Check component attribute exists

Question

How can I check whether a certain attribute is existing within my component?

Especially when I try to access a value dynamically like component.get("v." + fieldName) I want to make sure this is not throwing an exception. Unfortunately try/catch is not catching the frameworks default "Access Check failed"-exception nor I haven't found any suitable function in $A.util or the Component-API.

I am not 100% sure if this behavior changed with Summer17 but when I do a simple test on a component init handler:

onInit: function(component, event, helper) {
  if(component.get("v.testNotExistingAttribute")) {
    //never reach this code as exception is thrown
  }

this already gives me the following:

Answer 1

Another way of doing it is by using the util methods, such as:

var someAttribute = component.get('v.someAttribute');

if ($A.util.isUndefinedOrNull(someAttribute)) {
    // Do something when undefined or null
}
else {
    // Do something if defined and has a value
}

As sfdcfox wrote, undefined and null are falsy values and can be tested as such, but I prefer using the util methods because for documentation purposes for myself, it's a little more descriptive to me what I'm testing for.

Anyway, thought I'd toss the util method out there as an alternate way of checking for attributes.

Answer 2

Thanks for validating that not everybody gets the exception- so I found out that when Debug Mode is enabled (Setup -> Lightning Components -> Checkbox) the exception is raised to the user interface.

Does this mean checking an attribute for existence by using if (component.get("Attr")) is safe in production?

Answer 3

component.get("Attr") generally gives undefined if the attribute is not present. It doesn't throw an exception.

In JavaScript, the standard way to catch null and undefined simultaneously is this:

if (component.get("Attr")) {
    console.log("Happy");
}else{
    console.log("Sad");
}