GROUND
Problem
What becomes confusing, fragile, or impossible without understanding delegation? This lesson answers that through explanation, a worked example, two runnable exercises, and a reference solution. No teacher-supplied worksheet is required.
Browser APIs grew around the DOM to expose observation, storage, workers, files, history, devices, and offline capabilities under a permissioned security model.
LEARN
Concept explanation
Delegation belongs to “DOM events and ownership”. delegation is learned by comparing readable, constrained, and deliberately clever implementations against same evidence.
For delegation, trace concrete input, state transition, output, and failure through a permissioned browser capability with explicit ownership, lifecycle, event-loop, and storage boundaries.
The browser is a multi-process capability host. APIs cross security, event-loop, storage, and lifecycle boundaries. Apply that model to supplied normal, boundary, and failure cases; each case below names its input and expected evidence.
Evidence produced by the delegation experiment: output, state, trace, bytes, timing, or diagnostics.
Condition that must remain true while inputs or implementation of delegation change.
Point where delegation crosses ownership, representation, time, process, network, or trust.
Example bank
Compare normal, boundary, failure, and cross-layer cases. Predict each observation before revealing the explanation.
SETUPSolve delegation three ways using identical fixture: Log listeners on nested elements in capture and bubble phases.
OBSERVEAll versions preserve Log names currentTarget, target, phase, order, and default-action outcome for each case.; comparison records code size, predictability, and debugging cost.
WHY IT MATTERSThis isolates the normal contract of delegation; preserve its raw evidence as the control for every later comparison.
SETUPRun all three implementations against: Use delegation with dynamically added target and shadow boundary if available.
OBSERVERecord what remains invariant and the first representation, owner, size, or timing value that changes in Chromium Sources · Application · Performance · Media.
WHY IT MATTERSA boundary example is useful only when one named dimension changes and everything else stays comparable.
SETUPForce every implementation through: Prevent default or stop propagation at one point and predict missing effects.
OBSERVECapture the first divergence from the baseline, including exact input, diagnostic, state, and recovery result. Expected recovery: All versions preserve Log names currentTarget, target, phase, order, and default-action outcome for each case.; comparison records code size, predictability, and debugging cost.
WHY IT MATTERSThe diagnostic is part of the interface. Repair the proven cause, not the most visible symptom.
SETUPTrace delegation one layer below its usual abstraction through a permissioned browser capability with explicit ownership, lifecycle, event-loop, and storage boundaries.
OBSERVEUse DOM/event breakpoints, Application storage, observer logs, worker inspection, permissions, and lifecycle events.
WHY IT MATTERSThe lower layer is earned when it explains evidence the current layer cannot. Otherwise keep delegation at the simpler boundary.
SEE
Worked example
Start from supplied index.html. Focus: Solve delegation three ways using identical fixture: Log listeners on nested elements in capture and bubble phases.
- Run: Serve the directory locally, open index.html, click Run, then inspect relevant browser panels.
- Save baseline evidence. Use DOM/event breakpoints, Application storage, observer logs, worker inspection, permissions, and lifecycle events.
- Boundary case: Run all three implementations against: Use delegation with dynamically added target and shadow boundary if available.
- Failure case: Force every implementation through: Prevent default or stop propagation at one point and predict missing effects.
RESULT
All versions preserve Log names currentTarget, target, phase, order, and default-action outcome for each case.; comparison records code size, predictability, and debugging cost. Starter-level baseline: Button changes log from idle to a JSON record containing the lesson topic and a numeric timestamp.
START HERE
Starter material
PREREQUISITESNode.js 22+ and a modern browser. Local server is required for storage, workers, or permissioned APIs.
ONE-TIME SETUPSave index.html, then run npx --yes serve . and open the printed localhost URL.
Create index.html, paste this exact content, then run the command below.
<!doctype html>
<meta charset="utf-8">
<title>delegation</title>
<button id="run">Run delegation probe</button>
<pre id="log">idle</pre>
<script>
const log = document.querySelector('#log');
document.querySelector('#run').addEventListener('click', () => {
log.textContent = JSON.stringify({ topic: "delegation", time: performance.now() }, null, 2);
});
</script>Serve the directory locally, open index.html, click Run, then inspect relevant browser panels.STOP / CLEANUPReturn to server terminal and press Ctrl+C.
DO WITH GUIDANCE
Guided exercise
Solve under constraint: delegation
- Normal case: Solve delegation three ways using identical fixture: Log listeners on nested elements in capture and bubble phases.
- Write predicted evidence from this named case before running starter.
- Keep readable baseline, then remove one convenience without hiding behavior.
- Run exact normal case. Save commands, inputs, outputs, and diagnostics in notebook.
- Explain changed evidence using lesson mental model in no more than five sentences.
Concrete guided solution
- Copy the supplied index.html unchanged and run: Serve the directory locally, open index.html, click Run, then inspect relevant browser panels.
- Write this prediction before inspecting output: All versions preserve Log names currentTarget, target, phase, order, and default-action outcome for each case.; comparison records code size, predictability, and debugging cost.
- Perform only the named normal case: Solve delegation three ways using identical fixture: Log listeners on nested elements in capture and bubble phases.
- Save the raw output, then annotate input → transition → evidence. Use Chromium Sources · Application · Performance · Media to confirm the transition rather than inferring it.
- Compare prediction with evidence; if they differ, keep both and write the rule that explains the difference. Reference baseline: Button changes log from idle to a JSON record containing the lesson topic and a numeric timestamp.
DO ALONE
Independent exercise
Compare three versions: delegation
- Create second case from blank file: Run all three implementations against: Use delegation with dynamically added target and shadow boundary if available.
- Then create controlled failure: Force every implementation through: Prevent default or stop propagation at one point and predict missing effects.
- Use Chromium Sources · Application · Performance · Media to prove behavior, then repair controlled failure.
- Compare result against supplied acceptance checks and reference approach before marking complete.
Concrete independent solution
- Duplicate the starter into a clean comparison case; change only this boundary: Run all three implementations against: Use delegation with dynamically added target and shadow boundary if available.
- Save its evidence beside the baseline and identify the first changed value. Use DOM/event breakpoints, Application storage, observer logs, worker inspection, permissions, and lifecycle events.
- Create the exact controlled failure: Force every implementation through: Prevent default or stop propagation at one point and predict missing effects.
- Version A: use clearest native primitives and name every intermediate state for delegation.
- Version B: remove one convenience while preserving same input/output contract.
- Version C: compress only after A and B pass baseline: Log listeners on nested elements in capture and bubble phases.
- Run shared boundary and failure fixtures against all versions: Use delegation with dynamically added target and shadow boundary if available. / Prevent default or stop propagation at one point and predict missing effects.
- Keep A unless another version has measured benefit; expected invariant: Log names currentTarget, target, phase, order, and default-action outcome for each case.
- Rerun baseline, boundary, and repaired failure together. Accept only if all reproduce: All versions preserve Log names currentTarget, target, phase, order, and default-action outcome for each case.; comparison records code size, predictability, and debugging cost.
COMPARE
Expected result
- All versions preserve Log names currentTarget, target, phase, order, and default-action outcome for each case.; comparison records code size, predictability, and debugging cost.
- Button changes log from idle to a JSON record containing the lesson topic and a numeric timestamp.
- Controlled delegation failure produces captured evidence; repair restores stated invariant without hiding error.
PROVE
Acceptance checks
Lesson is complete only when every check is true. Each check is stored locally and travels with your JSON backup.
0/5 complete · saved on this device
UNSTICK
Hints
Reveal hints
- Start with supplied normal case exactly as written: Solve delegation three ways using identical fixture: Log listeners on nested elements in capture and bubble phases.
- For boundary case, change only named dimension: Run all three implementations against: Use delegation with dynamically added target and shadow boundary if available.
- If result is confusing, diff raw inputs and evidence before editing implementation.
- If tool shows nothing useful, move observation one boundary lower: representation, runtime, OS, or network.
VERIFY
Solution
Attempt both exercises before opening reference approach.
Reveal reference solution
- Run unmodified starter and preserve baseline evidence: Button changes log from idle to a JSON record containing the lesson topic and a numeric timestamp.
- Version A: use clearest native primitives and name every intermediate state for delegation.
- Version B: remove one convenience while preserving same input/output contract.
- Version C: compress only after A and B pass baseline: Log listeners on nested elements in capture and bubble phases.
- Run shared boundary and failure fixtures against all versions: Use delegation with dynamically added target and shadow boundary if available. / Prevent default or stop propagation at one point and predict missing effects.
- Keep A unless another version has measured benefit; expected invariant: Log names currentTarget, target, phase, order, and default-action outcome for each case.
PREDICT · INSPECT · BREAK · DEBUG · MEASURE
Interrogate reality
Prediction: write expected output, state transition, ordering, and failure evidence before running either exercise.
Inspection: Use event breakpoints, DOM breakpoints, Application storage panels, worker inspection, permissions, and performance traces.
Measurement: Measure main-thread blocking, storage cost, observer churn, worker transfer overhead, and lifecycle reliability.
Capture raw evidence before explaining.
Change one assumption and force controlled failure.
Find cause with Chromium Sources · Application · Performance · Media before editing fix.
MASTERY + FRONTIER + BOUNDARY
Own the knowledge
Explain delegation at beginner, intermediate, and senior depth.
Recreate smallest useful example from blank file without notes or AI.
Schedule recall for day 1, 7, 30, and 90.
Creative frontier lab
Try first without opening the solutions. The constraints invite invention; the reference gives one concrete direction, never the only valid answer.
Re-solve delegation by removing the most convenient abstraction. provide a reload-safe and offline-capable path before adding a server.
CONSTRAINTKeep the same inputs, observable result, and failure evidence; change the means, not the contract.
ORIGINAL IDEATurn subtraction into a design tool: the missing abstraction should reveal which responsibility it used to hide.
Reveal frontier solution
- Freeze the contract as three fixtures: Solve delegation three ways using identical fixture: Log listeners on nested elements in capture and bubble phases. / Run all three implementations against: Use delegation with dynamically added target and shadow boundary if available. / Force every implementation through: Prevent default or stop propagation at one point and predict missing effects.
- List every convenience used by the starter; remove the highest-level one while preserving Serve the directory locally, open index.html, click Run, then inspect relevant browser panels..
- Implement the smallest replacement using provide a reload-safe and offline-capable path before adding a server.
- Run all fixtures and compare raw evidence. Keep the simpler version unless the removed abstraction has a demonstrated benefit.
Build an explanation artifact for delegation: expose lifecycle, permission, event, storage, and cross-tab transitions in an on-page event journal.
CONSTRAINTA peer must be able to locate the first divergence without reading implementation code.
ORIGINAL IDEATreat the explanation itself as a product: make invisible transitions visible, replayable, and diffable.
Reveal frontier solution
- Create one row or timestamped event for each transition in: Solve delegation three ways using identical fixture: Log listeners on nested elements in capture and bubble phases.
- For every row record input, representation, owner, operation, output, and tool evidence from Chromium Sources · Application · Performance · Media.
- Replay Run all three implementations against: Use delegation with dynamically added target and shadow boundary if available.; highlight only changed rows.
- Replay Force every implementation through: Prevent default or stop propagation at one point and predict missing effects.; stop at the first divergent row and attach its recovery action.
Combine the boundary and failure into a new user-visible scenario for delegation. make two tabs coordinate without silently overwriting either user's state.
CONSTRAINTDo not merely add more input. Invent a recovery interaction, alternate representation, or self-checking behavior.
ORIGINAL IDEAMake the system teach its own limits: the artifact should expose the invariant and offer a safe next action when it breaks.
Reveal frontier solution
- Combine these two pressures without changing them: Run all three implementations against: Use delegation with dynamically added target and shadow boundary if available. AND Force every implementation through: Prevent default or stop propagation at one point and predict missing effects.
- Name the invariant that must survive and the user-visible evidence when it cannot: All versions preserve Log names currentTarget, target, phase, order, and default-action outcome for each case.; comparison records code size, predictability, and debugging cost.
- Implement this original direction: make two tabs coordinate without silently overwriting either user's state.
- Demonstrate baseline, combined failure, recovery, then baseline again; save the sequence as a regression fixture.
Capability frontier
Push delegation until another layer becomes justified. Record one robust technique, one contextual trade-off, and one labeled hack or historical curiosity.
CORE · PRACTICAL · CONTEXTUAL · HACK · FRAGILE · HISTORICAL · GOLF
Boundary
Local browser capabilities do not automatically provide shared truth, durable multi-device state, or trusted authority.
If this vanished tomorrow…
Use server round trips, file import/export, plain media elements, and lower-level platform primitives.
Why next layer is earned
Types are earned when growing browser state and contracts become difficult to keep valid by convention alone.