ECMAScript 4 Pragmas

ECMAScript 4 Netscape Proposal

Core Language

Pragmas

Tuesday, January 28, 2003

Pragmas allow a script writer to select program modes and options. Pragmas are lexically scoped and their effect does not extend past the end of the enclosing block, directive group, or substatement group.

Pragma use PragmaItems

PragmaItems

PragmaItem

| PragmaItems , PragmaItem

PragmaItem

PragmaExpr

| PragmaExpr ?

PragmaExpr

Identifier

| Identifier ( PragmaArgument )

PragmaArgument

true

| false

| Number

| - Number

| - NegatedMinLong

| String

The keyword use is followed by one or more PragmaItems, each of which consists of an identifier, an optional argument, and an optional ?.

The pragma identifiers below are currently defined. Implementations may define additional identifiers that have meaning as pragmas.

Identifier	Meaning
`ecmascript(n)`	Error if version `n` of ECMAScript is not supported; otherwise recommends but does not require that the implementation only support ECMAScript version `n` features.
`strict` `strict(true)`	Strict mode
`strict(false)`	Non-strict mode (default)

The pragma takes effect starting with the directive after the pragma and continues either until the end of the enclosing block or statement group or until overridden by another pragma in the same block or statement group, whichever comes first. If a pragma references the same identifier several times, the last reference takes precedence. The semicolon insertion rule changes implied by the strict pragma apply to the semicolon, if any, ending the use directive that contains that pragma.

If an implementation does not recognize a pragma identifier, then if the PragmaItem ends with a ? then that PragmaItem is ignored; if the PragmaItem does not end with a ? then an error occurs.

Strict Mode

Many parts of ECMAScript 4 are relaxed or unduly convoluted due to compatibility requirements with ECMAScript 3. Strict mode sacrifices some of this compatibility for simplicity and additional error checking. Strict mode is intended to be used in newly written ECMAScript 4 programs, although existing ECMAScript 3 programs may be retrofitted.

The opposite of strict mode is nonstrict mode, which is the default. A program can readily mix strict and nonstrict portions.

Strict mode has the following effects:

Line-break semicolon insertion is turned off. (Grammatical semicolon insertion remains turned on.)
[no line break] restrictions in grammar productions are ignored. Line breaks can be placed anywhere between input tokens.
Variables must be declared.
Definition scopes are not hoisted.
var and function declarations without attributes or types are initialized at the beginning of a scope.
FunctionDefinitions define constants rather than variables.
Calls to functions defined under strict mode are checked for the correct number of arguments except in functions that explicitly allow a variable number of arguments. (The mode of the call site does not matter.)
Implementations may choose to disable other compatibility extensions such as support for octal literals. These are not officially part of ECMAScript 4 but most implementations support these in nonstrict mode for compatibility with older programs.

An implementation does not have to implement strict mode; however, implementations are encouraged to do so.