11.2 Types of Source Code

There are four types of ECMAScript code:

Global code is source text that is treated as an ECMAScript Script. The global code of a particular Script does not include any source text that is parsed as part of a FunctionDeclaration, FunctionExpression, GeneratorDeclaration, GeneratorExpression, AsyncFunctionDeclaration, AsyncFunctionExpression, AsyncGeneratorDeclaration, AsyncGeneratorExpression, MethodDefinition, ArrowFunction, AsyncArrowFunction, ClassDeclaration, or ClassExpression.
Eval code is the source text supplied to the built-in eval function. More precisely, if the parameter to the built-in eval function is a String, it is treated as an ECMAScript Script. The eval code for a particular invocation of eval is the global code portion of that Script.
Function code is source text that is parsed to supply the value of the [[ECMAScriptCode]] and [[FormalParameters]] internal slots (see 10.2) of an ECMAScript function object. The function code of a particular ECMAScript function does not include any source text that is parsed as the function code of a nested FunctionDeclaration, FunctionExpression, GeneratorDeclaration, GeneratorExpression, AsyncFunctionDeclaration, AsyncFunctionExpression, AsyncGeneratorDeclaration, AsyncGeneratorExpression, MethodDefinition, ArrowFunction, AsyncArrowFunction, ClassDeclaration, or ClassExpression.

In addition, if the source text referred to above is parsed as:
- the FormalParameters and FunctionBody of a FunctionDeclaration or FunctionExpression,
- the FormalParameters and GeneratorBody of a GeneratorDeclaration or GeneratorExpression,
- the FormalParameters and AsyncFunctionBody of an AsyncFunctionDeclaration or AsyncFunctionExpression, or
- the FormalParameters and AsyncGeneratorBody of an AsyncGeneratorDeclaration or AsyncGeneratorExpression,
then the source text matched by the BindingIdentifier (if any) of that declaration or expression is also included in the function code of the corresponding function.
Module code is source text that is code that is provided as a ModuleBody. It is the code that is directly evaluated when a module is initialized. The module code of a particular module does not include any source text that is parsed as part of a nested FunctionDeclaration, FunctionExpression, GeneratorDeclaration, GeneratorExpression, AsyncFunctionDeclaration, AsyncFunctionExpression, AsyncGeneratorDeclaration, AsyncGeneratorExpression, MethodDefinition, ArrowFunction, AsyncArrowFunction, ClassDeclaration, or ClassExpression.

Note 1

Function code is generally provided as the bodies of Function Definitions (15.2), Arrow Function Definitions (15.3), Method Definitions (15.4), Generator Function Definitions (15.5), Async Function Definitions (15.8), Async Generator Function Definitions (15.6), and Async Arrow Functions (15.9). Function code is also derived from the arguments to the Function constructor (20.2.1.1), the GeneratorFunction constructor (27.3.1.1), and the AsyncFunction constructor (27.7.1.1).

Note 2

The practical effect of including the BindingIdentifier in function code is that the Early Errors for strict mode code are applied to a BindingIdentifier that is the name of a function whose body contains a "use strict" directive, even if the surrounding code is not strict mode code.

11.2.1 Directive Prologues and the Use Strict Directive

A Directive Prologue is the longest sequence of ExpressionStatements occurring as the initial StatementListItems or ModuleItems of a FunctionBody, a ScriptBody, or a ModuleBody and where each ExpressionStatement in the sequence consists entirely of a StringLiteral token followed by a semicolon. The semicolon may appear explicitly or may be inserted by automatic semicolon insertion (12.10). A Directive Prologue may be an empty sequence.

A Use Strict Directive is an ExpressionStatement in a Directive Prologue whose StringLiteral is either of the exact code point sequences "use strict" or 'use strict'. A Use Strict Directive may not contain an EscapeSequence or LineContinuation.

A Directive Prologue may contain more than one Use Strict Directive. However, an implementation may issue a warning if this occurs.

Note

The ExpressionStatements of a Directive Prologue are evaluated normally during evaluation of the containing production. Implementations may define implementation specific meanings for ExpressionStatements which are not a Use Strict Directive and which occur in a Directive Prologue. If an appropriate notification mechanism exists, an implementation should issue a warning if it encounters in a Directive Prologue an ExpressionStatement that is not a Use Strict Directive and which does not have a meaning defined by the implementation.

11.2.2 Strict Mode Code

An ECMAScript syntactic unit may be processed using either unrestricted or strict mode syntax and semantics (4.3.2). Code is interpreted as strict mode code in the following situations:

Global code is strict mode code if it begins with a Directive Prologue that contains a Use Strict Directive.
Module code is always strict mode code.
All parts of a ClassDeclaration or a ClassExpression are strict mode code.
Eval code is strict mode code if it begins with a Directive Prologue that contains a Use Strict Directive or if the call to eval is a direct eval that is contained in strict mode code.
Function code is strict mode code if the associated FunctionDeclaration, FunctionExpression, GeneratorDeclaration, GeneratorExpression, AsyncFunctionDeclaration, AsyncFunctionExpression, AsyncGeneratorDeclaration, AsyncGeneratorExpression, MethodDefinition, ArrowFunction, or AsyncArrowFunction is contained in strict mode code or if the code that produces the value of the function's [[ECMAScriptCode]] internal slot begins with a Directive Prologue that contains a Use Strict Directive.
Function code that is supplied as the arguments to the built-in Function, Generator, AsyncFunction, and AsyncGenerator constructors is strict mode code if the last argument is a String that when processed is a FunctionBody that begins with a Directive Prologue that contains a Use Strict Directive.

ECMAScript code that is not strict mode code is called non-strict code.

11.2.3 Non-ECMAScript Functions

An ECMAScript implementation may support the evaluation of function exotic objects whose evaluative behaviour is expressed in some host-defined form of executable code other than ECMAScript source text. Whether a function object is defined within ECMAScript code or is a built-in function is not observable from the perspective of ECMAScript code that calls or is called by such a function object.

ECMAScript® 2024 Language Specification

Draft ECMA-262 / February 15, 2024

11.2 Types of Source Code

11.2.1 Directive Prologues and the Use Strict Directive

11.2.2 Strict Mode Code

11.2.3 Non-ECMAScript Functions