() brackets, what does it do in the statement of ({__proto__: []} instanceof Array) in JavaScript

Answer 1

When the interpreter sees a {, by default, it will think that you're declaring a new block, such as

{
  console.log('foo');
}

As a result:

{
  __proto__: []
} instanceof Array

doesn't make much sense - you can't instanceof a block.

But when it's wrapped in parentheses, the interpreter knows to expect a value inside the parentheses, not a block - so it evaluates everything inside as an expression instead, and (properly) parses { __proto__: [] } as an object.

This is exactly the same reason why, when destructuring to already assigned variables, you have to put parentheses around the line:

let x, y;
({x, y} = { x: 'foo', y: 'bar'});
console.log('done ' + x);

works, but it doesn't without the ():

let x, y;
{x, y} = { x: 'foo', y: 'bar'};
console.log('done ' + x);