在 Javascript if else 语句中，只有 if 被执行，即使它不是真的

3回答

ABOUTYOU

您没有访问元素的值。您应该更改代码以包括从 HTML 元素中提取值。像这样：var field1 = document.getElementById("englishid").value;var field2 = document.getElementById("mathid").value;var field3 = document.getElementById("computerid").value;将.value提取 HTML 标记中的值。您还可以改进 if 语句。以下可能不会像您认为的那样做：if (field1 || field2 || field3 === NULL)这里实际发生了什么，它说的是： Is field1 set (not NULL, undefined, 0, NaN, "" or false) OR is field2 set (not NULL, undefined, 0, NaN, "" or false) OR is field3 NULL.如果您想检查其中任何一个是否为空，您需要执行以下操作： if (field1 === NULL || field2 === NULL || field3 === NULL)甚至更好，更容易：if (!field1 || !field2 || !field3)完整功能：function GradeCal() {      var field1 = document.getElementById("englishid").value;      var field2 = document.getElementById("mathid").value;      var field3 = document.getElementById("computerid").value;      var result;      if (!field1 || !field2 || !field3) {        alert("Please enter number");      } else {        result = (field1 + field2 + field3) / 3;        document.getElementById("markid").innerHTML = +result;      }    }

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守候你守候我

您的字段变量是您需要获取字段值以检查元素不能为空的元素 var field1 = document.getElementById("englishid").value;  var field2 = document.getElementById("mathid").value;  var field3 = document.getElementById("computerid").value;

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慕桂英3389331

GradeCal您可以稍微调整该函数并使其更具可扩展性，只需使用要处理的字段名称的数组和一些基本的数组方法即可。function GradeCal(){    let result=[];    let fields=[ 'english', 'math', 'computer' ];    let out=document.getElementById('markid');    fields.forEach( ( id, i )=>{        let field=document.querySelector( 'input[name="'+id+'"]' );        if( field && !isNaN( field.value ) && field.value!='' )result.push( parseFloat( field.value ) );        else alert( 'Please enter a number for '+id )    });    if( result.length === fields.length ) {        out.innerHTML = result.reduce(( a,v )=>{ return a + v } ) / fields.length;    } else {        out.innerHTML='Error';    }}function GradeCal(){let result=[];let fields=[ 'english', 'math', 'computer' ];let out=document.getElementById('markid');fields.forEach( ( id, i )=>{  let field=document.querySelector( 'input[name="'+id+'"]' );  if( field && !isNaN( field.value ) && field.value!='' )result.push( parseFloat( field.value ) );  else alert( 'Please enter a number for '+id )});if( result.length === fields.length ) {  out.innerHTML = result.reduce(( a,v )=>{ return a + v } ) / fields.length;} else {  out.innerHTML='Error';}}<h2>Grade Book Calculator</h2><h4> Welcome to Grade Book Calculator </h4><form id="my form">  <table>  <tr>    <td>Student ID:</td>    <td> <input id="studentid" value="bc180200783" oninvalid="alert(Please Enter Student ID)" type="text" place holder="Enter your VU ID" required="required" /></td>  </tr>  <tr>    <td>Name:</td>    <td> <input id="nameid" value="Hina" oninvalid="alert(Please Enter Name)" type="text" required="required" /></td>  </tr>  <tr>    <td>Father Name:</td>    <td> <input id="fathernameid" value="Zafar" oninvalid="alert(Please Enter Father Name)" type="text" required="required" /></td>  </tr>  <tr>    <td>CNIC:</td>    <td> <input id="cnicid" pattern="^[0-9+][5]-[0-9+][7]-[0-9][1]$" value="33100-1100321-5" oninvalid="alert(Please Enter CNIC)" required="required" /></td>  </tr>  <tr>    <td>English:</td>    <td> <input id="englishid" oninvalid="alert(Please Enter English Marks)" name="english" required="required" /></td>  </tr>  <tr>    <td>Math:</td>    <td> <input id="mathid" oninvalid="alert(Please Enter Math Marks)" name="math" required="required" /></td>  </tr>  <tr>    <td>Computer:</td>    <td> <input id="computerid" oninvalid="alert(Please Enter Computer Marks)" name="computer" required="required" /></td>  </tr>  <tr>    <td>Average Marks:</td>    <td> <output type="number" id="markid"> </output>    </td>  </tr>  <tr>    <td> <button type="button" onClick="GradeCal()"> Grade Book </button>    <button type="reset"> Reset</button></td>  </tr>  </table></form>

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