Whenever a script or program loads a global execution context is created. And every execution context goes through two phases:

  1. Creation Phase: In this phase memory space is set up for variables, funcions declarations etc within that context.

  2. Execution Phase: Where execution context is on the call stack and is executed.

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Creation Phase

Let’s talk about all parts of Global Execution Context in detail.

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Realm

The realm points to a realm record. Realm is an isolated environment is which our code runs. In browser a new realm is created whenever we open a new tab, refresh a page or use service works, web workers, iframes etc.

Realm consists of intrinsics, Global Object, and Global Environment Record.

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Intrinsics

Intrinsics provide all the standard build-in objects and functions that are essentially required to execute javascript. Eg: Array, Strings, Numbers, Syntax Errors etc.

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Global Object

These are properties of global objects that can be accessed from anywhere inside the script.

  1. Spec Properties: They are properties that exposes the intrinsics so that all the arrays, functions etc are present on the global object.
  2. Host Defined Properties: These are browser properties such as fetch, set timeout, document onject etc.
  3. User Defined Properties: As developers we can explicitly add properties to the global object or we can do it implicitly whenever we declare a function in the global scope or when we have a variable with var keyword in global scope.

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Global Environment Record

Environment records manages the identifier bindings within that context. In case of global environment record these values are accessible throughout our entire script. It contains:

  1. Object Record: It is just a direct reference to global object. Holds identifier bindings of var variable, function declarations etc. 2.Declarative Record: It contains all identifier bindings that aren’t a variable with var keyword, function, object declarations etc.
  2. Global “this” value: It contains the global “this” value which in most cases points to the global object.
  3. Outer Env: In global environment record it contains null but it is an important property used in scope, scope chain, closure. When a new Function Execution Context is created this Outer Env points to its outer environment.

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Lexical Environment

Lexical Environment points to the Environment Record that contains the bindings for everything except for variable with var keyword which is Global Environment Record -> Declarative Record.

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Variable Environment Record

Points to the record which stores the bindings that contains the refrences to the variable with the var keyword usually Global Environment Record -> Object Record.


In the next article we will try to understand this with an example.