Ionic 3 Cannot Read Property of Null
We have all seen many error messages while developing our Ionic apps. Some announced more frequent, others appear only on very special occasions. But peculiarly for beginners these messages tin be challenging equally the meaning of Ionic Problems is not always articulate.
The problem most of the time is that your app will run fine and the build is successful – the problems merely occur at runtime and therefore you lot are completely dependent on the hin inside the error bulletin and agreement those letters is an essential function of beingness a good developer.
In this commodity I'll outline 10 mutual error messages and problems that can happen quite often and how each of them can exist hands solved within your Ionic app!
1. Cannot read property 'xyz' of undefined
A very archetype error and one that tin exist hands tracked down although it'southward non immediately articulate where to look for the solution. This error comes upwardly when you take code similar this:
| // Inside your course item : cord ; // Inside your view { { item . myValue } } |
The holding of your object is not the trouble merely the object itself; you are trying to access a property of an element that's undefined (or in other cases null).
Bank check where you initialise your object or use the elvis operator to access properties safely without breaking your apps code:
| // ? operator prevents crashing here! { { item ? . myValue } } |
2. Uncaught (in hope): invalid link: MyPage
This error appears when you attempt to navigate to a page that's non existing. Mayhap yous spelled the proper name incorrect?
Especially when using lazy loading and passing page names as string, it can be tough to spot the problem as the linting won't requite you any indication that something is wrong.
Also when you go a message like "Nav Link error MyPage" information technology'southward related to the same obstacle, some navigation is simply not working because Ionic tin can't observe the folio yous want to navigate to (or which you lot take used as root for east.grand. a Tab bar).
As a solution, search your project for the proper name of the page of the mistake message and see if it's actually the existent proper noun of the folio you want to display!
3. _co.myFunction is non a function
This fault is quite easy to notice, it indicates you are calling a role from your template which you haven't implemented inside your class.
To prepare this, merely brand sure that you have a office alleged with the according name and that everything is spelled correctly. Again, this error won't come up upward through linting because JavaScript won't find errors within your template that are related to the class. Perchance i day we go this..
4. Cannot find control with proper name: 'myName'
If y'all are using reactive forms there are all kind of errors that can occur. This one is a very basic one and information technology indicates you lot are simply trying to employ a formControl which you haven't defined.
| // Inside your class this . registrationForm = this . fb . group ( { username : [ 'Simon' , [ Validators . required ] ] } ) ; // Inside your view < form [ formGroup ] = "registrationForm" > < ion - input type = "text" formControlName = "myName" > < / ion - input > < / form > |
In the lawmaking above you tin see that the form has a username control, but the name used inside the view is myName which is not role of the definition!
five. Type TestPage is part of the declarations of 2 modules
Now things get a bit more than catchy, as this error can be very hard to understand for beginners. What actually is a module?
When you use lazy loading for your pages, new generated pages will have their own module file correct side by side to them. Without lazy loading, pages don't have their own module file.
The play a joke on is: If you page doesn't have a module file (which means y'all are not using lazy loading) you need to reference all your pages within your app.module.ts file.
Merely if the page already has it'southward module and you import it somewhere else (for whatsoever reason) this error will come up and tell yous that y'all tin't have the page in 2 modules.
Therefore, bank check if you import your page in some place where information technology shouldn't exist and make sure it's just imported in ane module file!
6. No Provider for xyz
You retrieve you are pro structuring your code and outsourcing your concern logic to a provider, you inject it through your constructor but ffs why does this error appear?
This message is not really indicating the fix very good and therefore can be hard to find. The trouble is, you lot need to let Angular know most the providers of your app and then the internal Injector can actually put the correct provider into your course.
This error is related to dependency injection and in that location's a big internal process behind how you can go your providers through your constructor.
The fix?
Yous need to add your created Provider to the array of providers inside the app.module.ts so your app knows most information technology. With newer releases of the Ionic CLI this footstep should automatically happen, simply in case it'southward non you know how to fix it now.
7. Cannot find a differ supporting object '[object Object]' of type 'object'
Working with arrays inside your view and using *ngFor is an awesome feature of Angular, simply you lot can hands break things if you don't sentinel out. The fault message is not super helpful, but it has some additional data most of the time:
"But arrays and iterables are allowed Error: Cannot find a differ supporting object '[object Object]' of type 'object'. NgFor but supports binding to Iterables such as Arrays."
This should brand more than sense, but let'southward add a code example.
| // Inside your class items = { } ; // Inside your view < ion - list > < ion - item * ngFor = "let foo of items" > < / ion - detail > < / ion - list > |
The problem in the instance above is easy to spot: Y'all are trying to iterate over an object, simply ngFor expects an array!
While you can become all keys of an object, it is not meant to exist used this way.
Most of the time this error appears when you get data from a Remainder API and y'all are non sure almost the values. Perhaps you lot think something is an array, merely it really isn't.
So e'er check your responses, pick the right key where the result contains an array and make sure you are not passing a plain object to the ngFor!
8. Template parse errors: 'custom-component' is not a known chemical element
Now we got 2 errors that will immediately intermission your app once it starts, merely the trouble won't appear earlier (so build is still successful).
You tin can already note that Template parse errors means there'due south a problem within your template, your view file.
To get this error you might accept something similar this in your view:
| < ion - content > < special - component > < / special - component > < / ion - content > |
Y'all have created a Component and want to use it within your view. The problem is, by default the Ionic generated components are only added to a ComponentsModule, and yous have to import this module where yous need information technology!
This means, yous can either import it at the top level of your app in app.module.ts or with lazy loading import the module of your components only in the module file of your pages.
Then Angular will know how to reference this special tag of your view and insert the correct component that you've created before.
9. Template parse errors: Can't bind to 'enabled' since it isn't a known belongings of 'push'.
Another error that can occur with tags is this i, and it's actually pretty easy to track down.
Imagine a code like this:
| < push ion - button [ enabled ] = "imitation" > Exam < / button > |
Seem fine from the outside, but the problem is that enabled is non a property of button you can bind to!
A fix for this is to apply:
| < button ion - push [ disabled ] = "false" > Examination < / button > |
This is only one example, in general it simply means that you lot attempt to demark to something on the tag that's not really available. It cna happen for all tags, your components and all the things you use in your view.
If you are unsure most the backdrop of elements, simply check out the according API documentation and see what'due south available!
ten. cordova_not_available
To wrap this list up, let's end with a super classic i that hopefully everyone already knows about.
If you code your app and use the alive preview within your browser, most of the fourth dimension all is fine. But once you start to integrate Cordova plugins, you tin can't test or use those features within the simple preview.
The Cordova.js is just injected once y'all build your app on a device (or simulator), and therefore none of those features will piece of work inside the preview and you'll come across this error.
The set up is simple: Just run your app on a device or, a bit more than advanced way, is to mock your plugins.
You can override the Plugins so even on the browser the call returns a dummy issue and not an error. If you lot don't desire to write all your Mocks on your own, bank check out this awesome package from Chris Griffith to get a full set of Mock objects!
11. BONUS: Nothing works?
Every good list needs a petty bonus… And this one is rather simple.
Sometimes you lot fix all the problems, y'all are sure that everything is fine and all the same nothing works or nothing changes.
The solution?
Restart your livereload. Trust me, information technology has helped me and my students hundreds of times.
From fourth dimension to fourth dimension, Ionic releases can contain a few minor issues that break stuff. Things that worked don't work similar that should in the next release. It tin can happen.
Especially if you lot generate new files with the CLI I encountered this problem frequently, and the gear up was simply to close the command and start it once again.
If yous are lucky, everything will just piece of work again and then!
Anything missing?
Do you take any more than common issues y'all've stumbled upon while developing your next epic Ionic app?
Let us know in the comments below and I'll happily add together them to the listing to comprehend even more typical Ionic and Angular error letters!
Source: https://devdactic.com/10-ionic-problems/
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