ECMAScript® 2024 Language Specification

Draft ECMA-262 / February 15, 2024

16.1 Scripts

Syntax

Script : ScriptBodyopt ScriptBody : StatementList[~Yield, ~Await, ~Return]

16.1.1 Static Semantics: Early Errors

Script : ScriptBody ScriptBody : StatementList

16.1.2 Static Semantics: IsStrict

The syntax-directed operation IsStrict takes no arguments and returns a Boolean. It is defined piecewise over the following productions:

Script : ScriptBodyopt
  1. If ScriptBody is present and the Directive Prologue of ScriptBody contains a Use Strict Directive, return true; otherwise, return false.

16.1.3 Runtime Semantics: Evaluation

Script : [empty]
  1. Return undefined.

16.1.4 Script Records

A Script Record encapsulates information about a script being evaluated. Each script record contains the fields listed in Table 40.

Table 40: Script Record Fields
Field Name Value Type Meaning
[[Realm]] a Realm Record or undefined The realm within which this script was created. undefined if not yet assigned.
[[ECMAScriptCode]] a Script Parse Node The result of parsing the source text of this script.
[[LoadedModules]] a List of Records with fields [[Specifier]] (a String) and [[Module]] (a Module Record) A map from the specifier strings imported by this script to the resolved Module Record. The list does not contain two different Records with the same [[Specifier]].
[[HostDefined]] anything (default value is empty) Field reserved for use by host environments that need to associate additional information with a script.

16.1.5 ParseScript ( sourceText, realm, hostDefined )

The abstract operation ParseScript takes arguments sourceText (ECMAScript source text), realm (a Realm Record or undefined), and hostDefined (anything) and returns a Script Record or a non-empty List of SyntaxError objects. It creates a Script Record based upon the result of parsing sourceText as a Script. It performs the following steps when called:

  1. Let script be ParseText(sourceText, Script).
  2. If script is a List of errors, return script.
  3. Return Script Record { [[Realm]]: realm, [[ECMAScriptCode]]: script, [[LoadedModules]]: « », [[HostDefined]]: hostDefined }.
 Note

An implementation may parse script source text and analyse it for Early Error conditions prior to evaluation of ParseScript for that script source text. However, the reporting of any errors must be deferred until the point where this specification actually performs ParseScript upon that source text.

16.1.6 ScriptEvaluation ( scriptRecord )

The abstract operation ScriptEvaluation takes argument scriptRecord (a Script Record) and returns either a normal completion containing an ECMAScript language value or an abrupt completion. It performs the following steps when called:

  1. Let globalEnv be scriptRecord.[[Realm]].[[GlobalEnv]].
  2. Let scriptContext be a new ECMAScript code execution context.
  3. Set the Function of scriptContext to null.
  4. Set the Realm of scriptContext to scriptRecord.[[Realm]].
  5. Set the ScriptOrModule of scriptContext to scriptRecord.
  6. Set the VariableEnvironment of scriptContext to globalEnv.
  7. Set the LexicalEnvironment of scriptContext to globalEnv.
  8. Set the PrivateEnvironment of scriptContext to null.
  9. Suspend the running execution context.
  10. Push scriptContext onto the execution context stack; scriptContext is now the running execution context.
  11. Let script be scriptRecord.[[ECMAScriptCode]].
  12. Let result be Completion(GlobalDeclarationInstantiation(script, globalEnv)).
  13. If result is a normal completion, then
    1. Set result to Completion(Evaluation of script).
    2. If result is a normal completion and result.[[Value]] is empty, then
      1. Set result to NormalCompletion(undefined).
  14. Suspend scriptContext and remove it from the execution context stack.
  15. Assert: The execution context stack is not empty.
  16. Resume the context that is now on the top of the execution context stack as the running execution context.
  17. Return ? result.

16.1.7 GlobalDeclarationInstantiation ( script, env )

The abstract operation GlobalDeclarationInstantiation takes arguments script (a Script Parse Node) and env (a Global Environment Record) and returns either a normal completion containing unused or a throw completion. script is the Script for which the execution context is being established. env is the global environment in which bindings are to be created.

Note 1

When an execution context is established for evaluating scripts, declarations are instantiated in the current global environment. Each global binding declared in the code is instantiated.

It performs the following steps when called:

  1. Let lexNames be the LexicallyDeclaredNames of script.
  2. Let varNames be the VarDeclaredNames of script.
  3. For each element name of lexNames, do
    1. If env.HasVarDeclaration(name) is true, throw a SyntaxError exception.
    2. If env.HasLexicalDeclaration(name) is true, throw a SyntaxError exception.
    3. Let hasRestrictedGlobal be ? env.HasRestrictedGlobalProperty(name).
    4. If hasRestrictedGlobal is true, throw a SyntaxError exception.
  4. For each element name of varNames, do
    1. If env.HasLexicalDeclaration(name) is true, throw a SyntaxError exception.
  5. Let varDeclarations be the VarScopedDeclarations of script.
  6. Let functionsToInitialize be a new empty List.
  7. Let declaredFunctionNames be a new empty List.
  8. For each element d of varDeclarations, in reverse List order, do
    1. If d is not either a VariableDeclaration, a ForBinding, or a BindingIdentifier, then
      1. Assert: d is either a FunctionDeclaration, a GeneratorDeclaration, an AsyncFunctionDeclaration, or an AsyncGeneratorDeclaration.
      2. NOTE: If there are multiple function declarations for the same name, the last declaration is used.
      3. Let fn be the sole element of the BoundNames of d.
      4. If declaredFunctionNames does not contain fn, then
        1. Let fnDefinable be ? env.CanDeclareGlobalFunction(fn).
        2. If fnDefinable is false, throw a TypeError exception.
        3. Append fn to declaredFunctionNames.
        4. Insert d as the first element of functionsToInitialize.
  9. Let declaredVarNames be a new empty List.
  10. For each element d of varDeclarations, do
    1. If d is either a VariableDeclaration, a ForBinding, or a BindingIdentifier, then
      1. For each String vn of the BoundNames of d, do
        1. If declaredFunctionNames does not contain vn, then
          1. Let vnDefinable be ? env.CanDeclareGlobalVar(vn).
          2. If vnDefinable is false, throw a TypeError exception.
          3. If declaredVarNames does not contain vn, then
            1. Append vn to declaredVarNames.
  11. NOTE: No abnormal terminations occur after this algorithm step if the global object is an ordinary object. However, if the global object is a Proxy exotic object it may exhibit behaviours that cause abnormal terminations in some of the following steps.
  12. NOTE: Annex B.3.2.2 adds additional steps at this point.
  13. Let lexDeclarations be the LexicallyScopedDeclarations of script.
  14. Let privateEnv be null.
  15. For each element d of lexDeclarations, do
    1. NOTE: Lexically declared names are only instantiated here but not initialized.
    2. For each element dn of the BoundNames of d, do
      1. If IsConstantDeclaration of d is true, then
        1. Perform ? env.CreateImmutableBinding(dn, true).
      2. Else,
        1. Perform ? env.CreateMutableBinding(dn, false).
  16. For each Parse Node f of functionsToInitialize, do
    1. Let fn be the sole element of the BoundNames of f.
    2. Let fo be InstantiateFunctionObject of f with arguments env and privateEnv.
    3. Perform ? env.CreateGlobalFunctionBinding(fn, fo, false).
  17. For each String vn of declaredVarNames, do
    1. Perform ? env.CreateGlobalVarBinding(vn, false).
  18. Return unused.
 Note 2

Early errors specified in 16.1.1 prevent name conflicts between function/var declarations and let/const/class declarations as well as redeclaration of let/const/class bindings for declaration contained within a single Script. However, such conflicts and redeclarations that span more than one Script are detected as runtime errors during GlobalDeclarationInstantiation. If any such errors are detected, no bindings are instantiated for the script. However, if the global object is defined using Proxy exotic objects then the runtime tests for conflicting declarations may be unreliable resulting in an abrupt completion and some global declarations not being instantiated. If this occurs, the code for the Script is not evaluated.

Unlike explicit var or function declarations, properties that are directly created on the global object result in global bindings that may be shadowed by let/const/class declarations.