Enumerability

Problem

What becomes confusing, fragile, or impossible without understanding enumerability? 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

Enumerability belongs to “Objects are property systems”. enumerability must be connected to layer below by tracing representation, ownership, control, and failure. Objects are property tables governed by keys, descriptors, extensibility, and accessors.

For enumerability, 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: Trace enumerability end-to-end while running: 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: At every enumerability boundary, label owner and representation during: Compare own keys, for-in, JSON, spread, freeze, seal, and preventExtensions.
4. Failure case: Locate first layer where evidence diverges during: Write strict-mode read-only property and capture TypeError.

RESULT
Trace explains Inspection reports descriptor flags and each enumeration/copy operation includes exact expected keys. without skipping any conversion, queue, process, protocol, or storage boundary. 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 enumerability.
  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

Remove one abstraction: enumerability

1. Create second case from blank file: At every enumerability boundary, label owner and representation during: Compare own keys, for-in, JSON, spread, freeze, seal, and preventExtensions.
2. Then create controlled failure: Locate first layer where evidence diverges during: 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

• Trace explains Inspection reports descriptor flags and each enumeration/copy operation includes exact expected keys. without skipping any conversion, queue, process, protocol, or storage boundary.
• Two labeled records print in stable order; each exposes value, runtime type, and string representation.
• Controlled enumerability failure produces captured evidence; repair restores stated invariant without hiding error.

Hints

Solution

Attempt both exercises before opening reference approach.