scope = {window:undefined,...} and write with (scope) return Math.max(x,window,this). The with statement is only allowed in non-strict code. It adds an additional scope, so the chain is now: (0) the specified scope object we created, (1) the function scope which binds x, (2) the global scope. Because name 'window' is found in our scope object, it can never bind to the one in global scope.
Math will suffer the same fate.
We want to whitelist it! So we make our scope object {Math:global.Math, window:undefined, ...}
this is bound upon invocation of the function to the global object, window.
We don't want this! To prevent it, we call .bind({}) on the function, which wraps the function in a wrapper which sets this={}. Alas bind(null) didn't seem to bind.
Note: there's another possible construction of the scope object, using Proxy. It doesn't seem particularly better.
const scope = new Proxy({}, {
has: (obj, key) => !['x','y'].includes(key),
get: (obj, key) => (whitelist.includes(key)) ? this[key] : undefined,
set: (obj, key, value) => {},
deleteProperty: (obj, key) => {},
enumerate: (obj, key) => [],
ownKeys: (obj, key) => [],
defineProperty: (obj, key, desc) => {},
getOwnPropertyDescriptor: (obj, key) => undefined,
});

Is it possible to encapsulate P5 into a global variable like JQuery does with $

I was wondering if it's possible to remove the entire P5(or whatever other frame work/lib) from the global variable
and put it inside one single global variable without actually edit the P5.JS project it's self.
Just like JQuery does with the $ symbol. So you can actually declare variables named point, or mouseX that are P5 variable and will conflict with your variable.
So for example
window.P5 = (some way to get the all instance form the P5);
let point = {myPont:..., props....};
let canvasPoint = P5.point(...);

Sure. Use instance mode.
var s = function( sketch ) {
var x = 100;
var y = 100;
sketch.setup = function() {
sketch.createCanvas(200, 200);
};
sketch.draw = function() {
sketch.background(0);
sketch.fill(255);
sketch.rect(x,y,50,50);
};
};
var myp5 = new p5(s);

What advantages are there in wrapping data in a function call besides data hiding?

I'm reading "Javascript: The Definitive Guide", 6E, and I came across this example:
var cookies = (function() {
var cookies = {};
var all = document.cookie;
if ( all === "" )
return cookies;
var list = all.split("; ");
for ( var i = 0; i < list.length; i++ )
{
var cookie = list[ i ];
var p = cookie.indexOf( "=" );
var name = cookie.substring( 0, p );
var value = cookie.substring( p + 1 );
value = decodeURIComponent( value );
cookies[ name ] = value;
}
return cookies;
}());
I can see what he did here; he created a function and called it immediately. I just can't see why he might do that in this case. I've seen this idiom used in jQuery before to hide the "$" operator, but he's not hiding anything here; the only variable he creates is "cookies", and that's the var he's populating. I can't figure out how this is any different than:
var cookies = {};
var all = document.cookie;
if ( all !== "" )
{
var list = all.split("; ");
for ( var i = 0; i < list.length; i++ )
{
var cookie = list[ i ];
var p = cookie.indexOf( "=" );
var name = cookie.substring( 0, p );
var value = cookie.substring( p + 1 );
value = decodeURIComponent( value );
cookies[ name ] = value;
}
}
Other than the introduction of "all" in the global scope? Is there some deeper corner case he's sidestepping with this particular example that I'm just not aware of?

Javascript only has two scopes: Global and Function. Unlike many languages, it has no block scope.
Therefore, your alternative code puts all, list, i, cookie, p, name, and value into whatever scope cookies is being defined in.
Now, if your snippet is inside some function definition and cookies is being returned, that might not be so bad. But if it's inside a top level script, then you're looking at a lot of very common variable names that you're dumping into the global namespace.
So, immediate functions are useful when you need the scoping limits of a function, but don't really want to have a Function object kicking around afterwards.
Edit
To expand on other advantages.
Another common use of immediate functions is for doing on time set up, often when browser sniffing is involved:
var foo = (function(browser) {
if(isBar(browser) {
return function() {
/* Some implementation of foo that is compatible with bar */
};
} else if(isBaz(browser) {
return function() {
/* Some implementation of foo that is compatible with baz */
};
} else {
return function() {
/* Some generic implementation of foo */
};
}
}(browser_reference));
In one shot, you've defined a browser-compatible version of foo without cluttering up your scope with variables needed to determine what the browser is.

Javascript doesn't have a concept of namespaces. But it does have a concept of scope. The example code is a way of encapsulating code and emulating namespaces.
Basically, it keeps your code from accidentally overwriting someone else's code (and vice-versa).

How dangerous is modifying a function directly?

To get around what has proven to be a scope limitation to me (as answered here), I've written a piece of code which inserts a line in an anonymous function so that whoever writes the function doesn't have to do it themselves. It's a bit hacky (actually, it feels quite a lot hacky), and I really don't know what I'm doing, so I'd appreciate an expert eye to catch any errors I may have missed or point out any dangers I'm unaware of. Here's the code:
function myObj(testFunc) {
this.testFunc = testFunc;
this.Foo = function Foo(test) {
this.test = test;
this.saySomething = function(text) {
alert(text);
};
};
var Foo = this.Foo;
var funcSep = this.testFunc.toString().split("{");
funcSep.splice(0, 1);
funcSep = funcSep.join("{");
var compFunc = " var Foo = this.Foo;" + funcSep;
compFunc = compFunc.split("}");
compFunc.splice(compFunc.length - 1, 1);
compFunc.join("}");
var otherTestFunc = new Function(compFunc);
otherTestFunc.apply(this);
}
var test = new myObj(function() {
var test = new Foo();
test.saySomething("Hello world");
});
The function above evaluates as expected, and I don't need to force whoever writes the anonymous function to obtain access to Foo by using this.Foo. This approach feels iffy, though. Is what I'm doing acceptable, and if not, are there any ways to circumvent it?
Also, the only reason I didn't include this in my original question is that seems like something of a departure from the original context of the question.

You're trying to break the language. Don't do that. It's not Java.
Developers have certain expectations on the behaviour and scope of variables, and your approach would rather confuse them. Think about the following:
var Foo = SomeWonderfulClass;
var test = new myObj(function() {
var test = new Foo();
// ...
});
Now the developer wants to instantiate SomeWonderfulClass, but your magic messes around with that.
On the other hand, this would work fine, even with your trickery:
var test = new myObj(function() {
var Foo = SomeWonderfulClass;
var test = new Foo();
// ...
});
But the bigger problem is that the actual scope is lost:
var Bananas = SomeWonderfulClass;
var test = new myObj(function() {
var test = new Bananas(); // Error: Bananas is undefined!
});
Nobody expects such shenanigans.
That being said, there's some things about your code to be improved:
this.Foo is initialized with every new object. That's not necessary. Better use
myObj.prototype.Foo = function () {...}
The line var Foo = this.Foo; is not needed in myObj.
Your string magic is overly complex. How about
var otherTestFunc = new Function(testFunc.toString()
.replace(/^[^{]+{/, '{var Foo=this.Foo;'));
No need to remove the braces.
(testFunc does not accept any arguments, but I guess you know that.)
So that boils down to
function myObj(testFunc) {
this.testFunc = testFunc;
var otherTestFunc = new Function(testFunc.toString()
.replace(/^[^{]+{/, '{var Foo=this.Foo;'));
otherTestFunc.apply(this);
}
myObj.prototype.Foo = function Foo(test) {
this.test = test;
this.saySomething = function(text) {
alert(text);
};
};

I've been bothered by this method since I saw it in asp.net validation code (!). Not really safe for arbitrary function:
var f = (function () {
var closure = 1
return function (argument) {
alert(argument)
alert(closure)
}
})()
var f2 = new Function(f.toString().replace(/^function.*?{([\s\S]*)}$/, 'alert(1);$1'))
f2(1) // :(
Arguments could be saved though.

Create a new unique global variable each time a function is run in javascript [duplicate]

This question already has answers here:
Closed 11 years ago.
Possible Duplicate:
Javascript dynamic variable name
A very basic question. I want to create a new javascript global variable each time a function is called. The variable should contain the id of the element so that I can easily access it later.
id = 2347
//this function would be called multiple times, hopefully generating a new global each time
function (id)
{
var + id = something
// I want a variable that would be named var2347 that equals something, but the above line doesn't get it.
}
In a later function, I want to access the variable like so:
function two (id)
{
alert(var + id);
}
I'm sure I'm going to have a "doh!" moment when someone is kind enough to answer this.

How about...
var store = (function() {
var map = {};
return {
set: function ( name, value ) {
map[ name ] = value;
},
get: function ( name ) {
return map[ name ];
}
};
})();
Usage:
store.set( 123, 'some value' );
and then...
store.get( 123 ) // 'some value'
store.get( 456 ) // undefined
Live demo: http://jsfiddle.net/jZfft/
Programmers are highly advised to not declare global variables, since the browsers already ship with several hundreds of names in the global namespace. Using the global namespace for your own variables can lead to name-collisions which can then break the program or some of the browser's functionality. Creating new namespaces is free, so don't be shy to do it...

Global variables are properties of the window object, so window.lol and window['lol'] define a global variable lol which can be accessed in any of these ways.
The second, window['lol'], can also be used with variable names, like this:
var lol = 123;
var name = 'lol';
var content = window[name]; // window['lol'] == 123
content will now contain 123.
Pretty much anything can be put between square brackets [], so you can also do this:
var id = 123;
window['prefix' + id] = 'text';
var content = window['prefix' + id]; // prefix123 == text
Or, in your case:
var id = 2347;
function one(id) {
window['var' + id] = something;
}
function two(id) {
alert(window['var' + id]);
}

You can save your values to the global hash:
var g = {};
function (id)
{
g[id] = something;
}
function two (id)
{
alert(g[id]);
}

I would argue that you don't really want to be making lots of global variables. Rather, you can just make one global object or array and attach all your other variables to that. In this case, you probably want an object:
var myIds = {};
function makeSomething(id) {
// create something that goes with this id
myIds[id] = something;
}
Then, to fetch that information at some time later, you can retrieve it with this:
var something = myIds[id];
The reason for this suggestion is many-fold. First off, you want to minimize the number of global variables because every global is a chance for a naming collision with some other script you might be using. Second off, when keeping track of a bunch of related data, it's a better programming practice to keep it in one specific data structure rather than just throw it all in the giant global bin with all other data.
It's even possible to create an object that manages all this for you:
function idFactory() {
this.ids = {};
}
idFactory.prototype = {
makeSomething: function(id) {
// create something that goes with this id
this.ids[id] = something;
},
retrieveSomething: function(id) {
return(this.ids[id]);
},
clear: function() {
this.ids = {};
}
};
// then you would use it like this:
var myIds = new idFactory();
myIds.makeSomething(2347);
var value = myIds.retrieveSomething(2347);