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JavaScript · 10 Exceptions and Recovery Paths
Series contents · Technology map
Stage: DOM and asynchronous operations · Editable example · Storage ID: javascript-step-10
Goals and learning outcomes
- The busy state ends after both success and failure.
- Read valid input again after a failure.
Prerequisites
The work and experiment records from Unit 9, “Map and Set Indexes”
Key concepts and explanations
Exception Propagation
throw transfers control to the nearest catch. In catch, recover or add context and pass the exception on.
Guaranteed Cleanup
finally runs after success and failure. A return here can override the original return value or exception.
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 parse.js, prepare valid JSON and JSON with a missing closing bracket.
- Wrap JSON.parse in try/catch and turn off the progress indicator in finally.
- Run with valid JSON and JSON with a missing closing bracket. Results to check: The busy state ends after both success and failure. Read valid input again after a failure. Record expected and actual results separately.
Variables to observe
| Variable | Baseline | Change | Expected change / hypothesis |
|---|---|---|---|
| JSON | Valid | Missing bracket | Hypothesis before running: The progress indicator will turn off after the failure message. 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
- The busy state ends after both success and failure.
- Read valid input again after a failure.
Self-check
- What information does an empty catch lose?
- Why must you be careful with return in finally?
Official references
Linked example and execution scope
Example source · Runtime environments and limits
Compare the editable inputs with actual browser execution results.
Source code and records are not overwritten automatically. Resetting an existing example also preserves your notes.
Next learning
Previous: Map and Set Indexes · Next: Creating DOM Nodes and Inserting Text