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Web Workers · 07 Observing Main-Thread Responsiveness
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Stage: Messages and Execution Contexts · Editable example · Storage ID: web-workers-step-07
Goals and learning outcomes
- Learning outcome: explain the role of Separation. Input cases: the same array and clicks during computation.
- Deliverable: sums and input delays for both modes. Explain the rationale for handling Baseline experiment, linking it to the code.
Prerequisites
Prepare the experiment results for Task errors and Cleanup from Unit 6, “Error Response Contracts”.
Key concepts and explanations
Separation
A Worker moves CPU work off the main event loop. It does not guarantee a shorter total computation time.
Baseline experiment
Use the same input and computation on the main thread and Worker. Record UI input responsiveness together with result correctness.
Practice setup
First check the execution scope guidance on the example screen. The following describes the environment needed to extend the assignment in a separate project. The existing web-workers-01·02·03 are starting examples. Implement the new assignments in main.js and worker.js in a separate local HTTP project. Preserve CSP and iframe isolation. For SharedArrayBuffer, check COOP and COEP requirements on an external server you control; use the transfer path if they are not met. Measure performance yourself.
The linked example is a starting point. It does not implement every task below. Complete work beyond its supported scope in a separate project.
Practice tasks
- In src/day-7.js in a separate practice project, share the computation function and create an execution-mode control and a click counter. Refer to the existing Unit 1·14·28 examples only as starting points.
- Input conditions: the same array and clicks during computation. Baseline: Main thread. Run under the baseline conditions, then record observations and errors in notes/day-7.md.
- Variable: Mode; changed value: Worker. Run again through the same input path. Deliverable to save: sums and input delays for both modes. Hypothesis to check: Correctness is the same, while the pattern of input delays may differ. Compare it with the actual results, and record whether they agree and why they differ.
Variables to observe
| Variable | Baseline | Change | Expected change / hypothesis |
|---|---|---|---|
| Mode | Main thread | Worker | Correctness is the same, while the pattern of input delays may differ. |
Predictions in this table are not measured results. Change one condition at a time and record why the actual result differs.
Completion criteria
- Required deliverable: sums and input delays for both modes. Results are available for both the baseline and changed conditions, allowing comparison of 2 conditions; input values and the execution environment are recorded together.
- The explanation of differences between the two conditions connects both concepts: Separation and Baseline experiment. For unexpected items, at least 1 cause or reason they remain unresolved is recorded.
Self-check
- Which processing stage does Separation affect in this exercise? Input cases: the same array and clicks during computation.
- Variable: Mode. Hypothesis to check: Correctness is the same, while the pattern of input delays may differ. Did the actual results agree with the hypothesis? Explain the evidence in terms of Baseline experiment.
Official references
Linked example and execution scope
Example source · Runtime environments and limits
An interactive Observing Main-Thread Responsiveness experiment with cleanup of the resources it uses.
Source code and records are not overwritten automatically. Resetting an existing example also preserves your notes.
Next learning
Previous: Error Response Contracts · Next: Transfer Ownership