SOURCE

var isSubsequence = function(s, t) {
    const sLen = s.length
    const tLen = t.length
    let i = 0
    let j = 0
    while(i<sLen && j<tLen) {
        if(s[i] == t[j]) {
            i++
        }
        j++
    }
    return i == sLen
};