Proxy traps

Problem

What becomes confusing, fragile, or impossible without understanding Proxy traps? 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

Proxy traps belongs to “Metaprogramming without magic”. Symbols, Proxy, and Reflect expose metaprogramming hooks while preserving language invariants. This lesson turns Proxy traps into a working part of weekly fixture.

For Proxy traps, 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 Proxy traps around this exact fixture: Proxy get/set/ownKeys while delegating through Reflect and recording operations.

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 Proxy traps support for weekly boundary case: Use symbol key and revocable proxy.
4. Failure case: Make Proxy traps surface—not hide—this failure: Violate non-configurable ownKeys invariant and capture TypeError.

RESULT
Proxy traps handles baseline and boundary while preserving this observable result: Operation log matches access sequence; revocation and invariant enforcement are proven. 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 Proxy traps.
  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: Proxy traps

1. Create second case from blank file: Add Proxy traps support for weekly boundary case: Use symbol key and revocable proxy.
2. Then create controlled failure: Make Proxy traps surface—not hide—this failure: Violate non-configurable ownKeys invariant 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

• Proxy traps handles baseline and boundary while preserving this observable result: Operation log matches access sequence; revocation and invariant enforcement are proven.
• Two labeled records print in stable order; each exposes value, runtime type, and string representation.
• Controlled Proxy traps failure produces captured evidence; repair restores stated invariant without hiding error.

Hints

Solution

Attempt both exercises before opening reference approach.