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WebAssembly · 07 Traps and Input Validation
Series contents · Technology map
Stage: Modules and Numeric Contracts · External example · preview instructions · Storage ID: webassembly-step-07
Goals and learning outcomes
- Learning outcome: explain the role of Trap. Input cases: valid·negative length·out of bounds.
- Deliverable: rejection reasons and RuntimeError records. Explain the rationale for handling Preflight checks, linking it to the code.
Prerequisites
Prepare the experiment results for Numeric representation and Overflow from Unit 6, “I32 and JS Number”.
Key concepts and explanations
Trap
Out-of-bounds memory access can cause a runtime trap. Catch it as a JS exception and distinguish it from user errors.
Preflight checks
Validate pointers and lengths before calling. Do not overuse traps as normal input validation.
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-7.mjs in a separate practice project, add integer·negative-value·bounds checks to a sum call wrapper and classify errors. Refer to the existing Unit 1·14·28 examples only as starting points.
- Input conditions: valid·negative length·out of bounds. Baseline: none. Run under the baseline conditions, then record observations and errors in notes/day-7.md.
- Variable: validation; changed value: preflight checks. Run again through the same input path. Deliverable to save: rejection reasons and RuntimeError records. Hypothesis to check: Invalid inputs are rejected before the module call. Compare it with the actual results, and record whether they agree and why they differ.
Variables to observe
| Variable | Baseline | Change | Expected change / hypothesis |
|---|---|---|---|
| validation | none | preflight checks | Invalid inputs are rejected before the module call. |
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: rejection reasons and RuntimeError records. 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: Trap and Preflight checks. For unexpected items, at least 1 cause or reason they remain unresolved is recorded.
Self-check
- Which processing stage does Trap affect in this exercise? Input cases: valid·negative length·out of bounds.
- Variable: validation. Hypothesis to check: Invalid inputs are rejected before the module call. Did the actual results agree with the hypothesis? Explain the evidence in terms of Preflight checks.
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.