Clear localstorage values with certain prefix

Removing element while iterating is unsafe, so create an array to hold the keys that need to be removed. Then, iterate over that array to remove them:

var arr = []; // Array to hold the keys
// Iterate over localStorage and insert the keys that meet the condition into arr
for (var i = 0; i < localStorage.length; i++){
    if (localStorage.key(i).substring(0,3) == 'TM_') {
        arr.push(localStorage.key(i));
    }
}

// Iterate over arr and remove the items by key
for (var i = 0; i < arr.length; i++) {
    localStorage.removeItem(arr[i]);
}