Property keys and access

Problem

What becomes confusing, fragile, or impossible without understanding property keys and access? 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

Property keys and access belongs to “Objects are property systems”. Objects are property tables governed by keys, descriptors, extensibility, and accessors.

For property keys and access, 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: 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: Compare own keys, for-in, JSON, spread, freeze, seal, and preventExtensions.
4. Failure case: Write strict-mode read-only property and capture TypeError.

RESULT
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 property keys and access.
  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

Find the boundary: property keys and access

1. Create second case from blank file: Compare own keys, for-in, JSON, spread, freeze, seal, and preventExtensions.
2. Then create controlled failure: 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

• 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 property keys and access failure produces captured evidence; repair restores stated invariant without hiding error.

Hints

Solution

Attempt both exercises before opening reference approach.