Scope and Closures (2014)

Appendix A. Dynamic Scope

In Chapter 2, we talked about dynamic scope as a contrast to the lexical scope model, which is how scope works in JavaScript (and in fact, most other languages).

We will briefly examine dynamic scope, to hammer home the contrast. But, more important, dynamic scope actually is a near cousin to another mechanism (this) in JavaScript, which we cover in the this & Object Prototypes title of the You Don’t Know JS book series.

As we saw in Chapter 2, lexical scope is the set of rules about how the engine can look up a variable and where it will find it. The key characteristic of lexical scope is that it is defined at author time, when the code is written (assuming you don’t cheat with eval() or with).

Dynamic scope seems to imply, and for good reason, that there’s a model whereby scope can be determined dynamically at runtime, rather than statically at author time. That is in fact the case. Let’s illustrate via code:

function foo() {

console.log( a ); // 2

}

function bar() {

var a = 3;

foo();

}

var a = 2;

bar();

Lexical scope holds that the RHS reference to a in foo() will be resolved to the global variable a, which will result in value 2 being output.

Dynamic scope, by contrast, doesn’t concern itself with how and where functions and scopes are declared, but rather where they are called from. In other words, the scope chain is based on the call-stack, not the nesting of scopes in code.

So, if JavaScript had dynamic scope, when foo() is executed, theoretically the code below would instead result in 3 as the output.

function foo() {

console.log( a ); // 3 (not 2!)

}

function bar() {

var a = 3;

foo();

}

var a = 2;

bar();

How can this be? Because when foo() cannot resolve the variable reference for a, instead of stepping up the nested (lexical) scope chain, it walks up the call stack, to find where foo() was called from. Since foo() was called from bar(), it checks the variables in scope for bar(), and finds an a there with value 3.

Strange? You’re probably thinking so, at the moment.

But that’s just because you’ve probably only ever worked on (or at least deeply considered) code that is lexically scoped. So dynamic scoping seems foreign. If you had only ever written code in a dynamically scoped language, it would seem natural, and lexical scope would be the odd ball.

To be clear, JavaScript does not, in fact, have dynamic scope. It has lexical scope. Plain and simple. But the this mechanism is kind of like dynamic scope.

The key contrast: lexical scope is write-time, whereas dynamic scope (and this!) are runtime. Lexical scope cares where a function was declared, but dynamic scope cares where a function was called from.

Finally, this cares how a function was called, which shows how closely related the this mechanism is to the idea of dynamic scoping. To dig more into this, read the You Don’t Know JS title this & Object Prototypes.