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WebAssembly · 18 String Byte Contracts
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Stage: Deployment and Execution Structure · External example · preview instructions · Storage ID: webassembly-step-18
Goals and learning outcomes
- Learning outcome: explain the role of Encoding. Input cases: English·Korean·emoji.
- Deliverable: character counts·byte counts·round-trip strings. Explain the rationale for handling Passing length, linking it to the code.
Prerequisites
Prepare the experiment results for Separate regions and Ownership rules from Unit 17, “Laying Out Multiple Arrays”.
Key concepts and explanations
Encoding
JS string length differs from UTF-8 byte length. Korean characters and emoji use multiple bytes.
Passing length
A string address alone does not identify its end. Specify byteLength or a termination convention.
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-18.mjs in a separate practice project, write strings to memory with TextEncoder and restore the specified range with TextDecoder. Refer to the existing Unit 1·14·28 examples only as starting points.
- Input conditions: English·Korean·emoji. Baseline: character count. Run under the baseline conditions, then record observations and errors in notes/day-18.md.
- Variable: length basis; changed value: byteLength. Run again through the same input path. Deliverable to save: character counts·byte counts·round-trip strings. Hypothesis to check: Using byteLength restores the string without truncation. Compare it with the actual results, and record whether they agree and why they differ.
Variables to observe
| Variable | Baseline | Change | Expected change / hypothesis |
|---|---|---|---|
| length basis | character count | byteLength | Using byteLength restores the string without truncation. |
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: character counts·byte counts·round-trip strings. 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: Encoding and Passing length. For unexpected items, at least 1 cause or reason they remain unresolved is recorded.
Self-check
- Which processing stage does Encoding affect in this exercise? Input cases: English·Korean·emoji.
- Variable: length basis. Hypothesis to check: Using byteLength restores the string without truncation. Did the actual results agree with the hypothesis? Explain the evidence in terms of Passing length.
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: Laying Out Multiple Arrays · Next: Browser Loading and MIME