E0150: A write the browser can't run

An element's on… attribute holding statements is a chain of JS commands: each statement is a write of a ui.State — x = v with a value of the server's, x = !x for a bool, x++ or x-- for a number — or a call of a handler, which the chain pushes to the server after the writes before it. Anything else can't run in the browser: another assignment operator (x += 2), a value that reads a state (x = x + "!"), a write of a field that isn't a state, another statement.

Server state is written in a handler instead, and a tracked field (a .vuka.html struct's unexported one) is written through its own name, not a pointer to it: &p.rows would change it without the session seeing. Its fields and methods read through it (p.employer.Name), and a write into it (p.employer.Name = x, or a pointer method called as a statement, p.employer.Rename(x)) updates it; a pointer method called inside an expression (n := p.t.Inc() + 1) would change a copy, so it is this error too: call it as a statement of its own.

Wrong

<button onClick={count += 2}>+2</button>
<button onClick={label = label + "!"}>!</button>

Right

<button onClick={count++; count++}>+2</button>
<button onClick={AddBang()}>!</button>