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WebAssembly · 21 Workers and Wasm
Series contents · Technology map
Stage: Deployment and Execution Structure · External example · preview instructions · Storage ID: webassembly-step-21
Goals and learning outcomes
- Learning outcome: explain the role of Execution location. Input cases: the same large array and UI clicks.
- Deliverable: main-thread·Worker results and round-trip latency. Explain the rationale for handling Transfer cost, linking it to the code.
Prerequisites
Prepare the experiment results for Initialization cost and Concurrent requests from Unit 20, “Reusing Initialization”.
Key concepts and explanations
Execution location
A Wasm call occupies the thread on which it runs. Using Wasm does not automatically free the main thread for input.
Transfer cost
Sending arrays to a Worker requires copying or transfer. Account separately for copies into Wasm memory.
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 iframe preview is an experiment with WebAssembly.Memory and JS reference computations; do not present it as execution of an actual Wasm module. Practice with actual modules in a separate Node 22+ environment or a controlled local web project. Use kernel.wat·kernel.wasm·benchmark.mjs in the existing src/labs/webassembly-02·03 folders, and rebuild with the official wat2wasm tool when modifying WAT. Preserve CSP·isolation, and record measurements 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 day-21.mjs in a separate practice project, instantiate a module in an external Worker and return the result of processing the input array. Refer to the existing Unit 1·14·28 examples only as starting points.
- Input conditions: the same large array and UI clicks. Baseline: main thread. Run under the baseline conditions, then record observations and errors in notes/day-21.md.
- Variable: location; changed value: Worker. Run again through the same input path. Deliverable to save: main-thread·Worker results and round-trip latency. Hypothesis to check: Changes in UI responsiveness and total latency 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 |
|---|---|---|---|
| location | main thread | Worker | Changes in UI responsiveness and total latency 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: main-thread·Worker results and round-trip latency. 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: Execution location and Transfer cost. For unexpected items, at least 1 cause or reason they remain unresolved is recorded.
Self-check
- Which processing stage does Execution location affect in this exercise? Input cases: the same large array and UI clicks.
- Variable: location. Hypothesis to check: Changes in UI responsiveness and total latency may differ. Did the actual results agree with the hypothesis? Explain the evidence in terms of Transfer cost.
Official references
Linked example and execution scope
Example source · Runtime environments and limits
Standalone WAT + Node assertions; iframe CSP blocks Wasm compilation.
Source code and records are not overwritten automatically. Resetting an existing example also preserves your notes.
Next learning
Previous: Reusing Initialization · Next: Module Caching and Versions