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Web Workers · 03 Message Contracts
Series contents · Technology map
Stage: Messages and Execution Contexts · Editable example · Storage ID: web-workers-step-03
Goals and learning outcomes
- Learning outcome: explain the role of Tags. Input cases: valid input, a missing ID, and an array of strings.
- Deliverable: an acceptance/rejection table for each input. Explain the rationale for handling Validation, linking it to the code.
Prerequisites
Prepare the experiment results for Module Worker and Loading failure from Unit 2, “Module Worker Loading”.
Key concepts and explanations
Tags
type distinguishes message meanings. Interpreting every message as the same data shape causes errors.
Validation
Values received by a Worker also require validation. Check requestId and the array type before computing.
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-3.js in a separate practice project, create run, result, and error message structures and an incoming-message validation function. Refer to the existing Unit 1·14·28 examples only as starting points.
- Input conditions: valid input, a missing ID, and an array of strings. Baseline: Integer array. Run under the baseline conditions, then record observations and errors in notes/day-3.md.
- Variable: Input; changed value: Invalid object. Run again through the same input path. Deliverable to save: an acceptance/rejection table for each input. Hypothesis to check: Only the invalid request is rejected, and the next request is processed. Compare it with the actual results, and record whether they agree and why they differ.
Variables to observe
| Variable | Baseline | Change | Expected change / hypothesis |
|---|---|---|---|
| Input | Integer array | Invalid object | Only the invalid request is rejected, and the next request is processed. |
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: an acceptance/rejection table for each input. 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: Tags and Validation. For unexpected items, at least 1 cause or reason they remain unresolved is recorded.
Self-check
- Which processing stage does Tags affect in this exercise? Input cases: valid input, a missing ID, and an array of strings.
- Variable: Input. Hypothesis to check: Only the invalid request is rejected, and the next request is processed. Did the actual results agree with the hypothesis? Explain the evidence in terms of Validation.
Official references
Linked example and execution scope
Example source · Runtime environments and limits
An interactive Message Contracts experiment with cleanup of the resources it uses.
Source code and records are not overwritten automatically. Resetting an existing example also preserves your notes.