javascript - JS using eval to build the replace function is invalid

Question

Code meaning: Build a simple GADERYPOLUKI decoder

The GADERYPOLUKI is a simple substitution cypher used in scouting to encrypt messages. The encryption is based on short, easy to remember key. The key is written as paired letters, which are in the cipher simple replacement.

example:

encode("ABCD", "agedyropulik");             // => GBCE 

The code is as follows. I want to use the eval function to build a function that can replace characters, but it seems to be useless.

function decode(str,key) {

    key = key.split('')

    while (key.length>0) {
        let b = key.pop(), a = key.pop();
        eval(`str.replace(/${a}/g, "${b}")`)
        eval(`str.replace(/${a.toUpperCase()}/g, "${b.toUpperCase()}")`)
        eval(`str.replace(/${b}/g, "${a}")`)
        eval(`str.replace(/${b.toUpperCase()}/g, "${a.toUpperCase()}")`)
        console.log(a, b, str, `str.replace(/${a}/g, "${b}")`)
    }

    return str

}

console.log(decode("Hmdr nge brres", "gaderypoluki"))
console.log("Hmdr nge brres".replace(/g/g, "a"))

>>> k i Hmdr nge brres str.replace(/k/g, "i")
    l u Hmdr nge brres str.replace(/l/g, "u")
    p o Hmdr nge brres str.replace(/p/g, "o")
    r y Hmdr nge brres str.replace(/r/g, "y")
    d e Hmdr nge brres str.replace(/d/g, "e")
    g a Hmdr nge brres str.replace(/g/g, "a")
    Hmdr nge brres
    Hmdr nae brres

Answer 1

replace does not change the original value, but returns a new string.

In fact, you can use new RegExp(a, 'g') and there is no need for eval