Function prototype objects

Problem

What becomes confusing, fragile, or impossible without understanding function prototype objects? 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

Function prototype objects belongs to “Prototype lookup and construction”. Property lookup walks prototype links; new connects allocation, prototype, call, and returned value. This lesson turns function prototype objects into a working part of weekly fixture.

For function prototype objects, 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: Build function prototype objects around this exact fixture: Build chain with Object.create and inspect own versus inherited 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: Add function prototype objects support for weekly boundary case: Replace constructor prototype after one instance exists.
4. Failure case: Make function prototype objects surface—not hide—this failure: Create cycle attempt or broken constructor property and explain invariant.

RESULT
function prototype objects handles baseline and boundary while preserving this observable result: Lookup path, instanceof result, and simplified new implementation match native behavior for tested cases. 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 function prototype objects.
  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

Add one real case: function prototype objects

1. Create second case from blank file: Add function prototype objects support for weekly boundary case: Replace constructor prototype after one instance exists.
2. Then create controlled failure: Make function prototype objects surface—not hide—this failure: Create cycle attempt or broken constructor property and explain invariant.
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

• function prototype objects handles baseline and boundary while preserving this observable result: Lookup path, instanceof result, and simplified new implementation match native behavior for tested cases.
• Two labeled records print in stable order; each exposes value, runtime type, and string representation.
• Controlled function prototype objects failure produces captured evidence; repair restores stated invariant without hiding error.

Hints

Solution

Attempt both exercises before opening reference approach.