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WebAssembly · 15 In-place Transformation
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Stage: Deployment and Execution Structure · External example · preview instructions · Storage ID: webassembly-step-15
Goals and learning outcomes
- Learning outcome: explain the role of Overwriting. Input cases: arrays containing negative numbers·0·positive numbers.
- Deliverable: expected·actual values for every element. Explain the rationale for handling Repeat conditions, linking it to the code.
Prerequisites
Prepare the experiment results for Memory layout and View lifetime from Unit 14, “Arrays and Boundary Costs”.
Key concepts and explanations
Overwriting
transform modifies input memory directly. A comparison that needs the original requires a copy.
Repeat conditions
Feeding transformed results back in makes values keep growing. Restore the original on every iteration for a fair comparison.
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-15.mjs in a separate practice project, set source before the existing transform(0,n) and compare with the JS result for 2x+1. Refer to the existing Unit 1·14·28 examples only as starting points.
- Input conditions: arrays containing negative numbers·0·positive numbers. Baseline: none. Run under the baseline conditions, then record observations and errors in notes/day-15.md.
- Variable: input restoration; changed value: restore every iteration. Run again through the same input path. Deliverable to save: expected·actual values for every element. Hypothesis to check: Each iteration compares the same computation. 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 restoration | none | restore every iteration | Each iteration compares the same computation. |
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: expected·actual values for every element. 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: Overwriting and Repeat conditions. For unexpected items, at least 1 cause or reason they remain unresolved is recorded.
Self-check
- Which processing stage does Overwriting affect in this exercise? Input cases: arrays containing negative numbers·0·positive numbers.
- Variable: input restoration. Hypothesis to check: Each iteration compares the same computation. Did the actual results agree with the hypothesis? Explain the evidence in terms of Repeat conditions.
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: Arrays and Boundary Costs · Next: Aligned and Unaligned Access