TLDR

In non-bundled script-tag projects (<script src="...">), files can exist in the repository, be actively maintained, and be referenced in architecture comments while never actually executing in the browser because the <script> tag was omitted or deleted. When consuming modules use defensive fallbacks (window.SPECS || {}), the missing dependency fails silently without throwing console errors. Verifying dependency presence explicitly with dev-warnings or ES module imports eliminates this entire class of bugs.

Dependency Pattern	Runtime Error Behavior	Dev Experience	Structural Reliability
Defensive Silent Fallback	Silent fallback (		{})
Asserted Script Loading	Loud dev console warning	Immediate missing script feedback	Reliable & Self-Healing

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Problem statement: the unloaded lookup table defect

During rule engine development for component pin linting, we referenced window.COMPONENT_SPECS, a data table containing symbol pin counts, pin roles, and default parameters:

/* Component Specs — Electrical Domain Lookup Table
   Keyed by data-symbol attribute value.
   Loaded only in electrical/index.html; other domains are unaffected. */
window.COMPONENT_SPECS = {
  resistor: { pinCount: 2, pins: ['1', '2'] },
  capacitor: { pinCount: 2, pins: ['1', '2'] }
};

However, running a grep search across the codebase for script tag inclusions yielded zero matches:

$ grep -rn "component-specs.js" --include='.html' --include='.js' .
(Zero results found!)

The script file existed, but no HTML file loaded it. Features reading window.COMPONENT_SPECS hit defensive fallback paths (undefined), silently skipping spec validation checks for months.

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Technical failure mode: silent defensive fallbacks

Defensive runtime checks mask un-loaded dependency bugs:

// DEFECTIVE CONSUMER: Silent fallback hides missing script tag
function getComponentSpec(symbolType) {
  const specs = window.COMPONENT_SPECS || {}; // Silently evaluates to {}
  return specs[symbolType]; // Returns undefined silently!
}

Because specs evaluated to {} when component-specs.js was omitted, the application executed without throwing errors, while spec rules remained 100% inert.

---

The fix & architecture: explicit inclusions & Dev warnings

Step 1: include the script entry

Add the script tag to host HTML entry points prior to feature script execution:

<!-- Load component spec data table before consuming feature scripts -->
<script src="/assets/schema-editor/data/component-specs.js"></script>
<script src="/assets/schema-editor/js/features/ercEngine.js"></script>

Step 2: explicit consumer assertion warnings

Replace silent defensive fallbacks with explicit console warnings during development:

// REFACTORED CONSUMER: Loud development assertion
function getComponentSpec(symbolType) {
  if (!window.COMPONENT_SPECS) {
    console.warn('[ERC Engine] Warning: window.COMPONENT_SPECS is not loaded! Spec-based rules are inert.');
  }
const specs = window.COMPONENT_SPECS || {};   return specs[symbolType]; }

Step 3: migration to native ES modules

Migrate no-build script setups to native ES module imports to make dependencies toolchain-enforced:

// Modern ES Module Dependency (Enforced at runtime)
import { COMPONENT_SPECS } from '../data/component-specs.js';

Rule of thumb: Never rely on code comments to document script loading requirements. Enforce script dependencies using ES module import statements or loud runtime initialization warnings.