ECMA2623 in detail. Chapter 1. Execution Contexts.

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Introduction

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

Definitions

Every time when control is transferred to ECMAScript executable code, control is entered anexecution 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.

Types of executable code

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.

Global code

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

];

Function code

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';

  })();

})();

Eval code

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 callingcontext:

// 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(

  evalContext,

  callingContext: globalContext

);

 

// eval exited context

ECStack.pop();

 

// foo funciton call

ECStack.push(<foo> functionContext);

 

// eval('var y = 20');

ECStack.push(

  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.

Conclusion

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.

Additional literature

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