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How to use objects to work with data How to use the window and document objects How to use Textbox and Number objects How to use Date and String objects How to use the DOM to change the text for an element How to use functions How to create and call a function expression How to create and call a function declaration When and how to use local and global variables When and how to use strict mode How to handle events How to attach an event handler to an event How to use an onload event handler to attach other event handlers Two illustrative applications The Miles Per Gallon application The Email List application.
Note: you can get detailed information about lexical environment in this article.
Environment records differ by type. There are object environment records and declarative environment records. On top of the declarative record there are also function environment records, and module environment records. Each type of the record has specific only to it properties. An example of an object environment record can be the record of the global environment. Such record has also associated binding object , which may store some properties from the record, but not the others, and vice-versa.
The binding object can also be provided as this value. Notice, the binding object exists to cover legacy constructs such as var -declarations, and with -statements, which also provide their object as a binding object. These are historical reason when environments were represented as simple objects. We have already seen how environments are related via the parent link. Now we shall see how an environment can outlive the context which creates it.
As we see, the caller, that is the bar function, also provides the binding for x — with the value The use-case described above is known as the downwards funarg problem , i. This is solved by an agreement of using static scope , that is the scope of the creation time.
The static scope sometimes is also called lexical scope , hence the lexical environments naming. Technically the static scope is implemented by capturing the environment where a function is created. Note: you can read about static and dynamic scopes in this article. In our example, the environment captured by the foo function, is the global environment :. We can see that an environment references a function, which in turn reference the environment back. Further this environment is used for identifier resolution.
Note: a function is called in a fresh activation environment which stores local variables , and arguments. The parent environment of the activation environment is set to the closured environment of the function, resulting to the lexical scope semantics. The second sub-type of the Funarg problem is known as the upwards funarg problem.
The only difference here is that a capturing environment outlives the context which creates it. But we captured it, so it cannot be deallocated , and is preserved — to support static scope semantics. Often there is an incomplete understanding of closures — usually developers think about closures only in terms of the upward funarg problem and practically it really makes more sense. However, as we can see, the technical mechanism for the downwards and upwards funarg problem is exactly the same — and is the mechanism of the static scope.
As we mentioned above, similarly to prototypes, the same parent environment can be shared across several closures. This allows accessing and mutating the shared data:. Since both closures, increment and decrement , are created within the scope containing the count variable, they share this parent scope. Some languages may capture by-value , making a copy of a captured variable, and do not allow changing it in the parent scopes. However in JS, to repeat, it is always the reference to the parent scope.
Note: implementations may optimize this step, and do not capture the whole environment. Capturing only used free-variables, they though still maintain invariant of mutable data in parent scopes. You can find a detailed discussion on closures and the Funarg problem in the appropriate chapter. So all identifiers are statically scoped. The this value is a special object which is dynamically and implicitly passed to the code of a context. We can consider it as an implicit extra parameter , which we can access, but cannot mutate.
The major use-case is the class-based OOP. An instance method which is defined on the prototype exists in one exemplar , but is shared across all the instances of this class.
When the getX method is activated, a new environment is created to store local variables and parameters. In addition, function environment record gets the [[ThisValue]] passed, which is bound dynamically depending how a function is called. Another application of this , is generic interface functions , which can be used in mixins or traits. As an alternative, a mixin can also be applied at prototype level instead of per-instance as we did in the example above.
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Just to show the dynamic nature of this value, consider this example, which we leave to a reader as an exercise to solve:. Since only by looking at the source code of the foo function we cannot tell what value of this will it have in a particular call , we say that this value is dynamically scoped.