JSFiddle - React, Tailwind, and code Playground

HTML

Type number: <input /><button>Convert</button>
<div></div>

JavaScript

var map = 'abcdefg';

$('button').click(function() {
    var n = parseInt($('input').val());
    var out = '';
    if(n == 0) out = map.charAt(0) + out;
    while (n > 0) {
        var r = n % 7;
        n = Math.floor(n / 7);
        out = map.charAt(r) + out;
    }
    $('div').text(prepend(out, '-', 4));
});

function prepend(str, char, l) {
    if (str.length < l) {
        for (var i = l - str.length; i--;) {
            str = '-' + str;
        }
    }
    return str;
}