Lexical Environment

Introduction

A lexical environment is a specification type that defines the association between identifiers (variable names) and their values within a scope.

Structure

ExecutionContext = {
  lexicalEnvironment: {
    environmentRecord: {
      type: 'Object' or 'Declarative',
      // Variables, functions, let/const declarations
    },
    outerEnvironmentReference: // Parent scope
  },
  variableEnvironment: {
    // Similar but for var declarations
  },
  thisBinding: // Value of 'this'
}

Environment Record Types

Declarative: For functions, catch blocks — stores let, const, class, function declarations
Object: For global context — stores var declarations as properties

Outer Environment Reference

Each lexical environment has a reference to its outer (parent) environment, forming the scope chain:

const global = 'global';

function outer() {
  const outerVar = 'outer';

  function inner() {
    const innerVar = 'inner';
    console.log(innerVar); // own
    console.log(outerVar); // from outer reference
    console.log(global);   // from outer's outer reference
  }

  inner();
}

outer();

Visual

flowchart LR
    subgraph "Inner EC"
        A["innerVar: 'inner'"]
        B["outer →"]
    end

    subgraph "Outer EC"
        C["outerVar: 'outer'"]
        D["global →"]
    end

    subgraph "Global EC"
        E["global: 'global'"]
        F["null"]
    end

    B --> C
    D --> E
    F --> G["null (end of chain)"]

Lexical vs Variable Environment

LexicalEnvironment: Stores let, const, and function declarations
VariableEnvironment: Stores var declarations

They’re usually the same, except in catch and with blocks.

Summary

Lexical environment = environment record + outer reference
Forms the scope chain for variable lookup
Outer reference connects to parent scope
LexicalEnvironment (let/const) vs VariableEnvironment (var)
The chain ends at global environment’s outer reference (null)

Lexical Environment

Lexical Environment

Introduction

Structure

Environment Record Types

Outer Environment Reference

Visual

Lexical vs Variable Environment

Summary

Further Reading