Spread syntax can be used when all elements from an object or array need to be included in a list of some kind.

(c) MDN

🐊Putout plugin adds ability to remove useless spread syntax.

npm i @putout/plugin-remove-useless-spread

{
    "rules": {
        "remove-useless-spread/array": "on",
        "remove-useless-spread/object": "on",
        "remove-useless-spread/nested": "on"
    }
}

The thing is [...b] can be used for:

• copying an array;
• converting different value type like string to an array.

So better to be more concrete and use slice for copying and Array()/Array.from() for converting to decrease cognitive load. Also sometimes there is no need on any of this operations, and we can drop spread.

for (const a of [...b]) {}

const places = [...getPlaces()];

for (const a of b) {}

const places = getPlaces();

// Array constructor creates sparse array
[...Array(5)].map(Number);

const a = {
    ...fn(),
};

const a = fn();

Checkout in 🐊Putout Editor.

[
    ...[
        ...a,
        ...b,
    ],
    ...x,
];

[
    ...a,
    ...b,
    ...x,
];

MIT