*.js

var a = [1, 2, 3, 4, 5, 6, 7]
var b = ['a', 'b', 'c']

al = a.length
bl = b.length
console.log(al, bl) // 7 3

if (al < bl) {
    for (let i = 0, len = (bl - al); i < len; i++) {
        a.push('')
    }
} else if (al > bl) {
    for (let i = 0, len = (al - bl); i < len; i++) {
        b.push('')
    }
}

console.log(a) // [1, 2, 3, 4, 5, 6, 7]
console.log(b) // ["a", "b", "c", "", "", "", ""]