Reflect Module

Introduction

The reflect module provides runtime TypeScript/JavaScript analysis and introspection capabilities, allowing you to examine and understand code structure during execution.

Purpose

The reflect module helps you:

Analyze function signatures at runtime - Inspect parameters, names, and structure of functions without needing the source code
Extract parameter information - Determine which parameters are required vs optional, their names, and positions
Track code provenance - Keep track of where data and code originated from (file paths, line numbers, timestamps)
Inspect TypeScript source content - Parse and analyze TypeScript code to extract exports, functions, and other declarations

Directory Structure

lib/reflect/
├── callable.ts       # Function analysis and signature inspection
├── callable_test.ts  # Tests for callable functionality
├── provenance.ts     # Source tracking and origin information
├── provenance_test.ts
├── reflect.ts        # Core reflection utilities
├── reflect_test.ts
├── ts-content.ts     # TypeScript source code analysis
├── ts-content_test.ts
├── value.ts          # Runtime value inspection
├── value_test.ts
└── deps-test.ts      # Dependency tests

File Reference

callable.ts

Analyzes functions and callable objects to extract their signatures and parameter information.

Function Analysis

import { analyzeCallable } from "./callable.ts";

function greet(name: string, age?: number): string {
  return `Hello ${name}`;
}

const info = analyzeCallable(greet);
// Returns:
// {
//   name: "greet",
//   params: [
//     { name: "name", required: true },
//     { name: "age", required: false }
//   ]
// }

This is particularly useful when you need to understand function signatures at runtime, such as when building tools that work with arbitrary functions.

Use Cases

Generating CLI help from functions - Automatically create command-line help text based on function parameters
Validating function arguments - Check if the right arguments are being passed before calling a function
Building documentation - Auto-generate API documentation from actual function definitions

provenance.ts

Tracks where code and data originated, providing an audit trail for debugging and error reporting.

Tracking Sources

import { trackProvenance, getProvenance } from "./provenance.ts";

// Attach provenance information to data
const data = trackProvenance({ value: 42 }, {
  source: "config.json",
  line: 10
});

// Later, retrieve the origin information
const origin = getProvenance(data);
// Returns: { source: "config.json", line: 10 }

This is especially valuable in complex applications where data flows through multiple transformations and you need to trace back to the original source.

Use Cases

Error messages with source locations - Show users exactly where in their config file an error occurred
Debugging generated content - Track which template or generator created specific output
Audit trails - Maintain records of where data came from for compliance or debugging

reflect.ts

Core reflection utilities for runtime type checking and object inspection.

import { getTypeOf, isObject, isFunction } from "./reflect.ts";

console.log(getTypeOf(42));        // "number"
console.log(getTypeOf([]));        // "array" (not just "object")
console.log(isObject({}));         // true
console.log(isFunction(() => {})); // true

These utilities provide more accurate type detection than JavaScript's built-in typeof operator, which has limitations (e.g., typeof [] returns "object" instead of "array").

ts-content.ts

Analyzes TypeScript source code to extract structural information without executing it.

import { analyzeTypeScript } from "./ts-content.ts";

const source = `
export function hello(name: string): string {
  return "Hello " + name;
}
`;

const analysis = analyzeTypeScript(source);
// Returns:
// {
//   exports: ["hello"],
//   functions: [{ name: "hello", exported: true }]
// }

This is useful for build tools, documentation generators, or any system that needs to understand TypeScript code structure without executing it.

value.ts

Provides deep runtime value inspection, going beyond simple type checking to analyze structure.

import { inspectValue, describeType } from "./value.ts";

const info = inspectValue({ a: 1, b: [2, 3] });
// Returns detailed structure:
// {
//   type: "object",
//   keys: ["a", "b"],
//   properties: {
//     a: { type: "number" },
//     b: { type: "array", length: 2 }
//   }
// }

// More precise type descriptions
console.log(describeType(null));      // "null" (not "object")
console.log(describeType(undefined)); // "undefined"
console.log(describeType(42));        // "number"

Usage Examples

Function Documentation Generator

Automatically generate documentation for functions based on their signatures:

import { analyzeCallable } from "./callable.ts";

function buildTask(
  name: string,
  depends?: string[],
  options?: { timeout?: number }
): void {
  // Task implementation...
}

const info = analyzeCallable(buildTask);
console.log(`Function: ${info.name}`);
console.log("Parameters:");
for (const param of info.params) {
  const req = param.required ? "(required)" : "(optional)";
  console.log(`  - ${param.name} ${req}`);
}

// Output:
// Function: buildTask
// Parameters:
//   - name (required)
//   - depends (optional)
//   - options (optional)

Source Tracking for Better Error Messages

Track configuration sources so you can provide helpful error messages:

import { trackProvenance, getProvenance } from "./provenance.ts";

// When loading config, attach source information
const config = trackProvenance(
  JSON.parse(content),
  { source: filePath, timestamp: Date.now() }
);

// Later, when validation fails
function validateConfig(cfg: unknown) {
  if (!isValidConfig(cfg)) {
    const source = getProvenance(cfg);
    throw new Error(
      `Invalid configuration in ${source.source}\n` +
      `Loaded at ${new Date(source.timestamp)}`
    );
  }
}

Type Validation with Custom Assertions

Create type guards with better error messages:

import { getTypeOf, isObject } from "./reflect.ts";

function validate(input: unknown): asserts input is Record<string, unknown> {
  if (!isObject(input)) {
    throw new Error(
      `Expected object, got ${getTypeOf(input)}. ` +
      `Value: ${JSON.stringify(input)}`
    );
  }
}

// Usage
validate(someData);
// Now TypeScript knows someData is an object
someData.property = "value"; // Type-safe!

Integration Points

With axiom/

The reflect module integrates with the axiom parser to track source locations in error messages:

import { trackProvenance } from "../reflect/provenance.ts";

// During parsing, attach file location to AST nodes
const node = trackProvenance(parsed, { 
  source: filePath, 
  line: lineNum,
  column: colNum 
});

// Later, errors can reference the exact source location

With task/

Used for analyzing task functions to validate arguments and generate help:

import { analyzeCallable } from "../reflect/callable.ts";

// Analyze a task function to understand its parameters
const taskInfo = analyzeCallable(taskFn);

// Use this to validate task invocations
if (args.length < requiredParamCount) {
  throw new Error(`Task ${taskInfo.name} requires ${requiredParamCount} arguments`);
}

Design Notes

Runtime vs Static Analysis

This module performs runtime analysis, which has both advantages and limitations:

Advantages:

Works with actual values and functions as they exist during execution
No need for TypeScript compiler API or complex AST parsing
Can inspect values that were dynamically created

Limitations:

Cannot access private TypeScript type information (generics, union types, etc.) at runtime
Function parameter names may be minified in production builds
Complex TypeScript features are erased during compilation

For static analysis (parsing source code without execution), use ts-content.ts, which can analyze TypeScript files as text.

Performance Considerations

Reflection operations have some overhead:

Function analysis requires iterating over parameters
Provenance tracking adds metadata to objects
Deep value inspection can be expensive for large objects

Use caching when analyzing the same functions repeatedly, and consider lazy evaluation for expensive operations.

Limitations

Key Limitations:

No access to TypeScript type information - Types are erased at runtime, so you can't inspect generic parameters or union types
Minification affects parameter names - In production, function greet(name) might become function a(b)
Complex generics not captured - Runtime only sees JavaScript, not TypeScript's type system
Circular references - Deep value inspection may need special handling for circular object graphs

Testing

Run the test suite to verify functionality:

# Test all reflect modules
deno test lib/reflect/

# Test specific module
deno test lib/reflect/callable_test.ts

# Run with coverage
deno test --coverage=coverage lib/reflect/

Best Practices

Follow these best practices when using the reflect module:

Cache function analysis - If you analyze the same function multiple times, cache the results
Use type guards - Combine with TypeScript's type system for maximum safety
Provide context in errors - Use provenance tracking to give users helpful error messages
Document runtime limitations - Let users know when type information is unavailable
Test with minified code - Ensure your reflection-based code works in production builds

Summary

The Reflect module provides powerful runtime introspection capabilities for TypeScript/JavaScript code, enabling:

Function signature analysis
Source tracking and provenance
Deep value inspection
TypeScript source code analysis

These capabilities are essential for building tools that work with dynamic code and need to provide helpful debugging information.

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