Build a descriptor inspector

Problem

What becomes confusing, fragile, or impossible without understanding build a descriptor inspector? This lesson answers that through explanation, a worked example, two runnable exercises, and a reference solution. No teacher-supplied worksheet is required.

JavaScript began as a small browser scripting language, then accumulated object, functional, metaprogramming, module, and runtime capabilities while preserving early coercion rules.

Concept explanation

Build a descriptor inspector belongs to “Objects are property systems”. build a descriptor inspector ships as one thin, inspectable slice of an interactive object descriptor explorer.

For build a descriptor inspector, trace concrete input, state transition, output, and failure through a JavaScript semantic rule observable through values, property operations, scope, calls, or protocol steps.

Execution evaluates expressions against lexical environments. Objects are property tables linked through prototypes; functions are callable objects with closures. Apply that model to supplied normal, boundary, and failure cases; each case below names its input and expected evidence.

Example bank

Compare normal, boundary, failure, and cross-layer cases. Predict each observation before revealing the explanation.

Worked example

Start from supplied lab.mjs. Focus: Add build a descriptor inspector to an interactive object descriptor explorer; demo it with: Define enumerable, non-enumerable, writable, read-only, data, and accessor properties.

1. Run: node lab.mjs
2. Save baseline evidence. Use property descriptors, scope panels, call stacks, console probes, and deliberately tiny cases.
3. Boundary case: Document and test shipped slice under: Compare own keys, for-in, JSON, spread, freeze, seal, and preventExtensions.
4. Failure case: Add recovery behavior and a regression check for: Write strict-mode read-only property and capture TypeError.

RESULT
Fresh clone or blank directory can run documented slice and reproduce: Inspection reports descriptor flags and each enumeration/copy operation includes exact expected keys. Starter-level baseline: Two labeled records print in stable order; each exposes value, runtime type, and string representation.

Starter material

Create lab.mjs, paste this exact content, then run the command below.

const cases = [
  { label: "baseline", input: 0 },
  { label: "boundary", input: 1 },
];

function observe(value) {
  // Change only this function while investigating build a descriptor inspector.
  return { value, type: typeof value, text: String(value) };
}

for (const test of cases) console.log(test.label, observe(test.input));

STOP / CLEANUPScripts exit by themselves; no cleanup command required.

Independent exercise

Make it recoverable: build a descriptor inspector

1. Create second case from blank file: Document and test shipped slice under: Compare own keys, for-in, JSON, spread, freeze, seal, and preventExtensions.
2. Then create controlled failure: Add recovery behavior and a regression check for: Write strict-mode read-only property and capture TypeError.
3. Use Chromium Sources · console · hx · Node REPL to prove behavior, then repair controlled failure.
4. Compare result against supplied acceptance checks and reference approach before marking complete.

Expected result

• Fresh clone or blank directory can run documented slice and reproduce: Inspection reports descriptor flags and each enumeration/copy operation includes exact expected keys.
• Two labeled records print in stable order; each exposes value, runtime type, and string representation.
• Controlled build a descriptor inspector failure produces captured evidence; repair restores stated invariant without hiding error.

Hints

Solution

Attempt both exercises before opening reference approach.