javascript - 未捕获的TypeError：无法读取未定义的属性“值”(Uncaught TypeError: Cannot read property 'value' of undefined)

Question

I have some JavaScript code that gives this error(我有一些JavaScript代码会给出此错误)

Uncaught TypeError: Cannot read property 'value' of undefined

Code(码)

var i1 = document.getElementById('i1');
var i2 = document.getElementById('i2');
var __i = {'user' : document.getElementsByName("username")[0], 'pass' : document.getElementsByName("password")[0] };
if(  __i.user.value.length >= 1 ) { i1.value = ''; } else { i1.value = 'Acc'; }

if(  __i.pass.value.length >= 1 ) { i2.value = ''; } else { i2.value = 'Pwd'; }

What does this error mean?(这个错误是什么意思？)

  ask by John translate from so

Answer 1

Seems like one of your values, with a property key of 'value' is undefined.(好像您的值之一，属性键为“值”是未定义的。)

Test that i1 , i2 and __i are defined before executing the if statements:(执行if语句之前，测试__i已定义i1 ， i2和__i ：)

var i1 = document.getElementById('i1');
var i2 = document.getElementById('i2');
var __i = {'user' : document.getElementsByName("username")[0], 'pass' : document.getElementsByName("password")[0] };
if(i1 && i2 && __i.user && __i.pass)
{
    if(  __i.user.value.length >= 1 ) { i1.value = ''; } else { i1.value = 'Acc'; }

    if(  __i.pass.value.length >= 1 ) { i2.value = ''; } else { i2.value = 'Pwd'; }
}