Explanation Beginner

Chapter 1 Updated April 21, 2026

Execution Context

Memory + Code — the sealed container every JS program runs inside.

Full 6m
Revision 2m
Flow 2m

Everything in JS runs inside an Execution Context.

Two components: Memory (Variable Environment) + Code (Thread of Execution).
Memory stores variables and functions as key:value pairs.
Code runs one line at a time, in order.
A Global Execution Context is created the moment a program runs.

Why JS runs line by line.

Synchronous — one command executes at a time.
Single-threaded — there is one call stack, one thread of execution.
JS cannot move to the next line until the current one finishes.
Async behaviour comes from Web APIs + the callback queue + the event loop — not from the language itself.

Memory Creation → Code Execution.

Phase 1 — Memory Creation. Scan the code, allocate memory.
Variables get undefined; function declarations get their entire code.
Phase 2 — Code Execution. Run line by line, assign real values, invoke functions.
Hoisting is a consequence of Phase 1 — not a physical movement of code.

Each call creates a fresh context.

Invoking a function creates a brand new execution context pushed on top of the stack.
That context has its own two phases (Memory + Code) and its own variables.
return sends a value back and destroys the function’s context.
Multiple contexts can exist at once; only the top one is actively running.

Classic output traces.

console.log("A"); console.log("B"); console.log("C") → A → B → C (strict order).
var x = 1; console.log(x); x = 2; → prints 1 then the assignment runs.
Re-declaring var x in the same scope is silently allowed; only the last value wins.
Function declarations are “skipped” during execution — already stored in Phase 1.

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