输入框Java编号 [英] Number from input box Javascript

问题描述

我正在尝试从输入框中获取一个数字，但它会返回一个空字符串作为警报.我想念什么?

I'm trying to get a number from an input box, but it returns an empty string as an alert. What do I miss?

var button = document.getElementsByClassName("btn")[0];
var num = document.getElementById("input").value;
button.addEventListener("click", calculate);

function calculate() {
    alert(num);
}

<input type="number" id="input">
<input type="button" value="Calculate!" class="btn">
<p id="result">

推荐答案

您需要读取函数内部的输入值，而不是在页面加载期间.以下代码将起作用.

You need to read the value of the input inside the function, not during page load. The following code will work.

var button = document.getElementsByClassName("btn")[0];

button.addEventListener("click", calculate);

function calculate() {
  var num = document.getElementById("input").value;
  alert(num);
}

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