Build a Practical Unions and Narrowing Example in TypeScript Fundamentals

Define the build target

For Unions and Narrowing, model a success/error result as a discriminated union, narrow it in a function, and verify TypeScript rejects an impossible property access. Build the boundary case using this implementation lens: Use TypeScript source types, inference, unions, narrowing, interfaces, generics, compiler diagnostics, emitted JavaScript, and project configuration.

Keep the Unions and Narrowing build centered on these technical constraints: Static type checking versus runtime values. Union types and discriminants. Apply them through this path lens: Use TypeScript source types, inference, unions, narrowing, interfaces, generics, compiler diagnostics, emitted JavaScript, and project configuration. Use TypeScript source types, inference, unions, narrowing, interfaces, generics, compiler diagnostics, emitted JavaScript, and project configuration.

Implement the core behavior

Implement Unions and Narrowing around the module artifact—a working unions and narrowing exercise with a documented technical result—and keep the implementation specific to this path context: Use TypeScript source types, inference, unions, narrowing, interfaces, generics, compiler diagnostics, emitted JavaScript, and project configuration.

const rawAge = '27';
const age = Number(rawAge);
const profile = { age, active: true };
console.log(typeof age, profile);

node example.js

A concrete JavaScript value or error that demonstrates the module behavior.

practice/\n├── README.md\n├── unions-and-narrowing-build.js\n└── evidence/\n    └── expected-result.txt

Run the complete path

Run one realistic Unions and Narrowing case end to end and record the required evidence: the module-specific command, output, test, rendered state, query result, log, or measurement that proves the exercise worked. Interpret the result through this path context: Use TypeScript source types, inference, unions, narrowing, interfaces, generics, compiler diagnostics, emitted JavaScript, and project configuration.

Change one meaningful condition

Modify one condition central to Unions and Narrowing using this path context: Use TypeScript source types, inference, unions, narrowing, interfaces, generics, compiler diagnostics, emitted JavaScript, and project configuration. Predict the new result before rerunning the same workflow.

Verify the artifact

Your deliverable is a working unions and narrowing exercise with a documented technical result.

Common mistakes to avoid

• Using any removes useful checking.
• Type assertion used instead of validation.
• Union not narrowed before property access.
• Assuming TypeScript validates runtime JSON.