Javascript Cannot read property length of undefined when checking a variable s length

Question

I m building a node scraper that uses cheerio to parse the DOM  This is more or a vanilla javascript question though  At one part of my scrape  I m loading some content into a variable  then checking the variable s length  like so   var theHref     obj mainImg select  attr  href    if  theHref length         do stuff   else        do other stuff     This works just fine  until I came across a url for which   obj mainImg select  attr  href   didn t exist  I assumed that my theHref length check would account for this and skip through to the else  do other stuff statement  but instead I got   TypeError  Cannot read property  length  of undefined  What am I doing wrong here and how can I fix this

User · Answer

There s a difference between an empty string    and an undefined variable  You should be checking whether or not theHref contains a defined string  rather than its lenght   if theHref         ---     If you still want to check for the length  then do this   if theHref  amp  amp  theHref length

User · Answer

You can simply check whether the element length is undefined or not just by using   var theHref     obj mainImg select  attr  href    if  theHref        get the length here if the element is not undefined    elementLength   theHref length       do stuff   else         do other stuff

User · Answer

Why  You asked why it happens  let s see  The official language specificaion dictates a call to the internal   GetValue   method  Your  attr returns undefined and you re trying to access its length   If Type V  is not Reference  return V   This is true  since undefined is not a reference  alongside null  number  string and boolean   Let base be the result of calling GetBase V    This gets the undefined part of myVar length    If IsUnresolvableReference V   throw a ReferenceError exception   This is not true  since it is resolvable and it resolves to undefined   If IsPropertyReference V   then  This happens since it s a property reference with the   syntax  Now it tries to convert undefined to a function which results in a TypeError

User · Answer

If you aren t doing some kind of numeric comparison of the length property  it s better not to use it in the if statement  just do   if theHref         do stuff  else       do other stuff     An empty  or undefined  as it is in this case  string will evaluate to false  just like a length of zero would

User · Answer

As has been discussed elsewhere  the  length property reference is failing because theHref is undefined  However  be aware of any solution which involves comparing theHref to undefined  which is not a keyword in JavaScript and can be redefined   For a full discussion of checking for undefined variables  see Detecting an undefined object property and the first answer in particular

User · Answer

You can check that theHref is defined by checking against undefined   if  undefined     theHref  amp  amp  theHref length            theHref  is not undefined and has truthy property  length         do stuff   else          do other stuff     If you want to also protect yourself against falsey values like null then check theHref is truthy  which is a little shorter  if  theHref  amp  amp  theHref length            theHref  is truthy and has truthy property  length

User · Answer

In addition to others  proposals  there is another option to handle that issue   If your application should behave the same in case of lack of  href  attribute  as in case of it being empty  just replace this   var theHref     obj mainImg select  attr  href      with this   var theHref     obj mainImg select  attr  href            which will treat empty string      as the default  if the attribute has not been found   But it really depends  on how you want to handle undefined  href  attribute  This answer assumes you will want to handle it as if it was empty string

[javascript] Javascript "Cannot read property 'length' of undefined" when checking a variable's length

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