javascript sup()不工作

var ec = "1s2 2s4".split("");
for (var i = 0; i < ec.length; i++) {
    if (ec[i].match(/[a-z]/i)) {
        // I removed the call to sup, even though it is only deprecated
        // and has been for awhile it is still accessible. Feel free to
        // use it if you would like, i just opted not to use it.
        // The main issue with your code was this line because you weren't
        // assigning the value of your call to sup back to the original variable,
        // strings are immutable so calling on a function on them doesn't change
        // the string, it just returns the new value
        ec[i + 1] = '<sup>' + ec[i + 1] + '</sup>';
        // if you want to continue to use sup just uncomment this
        // ec[i + 1] = ec[i + 1].sup();

        // This is a big one that I overlooked too.
        // This is very important because when your regex matches you reach
        // ahead and modify the next value, you should really add some checks
        // around here to make sure you aren't going to run outside the bounds
        // of your array
        // Incrementing i here causes the next item in the loop to be skipped. 
        i++
    }
}

console.log(ec.join(''));