Optimizing with Javascript can be challenging due to the presence of eval, which has access to the local frame.

However, if compilers are intelligent enough to recognize that eval is not a factor, performance improvements can be significant.

Assuming only local variables, captured variables, and global variables are in play, and there is no interference with eval, theoretically:

• Accessing a local variable involves direct memory access with an offset from the local frame
• Accessing a global variable is a straightforward operation
• Accessing a captured variable requires double indirection

This means that when looking up a variable like x, it will always be either local or global, allowing for direct access without any lookup needed at runtime.

For captured variables, additional complexity arises due to lifetime considerations. In a typical implementation where captured variables are stored in cells and cell addresses are copied when closures are created, two extra indirection steps are necessary.

mov rax, [rbp]      ; Load closure data address into rax
mov rax, [rax+0x12] ; Load cell address into rax
mov rax, [rax]      ; Load actual value of captured var into rax

The differences in timing observed are influenced by various factors beyond just variable access, such as caching mechanisms and implementation specifics like garbage collector behavior.

It's worth noting that accessing a global variable using the window object can introduce additional overhead, especially considering that window is designed to function as a regular object as well.

Testing micro-optimizations has become increasingly challenging nowadays due to the optimization process carried out by JavaScript engines' JIT compilers. It's possible that some of the tests yielding incredibly fast results are influenced by the compiler removing "unused" code and optimizing loops.

There are two main concerns when it comes to this issue: "scope chain lookup" and code that hinders the JIT compiler from effectively compiling or simplifying the code. (The latter is quite intricate, so delving into some expert tips would be beneficial.)

The challenge with the scope chain lies in how the JS engine handles variables like x, searching through different scopes such as:

• local scope
• closure scopes (e.g., those created by IIFE)
• global scope

The "scope chain" functions as a linked list of these scopes. When looking up x, the engine first checks if it's a local variable. If not, it traverses up any closure scopes before ultimately checking the global context if needed.

In the given code snippet, console.log(a); initially seeks to find a within the local scope of innerFunc(). Upon not locating a local variable a, it progresses to search within its enclosing closure without success as well. In scenarios where additional nested callbacks introduce more closures, each one must be examined. Only after exhausting these options does the engine resort to verifying a within the global scope, where it does discover the variable.

var a = 1; // global scope
(function myIife(window) {
    var b = 2; // scope in myIife with closure implications due to reference within innerFunc
    function innerFunc() {
        var c = 3;
        console.log(a);
        console.log(b);
        console.log(c);
    }
    // invoking innerFunc
    innerFunc();
})(window);

Running the code snippet in Chrome shows that every alternative has minimal execution time, except for directly accessing window.x. The use of object properties is slower compared to using variables, raising questions about why window.x is even slower than x and other alternatives.

The assumption that x = 1; is equivalent to window.x = 1; is incorrect. It's important to note that window is not solely the global object; it acts as both a property and a reference to it, as demonstrated by window.window.window....

Exploring Environment Records

Each variable must be "registered" in an environment record, with two primary types: Declarative and object.

Declarative environment records are used in function scope, while global scope utilizes an object environment record. This means every variable in the global scope is also a property of the global object.

It's worth noting that although objects can be accessed through identifiers with matching names, they may not necessarily represent variables. The use of the with statement exemplifies this behavior.

Distinguishing Between x=1 and window.x = 1

Creating a variable differs from adding a property to an object, even if that object functions as an environment record. Running

Object.getOwnPropertyDescriptor(window, 'x')

in both scenarios reveals that when x is a variable, the property x is not configurable, restricting deletion.

In instances where only window.x is visible, distinguishing between a variable and a property becomes challenging without additional context. Variables reside in scopes, on stacks, or elsewhere. While a compiler could verify the presence of a variable x, the cost may outweigh simply executing window.x = window.x + 1. Additionally, the existence of window is exclusive to browsers; JavaScript engines operate in diverse environments with varying property structures.

The discrepancy in performance between window.x across browsers is intriguing—Chrome experiences significant lag compared to Firefox and Safari. While Firefox and Safari optimize object accesses efficiently, Chrome may face challenges accessing environment record objects, potentially explaining its slower performance in this scenario.

In my personal opinion (although I have not found a way to definitively prove any theory right or wrong), this issue is related to the fact that window serves as both the global scope and a native object with an extensive list of properties.

I've noticed that scenarios run more efficiently when the reference to window is stored once and then accessed through that reference within a loop. On the other hand, cases where window is involved in Left-hand Side (LHS) lookups during each iteration of the loop tend to be slower.

The question of why there are variations in timing across different cases remains unanswered, but it appears to be attributed to optimizations by the JavaScript engine. One plausible explanation is that different browsers exhibit varying time proportions. The unexpected superior performance of scenario #3 could possibly be explained by the assumption that it has been heavily optimized due to its widespread usage.

After conducting tests with certain modifications, the results were as follows: Moving window.x to

window.obj.x</code produced identical results. However, when <code>x

was placed in

window.location.x</code (given that <code>location

is another significant native object), the timings dramatically changed:

1. access x directly    - Completed in 4278ms
2. access window.x     - Completed in 6792ms
3. accessUnqualified() - Completed in 4109ms
4. accessWindowPrefix()- Completed in 6563ms
5. accessCacheWindow() - Completed in 4489ms
6. access IIFE window  - Completed in 4326ms
7. access IIFE x      - Completed in 4137ms