isSubsequence.js

/**
 * @param {string} s
 * @param {string} t
 * @return {boolean}
 */
var isSubsequence = function(s, t) {
    const len = s.length;
    if (len === 0) return true;
    
    let sIndex = 0;
    let pos = 0;
    
    while (sIndex < len) {
        pos = t.indexOf(s[sIndex], pos);
        if (pos === -1)
            return false;
        if (s[sIndex] === s[sIndex-1]) {
            pos += 1;
        }
        sIndex++;
    }
    
    return true;
};