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TypeScript · 13 Constraints and Minimal Contracts
Series contents · Technology map
Stage: Unions and Generics · Editable example · Storage ID: typescript-step-13
Goals and learning outcomes
- Additional field types are preserved for two kinds of objects.
- Record the diagnostic for input without id.
Prerequisites
Work and experiment records from Unit 12, “Generic Functions and Inference”
Key concepts and explanations
Generic Constraints
Use extends to constrain the structure a function needs. The return type can preserve a call's more specific type beyond the constraint.
Excessive Contracts
Requiring unused fields limits reuse. Constrain only the minimum properties the implementation needs.
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
- In constraints.ts, prepare conditions for a note object, a user object, and an object without id.
- Write a function that indexes objects with id while preserving additional field types.
- Run with a note object, a user object, and an object without id. Results to check: Additional field types are preserved for two kinds of objects. Record the diagnostic for input without id. Record expected and actual results separately.
Variables to observe
| Variable | Baseline | Change | Expected change / hypothesis |
|---|---|---|---|
| Input Structure | has id | no id | Pre-run hypothesis: the call will be rejected for violating the constraint. Keep 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
- Additional field types are preserved for two kinds of objects.
- Record the diagnostic for input without id.
Self-check
- What is restricted if the constraint is the entire Note type?
- Does returning the constraint type itself preserve specific information?
Official references
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.
Source code and records are not overwritten automatically. Resetting an existing example also preserves your notes.
Next learning
Previous: Generic Functions and Inference · Next: Generics and Boundary Validation