Some blogs still sharing outdated or misleading info about JS Environment in 2023.
Please be aware of!
(!!! VO and LE shouldn't appear simultaneously.)
(!!! VO is before ES5. Please Don't use it anymore.)
The terms are still improving for clarification of the JS mechanism.
Let's keep tracking!
concept to describe the environment:
- Before ES5:
Variable Object - ES5-ES11:
Lexical Environments - ES12- :
Environment Records
This is what we familiar with.
Lexical Environment:
=> Consists of:
Environment Record:
records the identifier bindings of the scope.- A
referenceto the outer Lexical Environment.
And this is the latest revision.
Environment Record:
=> Records the identifier bindings of the scope.
- A
referenceto the outer Environment Record.
10.1.3 Variable instantiation
- Every execution context has associated with it a
variable object.- Variables (and functions) declared in the source text are added as properties of the
variable object.
In ES5 the concept of variable object is replaced with lexical environments model.
- A
Lexical Environmentis a specification type used to define the association of Identifiers to specific variables and functions based upon the lexical nesting structure of ECMAScript code.- A Lexical Environment consists of an Environment Record and a possibly null reference to an outer
Lexical Environment.
An Environment Record records the identifier bindings that are created within the scope of its associated Lexical Environment.
For specification purposes Environment Record values can be thought of as existing in a simple objectoriented hierarchy where Environment Record is an abstract class with
twoconcrete subclasses, declarative environment record and object environment record.
- The global environment is a unique
Lexical Environmentwhich is created before any ECMAScript code is executed.- The global environment’s Environment Record is an object environment record whose binding object is the global object.
- The global environment’s outer environment reference is null
Table 19 —Execution Context State Components
- LexicalEnvironment: Identifies the
Lexical Environmentused to resolve identifier references made by code within this execution context.- VariableEnvironment: Identifies the
Lexical Environmentwhose environment record holds bindings created by VariableStatements and FunctionDeclarations within this execution context.
The LexicalEnvironment and VariableEnvironment components of an execution context are always
Lexical Environments.
(↓ removed from ES11)
When an execution context is created its LexicalEnvironment and VariableEnvironment components initially have the same value.
(↓ removed from ES6)
The value of the VariableEnvironment component never changes while the value of the LexicalEnvironment component may change during execution of code within an execution context.
In ES12 the concept of lexical environments is replaced with Environment Records model.
Environment Recordis a specification type used to define the association of Identifiers to specific variables and functions, based upon the lexical nesting structure of ECMAScript code.- Every Environment Record has an [[OuterEnv]] field, which is either null or a reference to an outer
Environment Record.
- Environment Records can be thought of as existing in a simple object-oriented hierarchy where Environment Record is an abstract class with
threeconcrete subclasses: declarative Environment Record, object Environment Record, and global Environment Record.- Function Environment Records and module Environment Records are subclasses of declarative Environment Record.
Table 25: Additional State Components for ECMAScript Code Execution Contexts
- LexicalEnvironment: Identifies the
Environment Recordused to resolve identifier references made by code within this execution context.- VariableEnvironment: Identifies the
Environment Recordthat holds bindings created by VariableStatements within this execution context.
The LexicalEnvironment and VariableEnvironment components of an execution context are always
Environment Records.