satisfies in TypeScript

Check a value against a type without widening it to that type.

type ThemeConfig = Record<string, { bg: string; dark: boolean }>;

const annotated: ThemeConfig = {
  monokai: { bg: "#272822", dark: true },
};

const checked = {
  monokai: { bg: "#272822", dark: true },
  paper: { bg: "#ffffff", dark: false },
} satisfies ThemeConfig;

// annotated.monokai is checked, but the keys are widened to string
const anyKey: string = Object.keys(annotated)[0]!;

// checked keeps its exact keys and value types
type ThemeName = keyof typeof checked;
const name: ThemeName = "paper";
const isDark: false = checked.paper.dark;

console.log(anyKey, name, isDark);

How it works

1. An annotation widens; satisfies keeps the literal types.
2. So you get both the check and the precise keys.
3. It is the right tool for a config object.

Keywords and builtins used here

• Object
• ThemeConfig
• ThemeName
• boolean
• const
• false
• string
• true
• type

The run, in numbers

Lines

20

Characters to type

584

Tokens

124

Three-star pace

100 tpm

At the three-star pace of 100 tokens a minute, this run takes about 74 seconds.

Type this snippet

Step 1 of 2 in satisfies & strictness, step 21 of 23 in Types & annotations.

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