JavaScript cannot read property bar of undefined

Question

I ve got a function that takes 3 parameters  The problem I have is one of the parameters is a property of a sometimes undefined value of an Object  i e  it takes in thing foo bar  and sometimes thing foo is undefined  so it can t access bar     What s a way around this  Within the function s declaration  I have a conditional checking   if   parameterName   but the browser  Chrome  is still throwing an error that it can t read the bar property of undefined

User · Answer

Just check for it before you pass to your function  So you would pass   thing foo   thing foo bar   undefined

User · Answer

Compound checking      if  thing foo  amp  amp  thing foo bar              thing foor bar exists

User · Answer

You can safeguard yourself either of these two ways   function myFunc thing        if  thing  amp  amp  thing foo  amp  amp  thing foo bar               safe to use thing foo bar here          function myFunc thing        try           var x   thing foo bar             do something with x       catch e               do whatever you want when thing foo bar didn t work           In the first example  you explicitly check all the possible elements of the variable you re referencing to make sure it s safe before using it so you don t get any unplanned reference exceptions   In the second example  you just put an exception handler around it   You just access thing foo bar assuming it exists   If it does exist  then the code runs normally   If it doesn t exist  then it will throw an exception which you will catch and ignore   The end result is the same   If thing foo bar exists  your code using it executes   If it doesn t exist that code does not execute   In all cases  the function runs normally   The if statement is faster to execute   The exception can be simpler to code and use in complex cases where there may be many possible things to protect against and your code is structured so that throwing an exception and handling it is a clean way to skip execution when some piece of data does not exist   Exceptions are a bit slower when the exception is thrown

User · Answer

If an object s property may refer to some other object then you can test that for undefined before trying to use its properties   if  thing  amp  amp  thing foo     alert thing foo bar     I could update my answer to better reflect your situation if you show some actual code  but possibly something like this   function someFunc parameterName       if  parameterName  amp  amp  parameterName foo         alert parameterName foo bar

[javascript] JavaScript "cannot read property "bar" of undefined

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