string_replace_LUT

const replaceAll = (string, substring, replacement) =>  string.split(substring).join(replacement)

const updateString = (string, lookup_json) => {
    for (key in lookup_json) {
        let substr_old = key
        let substr_new = lookup_json[key]
        string = replaceAll(json,
            substr_old, substr_new)
    }

    return string
}

var test_string = "ciao come va baby?"
var lut = '{ "a":1, "b":2, "c":3}'

const string_replaced = updateString(
    prompt("Paste your old string"),
    JSON.parse(prompt("Paste JSON look-up table"))
)

console.log(string_replaced)