REFERENCE / CORE API
isSignal()
True for values created by signal() or computed(), false for everything else. A type guard in TypeScript.
Signature
isSignal(v: unknown): v is Signal | ComputedParameters
PARAMETERTYPEDESCRIPTION
v
unknown
Anything.
Returns
true when v is a signal or a computed. It narrows the type, so v.value type-checks inside the branch.
When you need it
Mostly in components that take props which may or may not be reactive — the one place where "is this a value or a signal?" is a real question. Templates never need it: a hole already does the right thing with a constant, a signal or a function.
isSignal(signal(0)); // true
isSignal(computed(() => 1)); // true
isSignal(0); // false
isSignal(() => count.value); // false — a function hole is not a signal
// a component that accepts either a plain value or a reactive one
const Price = (amount) => html`<b>${isSignal(amount) ? () => fmt(amount.value) : fmt(amount)}</b>`;