Expressions

Expressions are JavaScript snippets provided as strings. They are parsed into an Abstract Syntax Tree (AST) using Acorn. The AST is then evaluated in a controlled environment against the forms value (onValueChange).

Parsing and Caching

• Parsing:
  An expression string is parsed to generate an AST.

• Caching:
  Parsed ASTs are cached to avoid reparsing the same expression multiple times. This improves performance when evaluating expressions repeatedly.

Supported Node Types

The evaluator supports a range of node types:

• Identifier
• Literal
• ArrayExpression
• UnaryExpression
• BinaryExpression
• LogicalExpression (supports &&, ||, and nullish coalescing ??)
• MemberExpression (property access with safety checks for null or undefined objects)
• ConditionalExpression (ternary operator)
• ObjectExpression
• SequenceExpression
• TemplateLiteral
• CallExpression for invoking safe methods
• ArrowFunctionExpression for simple arrow functions with expression bodies

Supported Operators

• Arithmetic Operators: +, -, *, /, %
• Comparison Operators: <, >, <=, >=
• Equality Operators: ==, !=, ===, !== (follow JavaScript behavior)
• Bitwise Operators: |, &, ^, <<, >>, >>>
• Logical Operations: &&, ||, ??
• Safe Method Calls: When calling methods on objects, the evaluator checks against a whitelist of safe methods (provided for strings, numbers, booleans, and arrays). This ensures that only approved operations are executed.

Limitations

• Restricted Syntax:
  Complex function bodies or block statements in arrow functions are not supported. Only simple expression-based arrow functions are allowed.

• Restricted Node Types:
  Some JavaScript features are not supported to maintain security and simplicity, such as update expressions, assignments, and using this or super.

• Controlled Context:
  The evaluation runs only in the context of the form object, avoiding global access and potential security vulnerabilities.