test.html

<!DOCTYPE html>
<html lang="en">

<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
</head>

<body>
    <input type="text">
    <button>解码</button>

    <script>
        var inp = document.getElementsByTagName('input')[0]
        var btn = document.getElementsByTagName('button')[0]
        btn.onclick = function() {
            var val = inp.value
            var start = val.indexOf('[')
            var end = val.indexOf(']')
            var s = changeStr(start, end, val)
            console.log(s);


        }

        function changeStr(start, end, val) {

            var str = ''
            if (start != -1 && end != -1) {
                var text = ''
                var sStr = ''
                if (val.slice(0, start - 2).indexOf('[') == -1) {
                    sStr = val.slice(0, start - 2)
                }
                var vStr = val.slice(start + 1, end)
                var num = val.slice(start - 1, start)
                    // console.log(num);

                for (var i = 0; i < num; i++) {
                    text += vStr
                }
                str = text
            } else {
                return val
            }
            return str
        }
    </script>
</body>

</html>