检查变量是否未定义将返回变量未定义

Question

I have a statement in my code:

if(!(typeof options.duration[i] === 'undefined'))

I have written it correctly, seems that there is no mistake but the console is throwing error that:

TypeError: options.duration is undefined

It should not show this error.It does not make any sense.

Answer 1

The variable options.duration is undefined, so accessing item i from it will result in this error. Perhaps try:

if(typeof options.duration !== 'undefined')

Or if you need to check both options.duration and options.duration[i] , try

if(typeof options.duration !== 'undefined' &&
   typeof options.duration[i] !== 'undefined')

Answer 2

为了使测试成功，还必须定义数组options.duration本身。

Answer 3

You get that error because the duration property doesn't exist.

Check if the property exists before you try to check items in it:

if('duration' in options && typeof options.duration[i] !== 'undefined')