The log ·
TIL: `satisfy` says 'check me, don't change me'
The difference between `as` and `satisfy` in TypeScript, in one sentence I finally got right.
1 min read#typescript#til
✎ by hand
No model was involved in drafting this entry.
satisfy validates that a value matches a type without widening it. That's the whole trick:
const theme = {
brand: "#2b5aa8",
background: "#faf6ef",
} satisfies Record<string, string>;
// theme.brand is still the literal "#2b5aa8", but a typo would be a compile error
With as, you're telling the compiler to shut up. With satisfy, you're asking it to check your work and then leave the value's precise type alone. Perfect for config objects, route maps, design tokens — anywhere I want both safety and narrow types.
Ten years of JavaScript and this one still took me embarrassingly long to internalize.