Global execution context in Javascript - Step-by-Step Execution

Concept Flow - Global execution context

Start: Script loads

↓

Create Global Execution Context

↓

Memory Creation Phase

↓

Variable & Function Declarations Stored

↓

Code Execution Phase

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Execute Code Line by Line

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Global Context Remains Active Until Script Ends

When a JavaScript file runs, the global execution context is created first, setting up memory for variables and functions, then runs the code line by line.

Execution Sample

Javascript

var a = 5;
function greet() {
  console.log('Hello');
}
greet();

This code creates a global variable and a function, then calls the function to print 'Hello'.

Execution Table

Step	Action	Memory State	Output
1	Create global execution context	Empty memory for variables and functions
2	Memory creation phase: store 'a' as undefined, store 'greet' function	a: undefined, greet: function
3	Code execution: assign 5 to 'a'	a: 5, greet: function
4	Code execution: call greet()	a: 5, greet: function	Hello
5	End of script	a: 5, greet: function

💡 Script ends, global execution context is removed

Variable Tracker

Variable	Start	After Step 2	After Step 3	Final
a	undefined	undefined	5	5
greet	undefined	function	function	function

Key Moments - 2 Insights

Why is variable 'a' initially undefined in memory before assignment?

During the memory creation phase (see step 2 in execution_table), variables are set to undefined before code runs. This is why 'a' is undefined before step 3 assigns 5.

Why can the function 'greet' be called before its code line?

Functions are fully stored in memory during the creation phase (step 2), so they can be called anytime after that, even before their code line appears.

Visual Quiz - 3 Questions

Look at the execution_table at step 3, what is the value of 'a'?

A undefined

B 5

C function

D null

💡 Hint

Check the 'Memory State' column at step 3 in the execution_table

At which step is the function 'greet' stored in memory?

A Step 1

B Step 3

C Step 2

D Step 4

💡 Hint

Look at the 'Memory State' column in execution_table rows to see when 'greet' appears

If we remove the function call 'greet()', what will be the output?

A No output

B undefined

C Hello

D Error

💡 Hint

Check the 'Output' column in execution_table, see when 'Hello' is printed

Concept Snapshot

Global Execution Context in JavaScript:
- Created when script starts
- Memory phase: variables set to undefined, functions stored
- Execution phase: code runs line by line
- Functions can be called anytime after memory phase
- Context lasts until script ends

Full Transcript

When JavaScript runs a script, it first creates the global execution context. This sets up memory for variables and functions. Variables start as undefined, and functions are stored fully. Then the code runs line by line, assigning values and calling functions. For example, variable 'a' starts undefined, then gets value 5. The function 'greet' is stored early and can be called anytime after. The output 'Hello' appears when greet() runs. The global context stays active until the script finishes.