Welcome to my mini-series - a series I’ll call “Understanding JavaScript” (though can we really understand JS 👀). These are meant to serve as a reference and also explaining my understanding of things I learn as I try to explore the JavaScript language on a deeper level.

We start off with the execution context - the “room” where the code runs.

Analogy: imagine every function call creates a sealed room. That room has a whiteboard (local variables), a door to the hallway outside (scope chain), and a nametag on the door(this binding). When the function finishes, the room is destroyed unless something still holds a reference to it ( that’s a closure).

The JS engine has two phases for every context before running a single line:

Creation Phase:

• var declarations are found and set to undefined (hoisting)
• function declarations are fully loaded in memory
• let/const are registered but untouchable

Execution Phase:

Code runs line by line and runs/‘executes’ each line - also known as the thread of execution.

This is why console.log(x) before var x = 5 gives undefined and not an error — x was created in the creation phase, just not assigned yet.