let a = [...new Array(n)].fill(sentinel);

let a = new Array(n).fill(sentinel);

let a = Array(n).fill(sentinel);

let a = Array.from({length: n}, () => sentinel);

let a = [...new Array(n)].map(x => sentinel);

let a = [];
a.length = n;
let i = 0;
while (i < n) {
  a[i] = sentinel;
  i++;
}

let a=[]; for(let i=0; i<n; i++) a[i]=sentinel;

let a=[]; for(let i=0; i<n; i++) a.push(sentinel);

let a = new Array(n); for (let i=0; i<n; ++i) a[i] = sentinel;

let a = new Array(n); for (let i=n; i--;) a[i] = sentinel;