TypeScript for JavaScript Developers: Add Contracts while Preserving Runtime Semantics
For JavaScript developers, TypeScript adds information about values without replacing JavaScript's underlying execution rules. Objects remain structurally described, and inference follows existing expressions. Gradual migration benefits from typing important boundaries first, then reducing unchecked assumptions while preserving knowledge of coercion, asynchronous behavior and other runtime semantics.
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Why Use TypeScript for JavaScript Developers: Add Contracts while Preserving Runtime Semantics
- Connect existing JavaScript values with inferred descriptions
- Understand compatibility through object structure
- Add contracts at module and function boundaries
- Replace unchecked assumptions with narrowing
- Keep JavaScript execution behavior distinct from types
Frequently Asked Questions
Does a type assertion convert a value?
A type assertion changes how the checker treats an expression under supported rules. It does not perform conversion or verify the value at execution time, so a separate operation is needed for either behavior.
What does structural compatibility mean?
Compatibility commonly depends on the members a value exposes rather than a required declaration name. This differs from nominal expectations and explains why independently described objects can satisfy the same contract.
Where should gradual migration begin?
Begin where an explicit description clarifies dependencies, such as exported functions or external-input boundaries. Those contracts help constrain later changes without requiring every existing local expression to be annotated immediately.
How does unknown differ from any?
Unknown requires suitable refinement before operations that depend on a specific shape. Any bypasses much checking for the value, making it easier for an unsupported assumption to propagate into surrounding code.
Does TypeScript change JavaScript coercion rules?
Type descriptions constrain accepted code but do not replace the coercion behavior of emitted JavaScript. Understanding the runtime operation remains necessary even when its surrounding variables have precise static descriptions.