Change string characters, except the last 4

Question

The idea of the function is to change the characters of a string (minus the last 4 characters) per "#". I created the function, but I believe I have problems in the logic of the code.

function criptografar(dados) {
    let result = ""
    if (dados.length > 4) {
        for (let index = 0; index >= dados.length - 4; index++) {
            result += dados.replace(dados[index], "#")
            return result
        }
    } else {
        return dados
    }
}

//let nome = "Alexandre"
//-> "#####ndre"

Answer 1

One of the problems was putting the condition index >= dados.length - 4. Like index starts at zero if dados has size 10, for example, the condition will be false, since zero is not greater than or equal to 6, then the for will not execute any iteration.

Another problem is that the return is inside the for, then in the first iteration it would already return, without iterating by the other characters.

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Anyway, you don’t have to use replace, because you only need to have an amount of # equal to string size less 4.

One way to do it is to fill in the result with # until dados.length - 4, and then pick up the last 4 characters of the string using slice:

function criptografar(dados) {
    let result = "";
    for (let index = 0; index < dados.length - 4; index++) {
        result += "#";
    }
    // juntar os # com os 4 últimos caracteres da string 
    return result + dados.slice(-4);
}

console.log(criptografar('Alexandre'));
console.log(criptografar('abcde'));
console.log(criptografar('ab'));

If the string has 4 characters or less, it does not enter the for, and slice returns the entire string.

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Another alternative is to use repeat to construct the string containing the #:

function criptografar(dados) {
    if (dados.length > 4) {
        return "#".repeat(dados.length - 4) + dados.slice(-4);
    }
    return dados;
}

console.log(criptografar('Alexandre'));
console.log(criptografar('abcde'));
console.log(criptografar('ab'));

If the string size is greater than 4, I create a string with "size - 4" characters # concatenated with the last 4 characters. If the size is less than or equal to 4, it does not enter the if and returns the entire string, no modifications.

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I also put a semicolon at the end of the lines. It may seem "freshness", but this avoids some bizarre situations which may occur if you don’t use them (see more about this here).

Answer 2

Can use .padStart(), which dispenses with the use of the loop for:

function criptografar(dados){
   if(dados.length > 4)
      return dados.slice(-4).padStart(dados.length, "#");
   return dados;
}

console.log(criptografar("Alexandre"));
console.log(criptografar("Alex"));
console.log(criptografar("Alexa"));