数组扁平化

[1, [2, [3]]].flat(2) // [1, 2, 3]

function flatten(arr) {
var result = [];
for (var i = 0, len = arr.length; i < len; i++) {
if (Array.isArray(arr[i])) {
result = result.concat(flatten(arr[i]))
} else {
result.push(arr[i])
}
}
return result;
}

function flatten(arr) {
while (arr.some(item => Array.isArray(item))) {
arr = [].concat(...arr);
}
return arr;
}