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WebAssembly · 06 I32 and JS Number
Series contents · Technology map
Stage: Modules and Numeric Contracts · External example · preview instructions · Storage ID: webassembly-step-06
Goals and learning outcomes
- Learning outcome: explain the role of Numeric representation. Input cases: small numbers and values near INT32_MAX.
- Deliverable: expected·actual sums for each numeric range. Explain the rationale for handling Overflow, linking it to the code.
Prerequisites
Prepare the experiment results for Import and Boundary cost from Unit 5, “Imports and Host Functions”.
Key concepts and explanations
Numeric representation
i32 performs 32-bit integer arithmetic. Its range differs from arithmetic on large integers with JS Number.
Overflow
i32 addition wraps at the defined bit width. The JS reference must use the same contract for a valid 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-6.mjs in a separate practice project, pass large integers to sum and apply explicit 32-bit conversion in the JS reference. Refer to the existing Unit 1·14·28 examples only as starting points.
- Input conditions: small numbers and values near INT32_MAX. Baseline: small positive integers. Run under the baseline conditions, then record observations and errors in notes/day-6.md.
- Variable: input range; changed value: overflow boundary. Run again through the same input path. Deliverable to save: expected·actual sums for each numeric range. Hypothesis to check: The difference between a regular JS sum and an i32 sum is exposed. 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 range | small positive integers | overflow boundary | The difference between a regular JS sum and an i32 sum is exposed. |
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 sums for each numeric range. 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: Numeric representation and Overflow. For unexpected items, at least 1 cause or reason they remain unresolved is recorded.
Self-check
- Which processing stage does Numeric representation affect in this exercise? Input cases: small numbers and values near INT32_MAX.
- Variable: input range. Hypothesis to check: The difference between a regular JS sum and an i32 sum is exposed. Did the actual results agree with the hypothesis? Explain the evidence in terms of Overflow.
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: Imports and Host Functions · Next: Traps and Input Validation