Semantics[ edit ] An assignment operation is a process in imperative programming in which different values are associated with a particular variable name as time passes. It is possible to put a value into a variable and later replace it with a new one.
A scope consists of a list or chain of objects. Each function object has an internal [[scope]] property which we will go into more detail about shortly that also consists of a list or chain of objects.
The scope that is assigned to the execution context of a function call consists of the list referred to by the [[scope]] property of the corresponding function object with the Activation object added at the front of the chain or the top of the list.
Then the process of "variable instantiation" takes place using an object that ECMA refers to as the "Variable" object. However, the Activation object is used as the Variable object note this, it is important: Named properties of the Variable object are created for each of the function's formal parameters, and if arguments to the function call correspond with those parameters the values of those arguments Unit 1 assignment an introduction assigned to the properties otherwise the assigned value is undefined.
Inner function definitions are used to create function objects which are assigned to properties of the Variable object with names that correspond 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 initialisation of local variables does not happen until the evaluation of the corresponding assignment expressions during the execution of the function body code.
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 was a function local variable.
Finally a value is assigned for use with the this keyword. If the value assigned refers to an object then property accessors prefixed with the this keyword reference properties of that object.
If the value assigned internally is null then the this keyword will refer to the global object. The global execution context gets some slightly different handling as it does not have arguments so it does not need a defined Activation object to refer to them.
The global object is used as the Variable object, which is why globally declared functions become properties of the global object. As do globally declared variables. The global execution context also uses a reference to the global object for the this object.
In ECMAScript functions are objects, they are created during variable instantiation from function declarations, during the evaluation of function expressions or by invoking the Function constructor.
Function objects created with the Function constructor always have a [[scope]] property referring to a scope chain that only contains the global object. Function objects created with function declarations or function expressions have the scope chain of the execution context in which they are created assigned to their internal [[scope]] property.
In the simplest case of a global function declaration such as: The global execution context has a scope chain consisting of only the global object. Thus the function object that is created and referred to by the property of the global object with the name "exampleFunction" is assigned an internal [[scope]] property referring to a scope chain containing only the global object.
A similar is assigned when a function expression is executed in the global context: But the creation of the function object still happens in the global execution context so the [[scope]] property of the created function object still only contains the global object in the assigned scope chain.
Inner function declarations and expressions result in function objects being created within the execution context of a function so they get more elaborate scope chains. Consider the following code, which defines a function with an inner function declaration and then executes the outer function: The scope of that new execution context becomes the chain consisting of the new Activation object followed by the chain refereed to by the outer function object's [[scope]] property just the global object.
Variable instantiation for that new execution context results in the creation of a function object that corresponds with the inner function definition and the [[scope]] property of that function object is assigned the value of the scope from the execution context in which it was created.
A scope chain that contains the Activation object followed by the global object. So far this is all automatic and controlled by the structure and execution of the source code.
The scope chain of the execution context defines the [[scope]] properties of the function objects created and the [[scope]] properties of the function objects define the scope for their execution contexts along with the corresponding Activation object.
The with statement then executes another statement that may itself be a block statement and then restores the execution context's scope chain to what it was before.Krooth & Altman LLP specializes in multifamily real estate, health care capital financing and related banking activities.
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