What execution context is in Javascript - Step-by-Step Execution

Concept Flow - What execution context is

Start Program

↓

Create Global Execution Context

↓

Execute Global Code

↓

Function Called?

No→End

Yes↓

Create Function Execution Context

↓

Execute Function Code

↓

Function Returns

↓

Destroy Function Execution Context

↓

Back to Previous Context

↩Loop if more functions called

Execution context is like a container that holds information about the environment where JavaScript code runs, starting with the global context and creating new ones for each function call.

Execution Sample

Javascript

let x = 10;
function add(y) {
  return x + y;
}
let result = add(5);
console.log(result);

This code runs in the global context, then creates a new context when the function add is called.

Execution Table

Step	Execution Context	Action	Variables in Context	Output
1	Global	Declare x=10 and function add	x=10, add=function
2	Global	Call add(5)	x=10, add=function
3	Function add	Create context with y=5	y=5
4	Function add	Calculate x + y	x=10 (from outer), y=5
5	Function add	Return 15	y=5
6	Global	Assign result=15	x=10, add=function, result=15
7	Global	console.log(result)	x=10, add=function, result=15	15
8	Global	End program	x=10, add=function, result=15

💡 Program ends after global context finishes executing all code.

Variable Tracker

Variable	Start	After Step 1	After Step 2	After Step 6	Final
x	undefined	10	10	10	10
add	undefined	function	function	function	function
y	undefined	undefined	5	undefined	undefined
result	undefined	undefined	undefined	15	15

Key Moments - 2 Insights

Why does the function add have access to variable x even though x is not declared inside it?

Because the function execution context can access variables from its outer (global) context, as shown in step 4 where x=10 is used inside the function.

What happens to the function's variable y after the function finishes?

The function execution context is destroyed after returning, so y is no longer accessible in the global context, as seen in variable_tracker where y becomes undefined again after step 6.

Visual Quiz - 3 Questions

Look at the execution table, what is the value of 'result' after step 6?

A 15

B 5

C undefined

D 10

💡 Hint

Check the 'Variables in Context' column at step 6 in the execution_table.

At which step is the function execution context created?

A Step 2

B Step 3

C Step 5

D Step 7

💡 Hint

Look for the step where 'Function add' context is mentioned in the execution_table.

If the function add did not return a value, what would be the output at step 7?

A 15

B NaN

C undefined

D Error

💡 Hint

Recall that if no return is given, functions return undefined by default.

Concept Snapshot

Execution context is the environment where JS code runs.
Global context is created first.
Each function call creates a new context.
Contexts hold variables and code info.
Contexts are destroyed after function ends.

Full Transcript

Execution context in JavaScript is like a container that holds all the information needed to run code. When a program starts, the global execution context is created. This context holds global variables and functions. When a function is called, a new execution context is created for that function. This function context has its own variables and parameters but can also access variables from outer contexts. After the function finishes running, its context is destroyed and control returns to the previous context. This process continues until the program ends.