Getting All Variables In Scope

Question

Is there a way to get all variables that are currently in scope in javascript?

Answer 1

Although everyone answer "No" and I know that "No" is the right answer but if you really need to get local variables of a function there is a restricted way.

Consider this function:

var f = function() {
    var x = 0;
    console.log(x);
};

You can convert your function to a string:

var s = f + '';

You will get source of function as a string

'function () {\nvar x = 0;\nconsole.log(x);\n}'

Now you can use a parser like esprima to parse function code and find local variable declarations.

var s = 'function () {\nvar x = 0;\nconsole.log(x);\n}';
s = s.slice(12); // to remove "function () "
var esprima = require('esprima');
var result = esprima.parse(s);

and find objects with:

obj.type == "VariableDeclaration"

in the result (I have removed console.log(x) below):

{
    "type": "Program",
    "body": [
        {
            "type": "VariableDeclaration",
            "declarations": [
                {
                    "type": "VariableDeclarator",
                    "id": {
                        "type": "Identifier",
                        "name": "x"
                    },
                    "init": {
                        "type": "Literal",
                        "value": 0,
                        "raw": "0"
                    }
                }
            ],
            "kind": "var"
        }
    ]
}

I have tested this in Chrome, Firefox and Node.

But the problem with this method is that you just have the variables defined in the function itself. For example for this one:

var g = function() {
    var y = 0;
    var f = function() {
        var x = 0;
        console.log(x);
    };
}

you just have access to the x and not y. But still you can use chains of caller (arguments.callee.caller.caller.caller) in a loop to find local variables of caller functions. If you have all local variable names so you have scope variables. With the variable names you have access to values with a simple eval.

Answer 2

No. "In scope" variables are determined by the "scope chain", which is not accessible programmatically.

For detail (quite a lot of it), check out the ECMAScript (JavaScript) specification. Here's a link to the official page where you can download the canonical spec (a PDF), and here's one to the official, linkable HTML version.

Update based on your comment to Camsoft

The variables in scope for your event function are determined by where you define your event function, not how they call it. But, you may find useful information about what's available to your function via this and arguments by doing something along the lines of what KennyTM pointed out (for (var propName in ____)) since that will tell you what's available on various objects provided to you (this and arguments; if you're not sure what arguments they give you, you can find out via the arguments variable that's implicitly defined for every function).

So in addition to whatever's in-scope because of where you define your function, you can find out what else is available by other means by doing:

var n, arg, name;
alert("typeof this = " + typeof this);
for (name in this) {
    alert("this[" + name + "]=" + this[name]);
}
for (n = 0; n < arguments.length; ++n) {
    arg = arguments[n];
    alert("typeof arguments[" + n + "] = " + typeof arg);
    for (name in arg) {
        alert("arguments[" + n + "][" + name + "]=" + arg[name]);
    }
}

(You can expand on that to get more useful information.)

Instead of that, though, I'd probably use a debugger like Chrome's dev tools (even if you don't normally use Chrome for development) or Firebug (even if you don't normally use Firefox for development), or Dragonfly on Opera, or "F12 Developer Tools" on IE. And read through whatever JavaScript files they provide you. And beat them over the head for proper docs. :-)

Answer 3

In ECMAScript 6 it's more or less possible by wrapping the code inside a with statement with a proxy object. Note it requires non-strict mode and it's bad practice.

function storeVars(target) {
  return new Proxy(target, {
    has(target, prop) { return true; },
    get(target, prop) { return (prop in target ? target : window)[prop]; }
  });
}
var vars = {}; // Outer variable, not stored.
with(storeVars(vars)) {
  var a = 1;   // Stored in vars
  var b = 2;   // Stored in vars
  (function() {
    var c = 3; // Inner variable, not stored.
  })();
}
console.log(vars);

The proxy claims to own all identifiers referenced inside with, so variable assignments are stored in the target. For lookups, the proxy retrieves the value from the proxy target or the global object (not the parent scope). let and const variables are not included.

Inspired by this answer by Bergi.

Answer 4

Yes and no. "No" in almost every situation. "Yes," but only in a limited manner, if you want to check the global scope. Take the following example:

var a = 1, b = 2, c = 3;

for ( var i in window ) {
    console.log(i, typeof window[i], window[i]);
}

Which outputs, amongst 150+ other things, the following:

getInterface function getInterface()
i string i // <- there it is!
c number 3
b number 2
a number 1 // <- and another
_firebug object Object firebug=1.4.5 element=div#_firebugConsole
"Firebug command line does not support '$0'"
"Firebug command line does not support '$1'"
_FirebugCommandLine object Object
hasDuplicate boolean false

So it is possible to list some variables in the current scope, but it is not reliable, succinct, efficient, or easily accessible.

A better question is why do you want to know what variables are in scope?

Answer 5

How much time do you have?

If you ḧ̸̬̦́̒͑̌͝ͅa̵͍͖͛t̴̪̟̬̖̾́̋͗̏ͅe̸̳̊̍̈́͌͝ your cpu you can bruteforce through every valid variable name, and eval each one to see if it results in a value!

The following snippet tries the first 1000 bruteforce strings, which is enough to find the contrived variable names in scope:

let alpha = 'abcdefghijklmnopqrstuvwxyz';
let everyPossibleString = function*() {
  yield '';
  for (let prefix of everyPossibleString()) for (let char of alpha) yield `${prefix}${char}`;
};
let allVarsInScope = (iterations=1000) => {  
  let results = {};
  let count = 0;
  for (let bruteforceString of everyPossibleString()) {
    if (!bruteforceString) continue; // Skip the first empty string
    try { results[bruteforceString] = eval(bruteforceString); } catch(err) {}
    if (count++ > iterations) break;
  }
  return results;
};

let myScope = (() => {
    
  let dd = 'ddd';
  let ee = 'eee';
  let ff = 'fff';
  
  ((gg, hh) => {
    
    // We can't call a separate function, since that function would be outside our
    // scope and wouldn't be able to see any variables - but we can define the
    // function in place (using `eval(allVarsInScope.toString())`), and then call
    // that defined-in-place function
    console.log(eval(allVarsInScope.toString())());
    
  })('ggg', 'hhh');
  
})();

This script will eventually (after a very long time) find all scoped variable names, as well as abc nifty and swell, some example variables I created. Note it will only find variable names consisting of alpha characters.

let preElem = document.getElementsByClassName('display')[0];
let statusElem = document.getElementsByClassName('status')[0];
let alpha = 'abcdefghijklmnopqrstuvwxyz';
alpha += alpha.toUpperCase();
let everyPossibleString = function*() {
  yield '';
  for (let prefix of everyPossibleString()) for (let char of alpha) yield `${prefix}${char}`;
};

(async () => {
  
  let abc = 'This is the ABC variable :-|';
  let neato = 'This is the NEATO variable :-)';
  let swell = 'This is the SWELL variable :-D';
  
  let ignoreRegex = /^(?:t(?:his|op)|self|window)$/;
  let results = {};
  let batch = 100;
  let waitMs = 25;
  let count = 0;
  let startStr = null;
  let t = null;
  
  for (let bruteStr of everyPossibleString()) {
    
    // Some property accesses cause problems within stackoverflow's iframe snippet
    // system - we'll bypass these problematic properties
    if (ignoreRegex.test(bruteStr)) continue;
    
    if (count === 0) {
      t = Date.now();
      startStr = bruteStr;
    }
    
    try { results[bruteStr] = eval(bruteStr); } catch(err) {}
    
    if (count++ >= batch) {
      
      count = 0;
      
      // Update the html
      statusElem.innerHTML = `Did batch of ${batch} from ${startStr} -> ${bruteStr} in ${waitMs.toFixed(0)}ms`;
      preElem.innerHTML = JSON.stringify(results, null, 2);
      
      // Optimize `batch` based on how well the CPU is holding up
      let dur = (Date.now() - t) + 2; // +2ms for breathing room
      let estimatedMaxBatch = batch * (waitMs / dur);
      batch = Math.round(batch * 0.8 + estimatedMaxBatch * 0.2);
      
      // 
      await new Promise(r => setTimeout(r, waitMs));
      
    }
            
  }
  
  console.log('Done...'); // Will literally never happen
  
})();

html, body { position: fixed; left: 0; top: 0; right: 0; bottom: 0; margin: 0; padding: 0; overflow: hidden }
.display {
  position: fixed;
  box-sizing: border-box;
  left: 0; top: 0;
  bottom: 30px; right: 0;
  overflow-y: scroll;
  white-space: pre;
  font-family: monospace;
  padding: 10px;
  box-shadow: inset 0 0 10px 1px rgba(0, 0, 0, 0.3);
}
.status {
  position: fixed;
  box-sizing: border-box;
  left: 0; bottom: 0px; right: 0; height: 30px; line-height: 30px;
  padding: 0 10px;
  background-color: rgba(0, 0, 0, 1);
  color: rgba(255, 255, 255, 1);
  font-family: monospace;
}

<div class="display"></div>
<div class="status"></div>

I am all too aware there is virtually no situation where this is practical

Answer 6

You can't.

Variables, identifiers of function declarations and arguments for function code, are bound as properties of the Variable Object, which is not accesible.

See also:

• Scope Chain and Identifier Resolution

Answer 7

I made a fiddle implementing (essentially) above ideas outlined by iman. Here is how it looks when you mouse over the second ipsum in return ipsum*ipsum - ...

The variables which are in scope are highlighted where they are declared (with different colors for different scopes). The lorem with red border is a shadowed variable (not in scope, but be in scope if the other lorem further down the tree wouldn't be there.)

I'm using esprima library to parse the JavaScript, and estraverse, escodegen, escope (utility libraries on top of esprima.) The 'heavy lifting' is done all by those libraries (the most complex being esprima itself, of course.)

How it works

ast = esprima.parse(sourceString, {range: true, sourceType: 'script'});

makes the abstract syntax tree. Then,

analysis = escope.analyze(ast);

generates a complex data structure encapsulating information about all the scopes in the program. The rest is gathering together the information encoded in that analysis object (and the abstract syntax tree itself), and making an interactive coloring scheme out of it.

So the correct answer is actually not "no", but "yes, but". The "but" being a big one: you basically have to rewrite significant parts of the chrome browser (and it's devtools) in JavaScript. JavaScript is a Turing complete language, so of course that is possible, in principle. What is impossible is doing the whole thing without using the entirety of your source code (as a string) and then doing highly complex stuff with that.

Answer 8

The Simplest Way to Get Access to Vars in a Particular Scope

1. Open Developer Tools > Resources (in Chrome)
2. Open file with a function that has access to that scope (tip cmd/ctrl+p to find file)
3. Set breakpoint inside that function and run your code
4. When it stops at your breakpoint, you can access the scope var through console (or scope var window)

Note: You want to do this against un-minified js.

The Simplest Way to Show All Non-Private Vars

1. Open Console (in Chrome)
2. Type: this.window
3. Hit Enter

Now you will see an object tree you can expand with all declared objects.

Answer 9

As everyone noticed: you can't. But you can create a obj and assign every var you declare to that obj. That way you can easily check out your vars:

var v = {}; //put everything here

var f = function(a, b){//do something
}; v.f = f; //make's easy to debug
var a = [1,2,3];
v.a = a;
var x = 'x';
v.x = x;  //so on...

console.log(v); //it's all there

Answer 10

If you just want to inspect the variables manually to help debug, just fire up the debugger:

debugger;

Straight into the browser console.

Answer 11

You can see scopes and their variables in [[Scopes]], even closure scopes using console.dir().

Example 1:

counterWithClosure = (function () {
    let internalVar = 0
    return function () {
        return ++internalVar
    }
})()

counterWithClosure() // 1
counterWithClosure() // 2
counterWithClosure() // 3

console.dir(counterWithClosure)

It shows you the variables in "[[Scopes]] > Closure", "[[Scopes]] > Global" and even "[[Scopes]] > Script" if the page have scripts reachable.

Image with output:

Even with nested closures you can see the nested scopes.

Example 2:

adderWithNestedClosures = (function () {
    let varLevel1 = 1
    return function (param1) {
        let varLevel2 = 10
        return function (param2) {
            let varLevel3 = 100
            return function (param3) {
                let varLevel4 = 1000
                return function (param4) {
                    ++varLevel1
                    ++varLevel2
                    ++varLevel3
                    ++varLevel4
                    return {
                        paramsSum: param1 + param2 + param3 + param4,
                        varsSum: varLevel1 + varLevel2 + varLevel3 + varLevel4
                    }
                }
            }
        }
    }
})()

adderWith123 = adderWithNestedClosures(1)(2)(3) // Preparing function with nested scopes
adderWith123(4) // {paramsSum:10,varsSum:1115}
adderWith123(4) // {paramsSum:10,varsSum:1119}
adderWith123(5) // {paramsSum:11,varsSum:1123}
console.dir(adderWith123) 

It shows all nested scopes in [[Scopes]] Image with output:

It works in Chromium-based browsers.

Answer 12

Writing an external function to do this would be impossible, and I'll demonstrate that with a proof by contradiction.

Let's assume we've written our magic function called getWhatsInScope. Now, let's say we want to call it from our function myFunction and see if it picks up our function variable myVariable. This is a contradiction because no matter how we write getWhatsInScope, it can't see myVariable.

function getWhatsInScope () {
  console.log(myVariable) // out of scope
}

function myFunction() {
  const myVariable = 42
  getWhatsInScope()
}