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TypeScript · 03 Inference and Literal Types
Series contents · Technology map
Stage: Type Checking and Data Models · Editable example · Storage ID: typescript-step-03
Goals and learning outcomes
- Record the inferred types of the three declarations.
- A call with an invalid state string fails to compile.
Prerequisites
Work and experiment records from Unit 2, “strict Settings and Executable Code”
Key concepts and explanations
Type Inference
Variable types are inferred from initial values. let and const may preserve literals differently because of reassignment.
Preserving Literals
as const preserves narrow types for a value. It does not freeze runtime objects.
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 literal.ts, prepare conditions for a function accepting only ready and three declaration styles.
- Declare a state string with let·const·an as const object, and pass it as an argument.
- Run with a function accepting only ready and three declaration styles. Results to check: Record the inferred types of the three declarations. A call with an invalid state string fails to compile. Record expected and actual results separately.
Variables to observe
| Variable | Baseline | Change | Expected change / hypothesis |
|---|---|---|---|
| Declaration | let mode | const mode | Pre-run hypothesis: the literal argument will be accepted. 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
- Record the inferred types of the three declarations.
- A call with an invalid state string fails to compile.
Self-check
- Is as const the same as Object.freeze?
- What information is lost when you declare a broad type?
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: strict Settings and Executable Code · Next: Object Contracts and Optional Fields