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Before `satisfies` (TypeScript 4.9), the only way to verify an object matched a type was to annotate it: `const x: MyType = { ... }`. But annotation widens the variable's type to `MyType`, losing the narrow literal type information. `satisfies` checks the type constraint without changing the inferred type. The variable stays narrowly typed, but TypeScript will error if the shape does not satisfy the constraint.
type Palette = Record<string, [number, number, number] | string>
// With annotation: TypeScript widens 'blue' to tuple | string
// so .toUpperCase() errors because string methods do not exist on tuples
const palette1: Palette = {
red: [255, 0, 0],
blue: "#0000ff",
}
// With satisfies: type is validated but NOT widened
// TS knows blue is string, red is [number, number, number]
const palette2 = {
red: [255, 0, 0],
blue: "#0000ff",
} satisfies Palette
palette2.blue.toUpperCase() // ✓
palette2.red.at(0) // ✓satisfies validates the shape against Palette without losing the narrow literal types
TypeScript 4.9: satisfies operator
The release note with the original motivation, examples and comparison to the annotation approach.
https://www.typescriptlang.org/docs/handbook/release-notes/typescript-4-9.html