Freeze, seal, and extensibility

Problem

What becomes confusing, fragile, or impossible without understanding freeze, seal, and extensibility? 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

Freeze, seal, and extensibility belongs to “Objects are property systems”. freeze, seal, and extensibility is inspected by introducing one realistic failure and proving root cause before repair.

For freeze, seal, and extensibility, 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: Capture healthy freeze, seal, and extensibility baseline from: 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: Record freeze, seal, and extensibility behavior at: Compare own keys, for-in, JSON, spread, freeze, seal, and preventExtensions.
4. Failure case: Diagnose this exact freeze, seal, and extensibility failure before editing: Write strict-mode read-only property and capture TypeError.

RESULT
Evidence identifies first divergence, repair changes only proven cause, and rerun restores: 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 freeze, seal, and extensibility.
  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

Prove root cause: freeze, seal, and extensibility

1. Create second case from blank file: Record freeze, seal, and extensibility behavior at: Compare own keys, for-in, JSON, spread, freeze, seal, and preventExtensions.
2. Then create controlled failure: Diagnose this exact freeze, seal, and extensibility failure before editing: 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

• Evidence identifies first divergence, repair changes only proven cause, and rerun restores: 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 freeze, seal, and extensibility failure produces captured evidence; repair restores stated invariant without hiding error.

Hints

Solution

Attempt both exercises before opening reference approach.