JavaScript: How to check if object method has valid result

Question

I have a very basic question but I couldn't find any similar questions. I have the following code:

var admin_li = helper.h('li', [fpu_link('/admin', 'control', false)]);
if (Discourse.User.current().admin) {
    menu_li.push(admin_li);
}

I get the following error in the DevTools:

 Uncaught TypeError: Cannot read property 'admin' of null

How can I check if the Discourse.User.current() has a property/method of admin in the if statement?

Possible duplicate of [Is there a standard function to check for null, undefined, or blank variables in JavaScript?](https://stackoverflow.com/questions/5515310/is-there-a-standard-function-to-check-for-null-undefined-or-blank-variables-in) — Heretic Monkey, Jun 04 '19 at 23:53
I don't think I've ever seen a method chained with a property? `Discourse.User.current().admin` or is it `Discourse.User.currentProp()` or `Discourse.User.current('admin')`? — zer00ne, Jun 04 '19 at 23:54
@zer00ne `if(document.getElementById('foo').classList.contains('bar') === false) { }` you see it every day. — Kaiido, Jun 04 '19 at 23:58
@Kaiido derp indeed I do, the obvious is always the least remembered. — zer00ne, Jun 05 '19 at 00:48

Dacre Denny · Accepted Answer · 2019-06-05T00:06:20.243

1

Something along the lines of this should work for you:

var admin_li = helper.h('li', [fpu_link('/admin', 'control', false)]);
// Get current from user and store in local variable
var currentUser = Discourse.User.current();

if(currentUser !== null && currentUser !== undefined) {
  // If currentUser not null and not undefined then it's safe to access
  // the admin field
  if (currentUser.admin) {
    menu_li.push(admin_li);
  }
}

Alternatively (as suggested by @MarkMeyer), the code above can be written in a more concise shorthand form as follows:

if(currentUser && currentUser.admin) {
  // If currentUser is "truthy" (not null, not undefined, etc), and 
  // currentUser.admin is "truthy", then update menu_li list
  menu_li.push(admin_li);
}

See this more information on the definition of "truthy" (and "falsey") values in JavaScript

edited Jun 05 '19 at 00:06

answered Jun 04 '19 at 23:52

Dacre Denny

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2

What if `undefined`? – Kaiido Jun 04 '19 at 23:56
1

It's probably more idiomatic to use `if (currentUser && currentUser.admin) { /*...*/}` and save yourself some nesting. – Mark Jun 05 '19 at 00:01
1

@MarkMeyer yeah I had considered that but decided against that approach for the sake of maximum clarity – Dacre Denny Jun 05 '19 at 00:02
@MarkMeyer thanks for your suggestion - I've just extended the answer to include that – Dacre Denny Jun 05 '19 at 00:07

JavaScript: How to check if object method has valid result

1 Answers1