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JavaScript · 16 Parallel Work and Partial Failure
Series contents · Technology map
Stage: Modules and data boundaries · Editable example · Storage ID: javascript-step-16
Goals and learning outcomes
- Distinguish completion order from the order of the returned array.
- The partial-failure screen retains 2 successful items.
Prerequisites
The work and experiment records from Unit 15, “Fetch Responses and HTTP Errors”
Key concepts and explanations
All Tasks Succeed
Promise.all returns successful results in input order. If one input rejects, the resulting Promise also rejects.
Partial Results
allSettled collects the success or failure of each operation. Decide whether to display only successful items as a product policy.
Practice setup
First check the execution scope guidance on the example screen. The following environment is for extending the assignment in a separate project. Implement it yourself in a separate local project and check it with browser developer tools. In a separate folder, connect your practice JS from index.html using script type="module" and open it through a local HTTP server. Use the Console, Network, and Elements panels.
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 batch.js, prepare different delays and a failure in the second operation.
- Run 3 delayed operations with all and allSettled, and output their result order and status.
- Run with different delays and a failure in the second operation. Results to check: Distinguish completion order from the order of the returned array. The partial-failure screen retains 2 successful items. Record expected and actual results separately.
Variables to observe
| Variable | Baseline | Change | Expected change / hypothesis |
|---|---|---|---|
| Number of failures | 0 | 1 | Hypothesis before running: It will be possible to separate 2 successes and 1 failure. Keep the other conditions fixed and record the actual result separately after running. |
Predictions in this table are not measured results. Change one condition at a time and record why the actual result differs.
Completion criteria
- Distinguish completion order from the order of the returned array.
- The partial-failure screen retains 2 successful items.
Self-check
- Does a failure in all cancel the remaining operations?
- How should partial failure be communicated?
Official references
Linked example and execution scope
Example source · Runtime environments and limits
Compare the editable inputs with actual browser execution results.
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Next learning
Previous: Fetch Responses and HTTP Errors · Next: Debouncing and Timer Cleanup