What does the satisfies operator add over plain annotations?

Short answer

satisfies validates a value against a type while preserving the value's inferred (often narrower) type. Plain annotations widen literals and lose specificity; satisfies keeps both safety and precision.

Detailed explanation

Annotating const config: Config = {...} checks compliance but *widens* the variable to Config — literal values become their general types ('dark' widens to string), losing autocomplete and exactness downstream. satisfies flips it: const config = {...} satisfies Config verifies every requirement while keeping inferred narrowness — config.theme stays 'dark' | 'light' instead of string, excess-property checking still applies during validation, and no duplicate type declaration is needed. Ideal cases: typed constant maps (routes, feature flags, theme tokens) where consumers benefit from precise literal types, and validating objects whose specific shape matters later. Limitations: it's expression-level (not for function returns — those still use annotations), available since TS 4.9, and codebases on older versions rely on as const + separate checks. Knowing when NOT to use it — public API boundaries where widening is desirable — shows balanced judgment.

Common mistakes

• Thinking replaces as const entirely — they solve different problems.
• Using satisfies where function return annotations are needed.