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RIA & Ajax: Article

JavaScript Execution Context, Closure, Eval and “this” Keyword

These are a few key concepts of JavaScript language that developers should know

These are a few key concepts of JavaScript language that developers should know: execution context, activation object, variable instantiation, scoping, closure, eval and “this” keyword. Knowing these would help one tremendously in Ajax development.

For example, when you write an inner function, you know that you can access the local variables defined in the outer function as if they were defined locally. You can also access the global variables. -Why? How does the host environment resolve such variables?

Another example: When you pass arguments to a function, you can access these arguments as if they were locally defined variables. How does this work?

A slightly more involved example that developers must have seem similar code a lot but may not know the “why”:

function outerFunc(outerArg){
   var localVar = 100;

   function innerFunc(innerArg){
      localVar+=100;
       return (outerArg +innerArg + localVar);
    }

   return innerFunc;
}

 var globalVar = outerFunc(200);
 alert(globalVar(300));	//this displays 700
 alert(globalVar(300));	//this displays 800
 globalVar = outerFunc(200)
 alert(globalVar(300));  //this displays 700 again

Why would the 2nd alert box show 800 (means the value of localVar is retained from the first call) while the third alert displays 700 (the value of localVar is not retained from previous calls)? How does it work?

    Key Concepts

  1. Execution Context:
    An execution context is an abstract concept used by the EcmaScript specification and can be conceived (and even implemented) as a set of objects with properties, though not publicly accessible, that encapsulate the context in which a piece of JavaScript code executes. The JavaScript code would rely on its execution context for things like variable resolution and scoping. 

    All JavaScript code executes in an execution context. Global code (code executed inline, normally as a JS file, or HTML page, loads) gets executed in a global execution context (in a browser, the global context is typically the window object), and each invocation of a function (possibly as a constructor) has an associated execution context. Code executed with the eval and setTimeout functions also gets a distinct execution context.

    When a JavaScript function is called, it enters an execution context. If another function is called (or the same function recursively), a new execution context is created and execution enters that context for the duration of the function call. When that called function returns, control returns to the original execution context and the just finished execution context is available for garbage collection except for cases like closure. Thus running JavaScript code forms a stack of execution contexts.

  2. Activation Object and Variable Object :

    1. Activation Object:
      When an execution context is created, an Activation Object is created. The Activation Object can be considered as a normal JavaScript object with accessible named properties, except that it has no prototype and it cannot be directly referenced by javaScript code.This Activation Object is used for storing context related information such as call arguments and variables, etc. so that they are accessible to the new execution context (see more info on this later in this post).

    2. Variable Object and Variable Instantiation:
      ECMA Specification refers to a “Variable Object” for variable instantiation. “Variable Instantiation” is a process that takes place when the JavaScript host environment processes a piece of script so that it can be executed but before executing the script. In this process, the JavaScript engine prepares(”instantiates”) the local variables, local functions declarations, and arguments to the function call etc. so that these variables are accessible during execution.

      The way that variable instantiation works is to instantiate each of the function’s formal parameters, local variables and inner function declarations as named properties of the Variable Object. In the process, named properties of the Variable object are created for each of the function’s formal parameters. If arguments to the function call correspond with those parameters, the values of those arguments are assigned to the properties (otherwise the assigned value is undefined). If there is an inner function declaration, a function object is created. This function object is assigned a property of the Variable object with name corresponding to the function name used in the function declaration. The last stage of variable instantiation is to create named properties of the Variable object that correspond with all the local variables declared within the function.

      The properties created on the Variable object that correspond with declared local variables are initially assigned undefined values during variable instantiation, the actual initialization of local variables does not happen until the evaluation of the corresponding assignment expressions during the execution of the function body code.

      However, it is very important to point out that the Activation object is used as the Variable object. Though conceptually they are called out as different objects, they are the same object in reality. This is important as you can see quickly in “scope and scope chain” and “variable resolution” later on.

      It is the fact that the Activation object, with its arguments property, and the Variable object, with named properties corresponding with function local variables, are the same object, that allows the identifier arguments to be treated as if it were a function local variable.

More Stories By Coach Wei

Coach Wei is founder and CEO of Yottaa, a web performance optimization company. He is also founder and Chairman of Nexaweb, an enterprise application modernization software company. Coding, running, magic, robot, big data, speed...are among his favorite list of things (not necessarily in that order. His coding capability is really at PowerPoint level right now). Caffeine, doing something entrepreneurial and getting out of sleeping are three reasons that he gets up in the morning and gets really excited.

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