ECMA-262-3 in detail. Chapter 1. Execution Contexts.

Read this article in: Russian, Chinese (version 1, version 2), Arabic, Japaneses, Korean.

In this note we will mention execution contexts of ECMAScript and types of executable code related with them.

Every time when control is transferred to ECMAScript executable code, control is entered an execution context.

Execution context (abbreviated form — EC) is the abstract concept used by ECMA-262 specification for typification and differentiation of an executable code.

The standard does not define accurate structure and kind of EC from the technical implementation viewpoint; it is a question of the ECMAScript-engines implementing the standard.

Logically, set of active execution contexts forms a stack. The bottom of this stack is always a global context, the top — a current (active) execution context. The stack is modified (pushed/popped) during the entering and exiting various kinds of EC.

With abstract concept of an execution context, the concept of type of an executable code is related. Speaking about code type, it is possible in the certain moments to mean an execution context.

For examples, we define the stack of execution contexts as an array:

ECStack = [];

The stack is pushed every time on entering a function (even if the function is called recursively or as the constructor), and also at built-in eval function work.

This type of code is processed at level Program: i.e. the loaded external .js-file or the local inline-code (inside the <script></script> tags). The global code does not include any parts of a code which are in bodies of functions.

At initialization (program start), ECStack looks like:

ECStack = [
  globalContext
];

On entering the function code (all kinds of functions), ECStack is pushed with new elements. It is necessary to notice that the code of concrete function does not include codes of the inner functions.

For example, let’s take the function which calls itself recursively once:

(function foo(flag) {
  if (flag) {
    return;
  }
  foo(true);
})(false);

Then, ECStack is modified as follows:

// first activation of foo
ECStack = [
  <foo> functionContext
  globalContext
];
 
// recursive activation of foo
ECStack = [
  <foo> functionContext – recursively 
  <foo> functionContext
  globalContext
];

Every return from a function exits the current execution context and ECStack popped accordingly — consecutively and upside-down — quite natural implementation of a stack. After the work of this code is finished, ECStack again contains only globalContext — until the program end.

A thrown but not caught exception may also exit one or more execution contexts:

(function foo() {
  (function bar() {
    throw 'Exit from bar and foo contexts';
  })();
})();

Things are more interesting with eval code. In this case, there is a concept of a calling context, i.e. a context from which eval function is called.

The actions made by eval, such as variable or function definition, influence exactly the calling context:

// influence global context
eval('var x = 10');

(function foo() {
  // and here, variable "y" is
  // created in the local context
  // of "foo" function
  eval('var y = 20');
})();
 
alert(x); // 10
alert(y); // "y" is not defined

Note, in the strict-mode of ES5, eval already does not influence the calling context, but instead evaluates the code in the local sandbox.

For the example above we have the following ECStack modifications:

ECStack = [
  globalContext
];
 
// eval('var x = 10');
ECStack.push({
  context: evalContext,
  callingContext: globalContext
});

// eval exited context
ECStack.pop();

// foo funciton call
ECStack.push(<foo> functionContext);

// eval('var y = 20');
ECStack.push({
  context: evalContext,
  callingContext: <foo> functionContext
});

// return from eval 
ECStack.pop();

// return from foo
ECStack.pop();

I.e. quite casual and logical call-stack.

In old SpiderMonkey implementations (Firefox), up to version 1.7, it was possible to pass a calling context as a second argument for eval function. Thus, if the context still exists, it is possible to influence private variables:

function foo() {
  var x = 1;
  return function () { alert(x); };
};

var bar = foo();

bar(); // 1

eval('x = 2', bar); // pass context, influence internal var "x"

bar(); // 2

However, due to security reasons in modern engines it was fixed and is not significant anymore.

This theoretical minimum is required for the further analysis of details related with execution contexts, such as variable object or scope chain, which descriptions can be found in the appropriate chapters.

Corresponding section of ECMA-262-3 specification — 10. Execution Contexts.


Translated by: Dmitry Soshnikov.
Published on: 2010-03-11

Originally written by: Dmitry Soshnikov [ru, read »]
Originally published on: 2009-06-26

Tags: , ,

 
 
 

19 Comments:

  1. Gravatar of John Merge John Merge
    15. March 2010 at 21:29

    Great article, as usual :)


  2. Gravatar of Robert Polovsky Robert Polovsky
    17. March 2010 at 22:30

    Thanks for these excellent articles! Your blog is very professional and useful for every JavaScript programmer.


  3. Gravatar of Dmitry A. Soshnikov Dmitry A. Soshnikov
    8. April 2010 at 16:15

    @John Merge, @Robert Polovsky, thanks colleagues.


  4. Gravatar of justin justin
    15. April 2010 at 20:41

    It’s an most excellent articles I have read.
    so I have tranclated it into Chinese,let more Chinese programmers see it.I would like to tranclate all articles of the set of “ECMA-262-3 in detail”.
    you can visit http://www.cnblogs.com/justinw/archive/2010/04/16/1713086.html
    I wish you like it.

    thanks for the share again


  5. Gravatar of Dmitry A. Soshnikov Dmitry A. Soshnikov
    15. April 2010 at 23:01

    @justin

    Thanks, justin. As Google translate shows me (unfortunately I can’t read Chinese myself :)), your translation is good. I am glad to see Chinese colleagues interested in deep JavaScript; that’s great.

    I added a link to your translation.

    Dmitry.


  6. Gravatar of Hans Höglund Hans Höglund
    2. September 2010 at 03:57

    Thanks for these series. An excellent overview of the specification.


  7. Gravatar of Alex Bars Alex Bars
    1. December 2010 at 21:24

    Thanks, I am trying hard to understand EC, now I know why them call it “Abstract” lol!!


  8. Gravatar of David Lee David Lee
    30. September 2011 at 18:25

    It is a great article. I want to translate to Korean so that other my friends can take it quickly.
    I have a question regarding below:

    “In SpiderMonkey implementation (built into Firefox, Thunderbird), up to version 1.7, it is possible to pass a calling context as a second argument for eval function.”

    I would like to clarify “up to version 1.7″. Did you mean after that version it doesn’t support the feature?

    And also did the version mean JavaScript Version? As far as I know there is 1.8.x version of JavaScript.

    Thanks.


  9. Gravatar of Dmitry A. Soshnikov Dmitry A. Soshnikov
    30. September 2011 at 18:36

    @Hans Höglund, @Alex Bars, thanks!

    @David Lee, thanks,

    I want to translate to Korean

    Yes, great. Please contact me when the translation will be ready, I’ll give a link from this article.

    I would like to clarify “up to version 1.7″. Did you mean after that version it doesn’t support the feature?

    Yes, because it was sort of a hack, it was banned. But you still make test in in older Firefox (2) and Rhino versions.

    And also did the version mean JavaScript Version? As far as I know there is 1.8.x version of JavaScript.

    Yes, exactly. The version of Mozilla’s SpiderMonkey is meant.


  10. Gravatar of Paul Paul
    18. November 2011 at 07:55

    Thank you for the translation. I can’t read Russian and it was nice you take the time in giving this to us in English.

    Very helpful.


  11. Gravatar of Michael Michael
    1. April 2012 at 00:46

    When is created globalContext in the browser? It is diffrent for EC3 and EC5? Scripts in the browser can load and execute at different times and earlier changes in the global object can be seen later.


  12. Gravatar of Dmitry Soshnikov Dmitry Soshnikov
    1. April 2012 at 21:17

    @Paul, thanks, glad it’s useful.

    @Michael, global context corresponds to evaluation of the script. Every script tag enters its own global execution context and is treated as separate program. However, all of these scripts are share the same global object.


  13. Gravatar of James Li James Li
    11. April 2012 at 16:16

    thanks for the great article. here’s a question: since eval code using a calling context, why the stack push addtional eval context above, while pop only the calling one? what’s the difference?


  14. Gravatar of Dmitry Soshnikov Dmitry Soshnikov
    12. April 2012 at 09:21

    @James Li

    It seems just not the best notation I chose for eval context pushing (there is a confusion with the real push method of arrays which accepts several parameters). There only one context is pushed — the eval‘s context, and the calling context is just a property:

    ECStack.push({
      context: evalContext,
      caller: globalContext
    });

  15. Gravatar of James Li James Li
    12. April 2012 at 18:23

    now it’s clear, thank u.


  16. Gravatar of kevin wang kevin wang
    7. May 2012 at 00:35

    If there is only one eval context being pushed, then I guess you miss putting the globalContext at the bottom, does it look like

    ECStack.push({
      context: evalContext,
      caller: globalContext
    },
    globalContext
    );

    And one more question, as you said, each script file has its own global context and all script files share another global context which is different from the script one, is it a purely implementation choice?


  17. Gravatar of kerry chen kerry chen
    13. July 2012 at 20:55

    awesome article~~thanks for your summary on specification~~only one question with your comment

    global context corresponds to evaluation of the script. Every script tag enters its own global execution context and is treated as separate program. However, all of these scripts are share the same global object.

    what is the difference between the global execution context into which scipt tags enter and the global object shared by these scripts?
    hope for your reply~~


  18. Gravatar of wuyingyou wuyingyou
    15. February 2013 at 19:02

    Hi,Dmitry A. Soshnikov
    It’s my honor to read your articles.But I have some problems as following.
    For exmaple,how to unstanding the globle code.There is a javascript code.Which’s the global code?Line one?

     
            1.var a = 10;
            2.var b = "Dimtry";
            3.var c = new Object();
            4.var d = { a: 123, b: 'aaa' };
            5.function D() {
                alert("I don't know how to unstanding the global code.");
            }
            6.D();
            7.b = a + b;
        

  19. Gravatar of rolfe renvyle rolfe renvyle
    19. February 2013 at 13:12

    Thank you for these very informative “ECMA-262-3 in Detail” articles. They have done so much to clear up my confusion on ECMA script.


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