what is GEC

What is GEC?

The Global Execution Context is the first execution context created when a JavaScript program starts.
It consists of two main phases:

1. Creation Phase (Memory Creation Phase)

   • JavaScript sets up memory for variables and functions.

   • Variables (var) are hoisted and set to undefined.

   • Functions are hoisted with their entire definition.

2. Execution Phase

   • Code is executed line by line.

   • Variables are assigned their actual values.

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Example:

var a = 10;
function greet() {
    console.log("Hello, World!");
}
greet();
console.log(a);

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Step 1: Creation Phase

Before execution starts, JavaScript scans the code and allocates memory.

• a → created in memory, initial value = undefined

• greet → entire function stored in memory

So, memory looks like:

a: undefined
greet: ƒ greet() { console.log("Hello, World!"); }

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Step 2: Execution Phase

Now the code runs line by line.

1. var a = 10;
   → a is assigned 10.

2. greet();
   → Function is invoked, creates Function Execution Context (FEC) for greet.
   → Inside it, console.log("Hello, World!"); prints:

    Hello, World!
   

3. console.log(a);
   → Prints:

    10
   

   Example with Nested Functions

    var a = 10;
   
    function first() {
        var b = 20;
        second();
        console.log("Inside first, b =", b);
    }
   
    function second() {
        var c = 30;
        console.log("Inside second, c =", c);
    }
   
    first();
    console.log("In global, a =", a);
   

   ---

   Step-by-Step Execution (with GEC and FEC)

   1. Creation Phase (Global Execution Context - GEC)

   Memory allocation before any code runs:

    a: undefined
    first: ƒ first() { ... }
    second: ƒ second() { ... }
   

   ---

   2. Execution Phase

   • Line var a = 10; → assigns 10 to a.

   • Next, first(); → a new Function Execution Context (FEC) is created for first().

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3. Inside first()

Creation phase for first():

    b: undefined

Execution:

• b = 20;

• second(); → another FEC is created for second(), pushed on top of the Call Stack.

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4. Inside second()

Creation phase for second():

    c: undefined

Execution:

• c = 30;

• console.log("Inside second, c =", c);

    Inside second, c = 30
  

After execution, second() FEC is removed (popped from Call Stack).

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5. Back to first()

• Next line: console.log("Inside first, b =", b);

    Inside first, b = 20
  

first() FEC is now destroyed.

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6. Back to Global Context

• Last line: console.log("In global, a =", a);

    In global, a = 10
  

GEC is removed after full execution ends.

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Call Stack Representation

    Start: [ GEC ]

    first() called → [ GEC → first() FEC ]

    second() called → [ GEC → first() FEC → second() FEC ]

    second() ends → [ GEC → first() FEC ]

    first() ends → [ GEC ]

    End: [ ]