Adding a new element into an array — it will no longer work properly [duplicate]

I have this code which works:

for (let b of buttons) {
    b.disabled = true;
}

However, this one won’t:

for (let b of [buttons, searchBtn]) {
    b.disabled = true;
}

Meaning, I wanted to add an additional button into an array of them dynamically.

Only searchBtn will get disabled.

Why? How to fix it?