const result = Array(arr.length);
for (let i = 0; i < arr.length; i++) {
result[i] = arr[i][0] || '';
}
// или
const result = [];
for (const [ n = '' ] of arr) {
result.push(n);
}
// или
const result = [];
while (result.length < arr.length) {
result[result.length] = arr[result.length].slice(0, 1);
}
// или
const result = arr.map(n => n.charAt(0));