var x = [a, b, ...[c, d]]

implies assumption that global Array.prototype[Symbol.iterator] has no side effects

arrays can be in-lined even if there are accessors as there is no behavior change

var x = {
  get b() { console.log('called b!'); return 2; },
  get c() { console.log('called c!'); return 3; }
};
console.log([1, x.b, ...[x.c, 4]]);

sparse arrays

elision in array spread is evaluated to undefined, this changes behavior of map and forEach

console.log([...[1, , 3]]) // [ 1, undefined, 3 ]
console.log([1, , 3]) // [ 1, <1 empty slot>, 3 ]