I have this code:

  if (window.content.document.getElementById("error-msg") != null )
  {
    if (window.content.document.getElementById("error-msg").offsetParent !== null) 
    {
...
    }
  }

Can it be written in one if statement?

I tried the following...

if ( (window.content.document.getElementById("error-msg") != null) || (window.content.document.getElementById("error-msg").offsetParent !== null) ) {}

But, it didn't work, and produces an error:

TypeError: window.content.document.getElementById(...) is null

I have this code:

  if (window.content.document.getElementById("error-msg") != null )
  {
    if (window.content.document.getElementById("error-msg").offsetParent !== null) 
    {
...
    }
  }

Can it be written in one if statement?

I tried the following...

if ( (window.content.document.getElementById("error-msg") != null) || (window.content.document.getElementById("error-msg").offsetParent !== null) ) {}

But, it didn't work, and produces an error:

TypeError: window.content.document.getElementById(...) is null

• javascript
• if-statement

• 3 You need &&, not ||. – Amadan Commented Apr 21, 2015 at 2:00
• If only we had the existential operator! – Knu Commented Apr 21, 2015 at 2:08

2 Answers 2

The mon idiom is to use the && operator like this

var errorMsg = window.content.document.getElementById("error-msg");

if (errorMsg && errorMsg.offsetParent) {
    ...
}

Here, JavaScript will evaluate errorMsg first and if it is Truthy, then it will evaluate the errorMsg.offsetParent part. The condition will be satisfied only if both the expressions in && are Truthy.

Note: The Truthy evaluation will return false, if the expression being tested is 0, false etc (See the list of Falsy values here). So, if you want to test if they are not null, just write that explicitly, like this

if (errorMsg !== null && errorMsg.offsetParent !== null) {
    ...
}

On the other hand, the || operator will evaluate the second operator only if the first expression is Falsy. In your case, if

(window.content.document.getElementById("error-msg") != null) 

is true, it means that getElementById("error-msg") returns null. Since the first expression is evaluated to be Truthy, it evaluates the other expression and it effectively tries to check

null.offsetParent !== null

That is why it fails.

Maybe you want to use &&

if (a != null && b != null) {
    // Do something.
}