无法读取null的属性* 0 * [英] Cannot read property *0* of null

问题描述

我收到此错误无法读取null的属性'0'."

I get this error "Cannot read property '0' of null."

我有一张桌子

somename.html

somename.html

<table>  
    <tr>  
    <td id="td1">text</td>  
    </tr>  
    </table>

somename.js

somename.js

function test(){  
    var date=new Date();  
    if(date.getDay()==1 && date.getHours() >=13  && date.getMinutes() >=3 ){ //8-9AM  
        document.getElementById('td1')[0].style.color = 'blue';  
    }else if(date.getDay()==1 && date.getHours() == 14  && date.getMinutes() <= 20){  
            document.getElementById('td1')[0].style.color = 'blue';  
    }else{  
      //nothing  
    }  
}  
setInterval(test(),1000);

如果我删除[0]无法读取null的属性'样式'，则为新错误"

new error if i remove [0] "Cannot read property 'style' of null"

推荐答案

getElementById 返回 null .(这将导致您准确地收到错误消息).

getElementById returns null if there is no match in the document. (Which then leads to exactly your error message).

这意味着您在选择器中输入了错字，或者在将元素包含到dom中之前执行了html或js.

This means that either you have a typo in your selector or the html or the js gets executed before your element is included into the dom.

此外， getElementById 返回单个元素而不是数组(准确地说是 Node List )，因此正确的调用将是:

Also, getElementById returns a single element and not an array (Node List to be precise), so the correct call would be:

document.getElementById('td1').style.color = 'blue';

第三个问题:

setInterval(test(),1000);
//              /\
// this will immeditately execute the function and
// hands over the *return value* to setInterval

将无法正常工作，它必须是

will not work as intended, it needs to be

setInterval(test,1000);
//            /\
// hand over the *function reference* to be executed after 1 second

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