EVAL
function executeEval(){
let evalCodeElt = document.getElementById("eval_code");
let evalCode = evalCodeElt.value;
// 如果不把开头的eval去掉的话直接执行会被执行两遍
evalCode = evalCode.replace(/^eval/, "");
try{
evalCodeElt.value = eval(evalCode);
}catch (e) {
alert("执行报错了:" + e);
}
}