[Typescript 4.9] TypeScript 4.9: satisfies operator

Previously, we have problem for such code:

type RGB = readonly [red: number, green: number, blue: number];
type Color = { value: RGB | string };

const myColor: Color = { value :'red' };

myColor.value.toUpperCase() // Property 'toUpperCase' does not exist on type 'string | RGB'.

With TS 4.9, it works

type RGB = readonly [red: number, green: number, blue: number];
type Color = { value: RGB | string };

const myColor = { value :'red' } satisfies Color;

myColor.value.toUpperCase() // works, now myColor.value is narrowing down to string type

Working with as const

type RGB = readonly [red: number, green: number, blue: number];
type Color = RGB | string;

const palette = {
    red: [255, 0, 0],
    green: "#00ff00",
    blue: [1,2,3],
} satisfies Record<string, Color>;

console.log(palette.green); // green is a string type

const constantPalette = {
    red: [255, 0, 0],
    green: "#00ff00",
    blue: [1,2,3],
} as const satisfies Record<string, Color>;

console.log(constantPalette.green);  // green is "#00ff00"

Blog