kad3nce

You Don't Know JS: this & Object Prototypes

Chapter 5: Prototypes

In the Chapters 3 and 4, we mentioned the [[Prototype]] chain several times, but haven't said what exactly it is. We will now examine prototypes in detail.

**Note: All of the attempts to emulate class-copy behavior, as described previously in Chapter 4, labeled as variations of "mixins", completely circument the [[Prototype]] chain mechanism we examine here in this chapter.

Links

Objects in JavaScript have an internal property, denoted in the specification as [[Prototype]], which is simply a reference to another object. Almost all objects are given a non-null value for this property, at the time of their creation.

kad3nce

You Don't Know JS: Scope & Closures

Chapter 1: What is Scope?

One of the most fundamental paradigms of nearly all programming languages is the ability to store values in variables, and later retrieve or modify those values. In fact, the ability to store values and pull values out of variables is what gives a program state.

Without such a concept, a program could perform some tasks, but they would be extremely limited and not terribly interesting.

But the inclusion of variables into our program begets the most interesting questions we will now address: where do those variables live? In other words, where are they stored? And, most importantly, how does our program find them when it needs them?

These questions speak to the need for a well-defined set of rules for storing variables in some location, and for finding those variables at a later time. We'll call that set of rules: Scope.

kad3nce

kad3nce

You Don't Know JS: Scope & Closures

Chapter 1: What is Scope?

One of the most fundamental paradigms of nearly all programming languages is the ability to store values in variables, and later retrieve or modify those values. In fact, the ability to store values and pull values out of variables is what gives a program state.

Without such a concept, a program could perform some tasks, but they would be extremely limited and not terribly interesting.

But the inclusion of variables into our program begets the most interesting questions we will now address: where do those variables live? In other words, where are they stored? And, most importantly, how does our program find them when it needs them?

These questions speak to the need for a well-defined set of rules for storing variables in some location, and for finding those variables at a later time. We'll call that set of rules: Scope.

kad3nce

kad3nce

You Don't Know JS: Scope & Closures

Chapter 1: What is Scope?

One of the most fundamental paradigms of nearly all programming languages is the ability to store values in variables, and later retrieve or modify those values. In fact, the ability to store values and pull values out of variables is what gives a program state.

Without such a concept, a program could perform some tasks, but they would be extremely limited and not terribly interesting.

But the inclusion of variables into our program begets the most interesting questions we will now address: where do those variables live? In other words, where are they stored? And, most importantly, how does our program find them when it needs them?

These questions speak to the need for a well-defined set of rules for storing variables in some location, and for finding those variables at a later time. We'll call that set of rules: Scope.

kad3nce

You Don't Know JS: Scope & Closures

Chapter 1: What is Scope?

One of the most fundamental paradigms of nearly all programming languages is the ability to store values in variables, and later retrieve or modify those values. In fact, the ability to store values and pull values out of variables is what gives a program state.

Without such a concept, a program could perform some tasks, but they would be extremely limited and not terribly interesting.

But the inclusion of variables into our program begets the most interesting questions we will now address: where do those variables live? In other words, where are they stored? And, most importantly, how does our program find them when it needs them?

These questions speak to the need for a well-defined set of rules for storing variables in some location, and for finding those variables at a later time. We'll call that set of rules: Scope.

kad3nce

You Don't Know JS: Scope & Closures

Chapter 1: What is Scope?

One of the most fundamental paradigms of nearly all programming languages is the ability to store values in variables, and later retrieve or modify those values. In fact, the ability to store values and pull values out of variables is what gives a program state.

Without such a concept, a program could perform some tasks, but they would be extremely limited and not terribly interesting.

But the inclusion of variables into our program begets the most interesting questions we will now address: where do those variables live? In other words, where are they stored? And, most importantly, how does our program find them when it needs them?

These questions speak to the need for a well-defined set of rules for storing variables in some location, and for finding those variables at a later time. We'll call that set of rules: Scope.

kad3nce

You Don't Know JS: Scope & Closures

Chapter 2: Lexical Scope

In Chapter 1, we defined "scope" as the set of rules that govern how the Engine can look up a variable by its identifier name and find it, either in the current Scope, or in any of the Nested Scopes it's contained within.

There are two predominant models for how scope works. The first of these is by far the most common, used by the vast majority of programming languages. It's called Lexical Scope, and we will examine it in-depth. The other model, which is still used by some languages (such as Bash scripting, some modes in Perl, etc) is called Dynamic Scope.

Dynamic Scope is covered in Appendix A. I mention it here only to provide a contrast with Lexical Scope, which is the scope model that JavaScript employs.

Lex-time

kad3nce

You Don't Know JS: Scope & Closures

Chapter 2: Lexical Scope

In Chapter 1, we defined "scope" as the set of rules that govern how the Engine can look up a variable by its identifier name and find it, either in the current Scope, or in any of the Nested Scopes it's contained within.

There are two predominant models for how scope works. The first of these is by far the most common, used by the vast majority of programming languages. It's called Lexical Scope, and we will examine it in-depth. The other model, which is still used by some languages (such as Bash scripting, some modes in Perl, etc) is called Dynamic Scope.

Dynamic Scope is covered in Appendix A. I mention it here only to provide a contrast with Lexical Scope, which is the scope model that JavaScript employs.

Lex-time