SOURCE

var key = document.getElementById("key");
//key.onkeydown =inputHandler;  //注册keydown事件处理函数
key.onkeyup = inputHandler;  //注册keyup事件处理函数
// key.onkeypress = inputHandler;  //注册keypress事件处理函数


var flagStart = false;
var flagEnd = false;

var startWithStar = 0;
var endWithStar = 0;
var keyCodeArr = []


function inputHandler(e) {
    var e = e || window.event;  //标准化事件处理
    var s = e.type + " " + e.keyCode;  //获取键盘事件类型和按下的值
    // console.log(this.value)
    var keyCode = e.keyCode;

    if (keyCode == '16') return;

    if (keyCode == '56') {
        console.log(222)
        startWithStar++

        if (flagStart) {
            endWithStar++
            if (endWithStar == 2) {
                flagEnd = true;
            } else {
                flagEnd = false;
            }
        }

        if (startWithStar == 2) {
            flagStart = true;
        }

    } else {
        startWithStar = 0
    }

    if (flagStart) {
        if (keyCode != '56') {
            keyCodeArr.push(keyCode)
        }

        if (flagEnd) {

            //   console.log(keyCodeArr)
            var str = '';
            for (let i = 0; i < keyCodeArr.length; i++) {
                str += String.fromCharCode(keyCodeArr[i])
            }

            flagStart = false;
            flagEnd = false;
            startWithStar = 0;
            endWithStar = 0;
            keyCodeArr = []
            console.log(`<b> ${str} </b>`)


        }
    }

    //  console.log(flagStart)



    //key.value = s;


}

<textarea id="key"></textarea>