SOURCE

function add(a, b) {
    return a + b
}
function minus(a, b) {
    return a - b
}
function mul(a, b) {
    return a * b
}
function add(a, b) {
    if (b != 0) {
        return a / b
    } else {
        alert("除数不能为0")
    }
}
function caculater(a, b, f) {
    return f(parseInt(a), parseInt(b))
}
//caculater(10,10,minus)

function jisuan() {
    var num1 = document.getElementById("num1").value
    var op = document.getElementById("op").value
    var num2 = document.getElementById("num2").value
    switch (op) {
        case "add":
            document.getElementById("res").value = caculater(num1, num2,add)
            break
        case "minus":
            document.getElementById("res").value = caculater(num1, num2,minus)
            break
        case "mul":
            document.getElementById("res").value = caculater(num1, num2,mul)
            break
        case "div":
            document.getElementById("res").value = caculater(num1, num2,div)
            break
    }
}

var dt = new Date();
console.log(dt)
//取整数
var x = 10.2
console.log(Math.round(x))
//取随机0-10数
var r = Math.random()
console.log(Math.floor(r * 10))

<form>
    <input id="num1"/>
    <select id="op">
        <option value="add">+</option>
        <option value="minus">-</option>
        <option value="mul">*</option>
        <option value="div">/</option>
    </select>
    <input id="num2"/>
    <label>=</label>
    <input id="res"/>
    <input type="button" value="计算" onclick="jisuan()"/>

</form>