JSFiddle - React, Tailwind, and code Playground

JavaScript

function permutate(inputString, outputString) {
    if (!inputString.length) console.log(outputString);
    else for (var i = 0; i < inputString.length; ++i) {
        permutate(inputString.substring(0, i) +
                  inputString.substring(i + 1),
                  outputString + inputString[i]);
    }
}

var inputString = "abcd";
var outputString = "";
permutate(inputString, outputString);