Javascript 補0

需求

‘1’ -> ‘0001’

方法1

function prefix(num, val) {
    return val.padStart(num, '0');
}
prefix(4, '1')
 

方法2

function prefix(num, val) {
    return (new Array(num + 1).join('0') + val).slice(-num);
}
prefix(4, '1')