Grammar

Grammar (informal)

Program     := Statement*
Statement   := Block | VarDecl | ExprStmt | If | While | For | Return | Break | Continue | FunDecl | Import | Export
Import      := 'import' ( '{' ImportSpec+ '}' | '*' 'as' Ident | Ident ) 'from' String
Export      := 'export' ( 'default' Expr | 'const' VarDecl | 'let' VarDecl | FunDecl )
Block       := Indent Statement* Dedent  |  '{' Statement* '}'
VarDecl     := ('let' | 'const') Ident TypeAnn? ('=' Expr)? ';'?
ExprStmt    := Expr ';'?
If          := 'if' '(' Expr ')' Statement ('else' Statement)?
While       := 'while' '(' Expr ')' Statement
For         := 'for' '(' Init? ';' Cond? ';' Update? ')' Statement
            |  'for' '(' ('let'|'const') Ident 'of' Expr ')' Statement
Return      := 'return' Expr? ';'?
FunDecl     := ('fn' | 'function') Ident '(' TypedParams? ')' TypeAnn? '=' Expr
            |  ('fn' | 'function') Ident '(' TypedParams? ')' TypeAnn? Block
Expr        := Assign | NullishCoalesce | Or | ...
Assign      := Ident '=' Expr
NullishCoalesce := Or ('??' Or)*

TypeAnn     := ':' Type
Type        := Ident | Type '[]' | '{' (Ident TypeAnn ',')* '}' | '(' (Type ',')* ')' '=>' Type | Type '|' Type
TypedParams := TypedParam (',' TypedParam)*
TypedParam  := Ident TypeAnn?

Indentation

• Blocks can use indentation instead of braces.
• Tab and space are normalized: 1 tab = 1 level; 2 spaces = 1 level.
• Consistent level matters; mixing styles is allowed.