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TypeScript · 23 Async Functions and Error Types

Series contents · Technology map

Stage: Module Contracts and Plugins · Editable example · Storage ID: typescript-step-23

Goals and learning outcomes

• Use a valid Note without assertions.
• Neither exception form leaves an unhandled failure.

Prerequisites

Work and experiment records from Unit 22, “DOM Types and null Checks”

Key concepts and explanations

Promise Contracts

An async function wraps its result in a Promise. External data still needs structural validation after await.

catch Values

A thrown value is not guaranteed to be an Error. Narrow unknown by checking for Error to create a message.

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 and tsc --noEmit. Record the installed tsc version in a separate TypeScript project. Enable strict in tsconfig.json and check with tsc --noEmit, then open the runtime exercise using compiled output or a local development server. Do not assume the browser executes TS type annotations directly.

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

1. In async.ts, prepare conditions for a valid response, an Error throw, and a string throw.
2. Create a loader returning Promise<Result<Note, string>> and handle Error and string throws.
3. Run with a valid response, an Error throw, and a string throw. Results to check: Use a valid Note without assertions. Neither exception form leaves an unhandled failure. Record expected and actual results separately.

Variables to observe

Predictions in this table are not measured results. Change one condition at a time and record why the actual result differs.

Completion criteria

• Use a valid Note without assertions.
• Neither exception form leaves an unhandled failure.

Self-check

1. Does a return type validate response JSON?
2. Why is reading message directly in catch risky?

Official references

• TypeScript Handbook

Linked example and execution scope

Example source · Runtime environments and limits

The preview is a JavaScript runtime boundary experiment. It does not run TypeScript diagnostics. Check lesson.ts locally with tsc --strict --noEmit --lib ES2023,DOM.

• example.js
• lesson.ts

Source code and records are not overwritten automatically. Resetting an existing example also preserves your notes.

Next learning

Previous: DOM Types and null Checks · Next: Modules and type-only import

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