Javascript'未捕获的TypeError：无法读取未定义'的属性'blah'

Question

I'm trying to test if an array element is undefined:

  if(typeof selected[i].facet != 'undefined')
  {
       //do stuff
  }

gives me

Javascript 'Uncaught TypeError: Cannot read property 'facet' of undefined '

Answer 1

您必须同时测试数组索引和属性：

if (selected[i] && selected[i].facet !== undefined) { // ...

Answer 2

if((selected[i]) && (selected[i].facet != undefined))
  {
       //do stuff
  }

Answer 3

You should just do your loop properly

for (var i = 0, len = selected.length; i < len; ++i) {
    //selected[i] will always be a valid index in the array
}

Should it be the case that selected[i] is a valid index in the array, but undefined regardless, then you don't semantically need an array but a dictionary with integers as keys. In that case, you can loop through it with for( var key in map ) . Aim to fix the source of errors instead of fighting the symptoms.