SIGX500 · Write to a read-only property signal#

Dev message

[sigx] toSignal: cannot write to read-only property "count".

What happened#

A toSignal view was assigned to, and the underlying object refused the write.

toSignal(source, key) is a two-way view onto one property, not a copy of it. Reading view.value reads source[key]; assigning view.value assigns source[key]. When that assignment cannot land, the write is a silent data loss — the signal would report a value the object never took — so SignalX throws instead.

The property rejects the write when it is:

  • non-writable — defined with writable: false via Object.defineProperty
  • getter-only — an accessor with a get but no set
  • frozen or sealed — the whole object went through Object.freeze()
  • a Proxy trap that returns false from its set handler
TypeScript 
const state = Object.freeze({ count: 0 });
const count = toSignal(state, 'count');

count.value;       // 0 — reading is fine
count.value = 1;   // SIGX500

Fix it#

If the value is meant to change, make the source writable — usually by building it with signal() rather than freezing a plain object:

TypeScript 
const state = signal({ count: 0 });
const count = toSignal(state, 'count');

count.value = 1;   // fine — and `state.count` is now 1

For a getter-only property, give it a setter, or point toSignal at the backing field the getter reads.

If the value is genuinely read-only, don't route writes through it. Derive with computed() instead — a computed has no setter to misuse:

TypeScript 
const total = computed(() => config.items.length);

Not a SigxError#

This one throws a plain Error, not a SigxError — @sigx/reactivity sits below the error class and does not depend on it. There is no error.code to branch on here; match on the message, or better, fix the write.