JAVASCRIPT ENGINEERING
Performance & Debugging Master Lab
A sales dashboard slows down as records grow. Measure the baseline, edit the aggregation code, then prove the faster version returns the same totals.
Root cause, repair and extension
Source editor
Execution inspector
The worker records actual function entry, return, exceptions and explicit trace calls. Add trace("name", value) to inspect a variable or event.
Regression and timing results
Explain and reflect
Pass the case’s checks and explain your result before saving.
AI prompt studio
DOM performance
Compare repeated DOM writes with layout reads against a single fragment insertion. Both paths render the same rows.
Event loop and retention
Registry entries retain references until removed. The count represents retained references. It is not a heap measurement.
Network debugging
Requests and response objects are real. The transport is a local simulation with no network calls.
Case library
Portfolio & export
Weekly debugging case
Verified repairs
Saved investigations
Downloads
Learn to verify a JavaScript repair
Work with instrumented JavaScript, browser APIs, and before-and-after correctness checks.
Browse interactive guides · JavaScript learning path