Typeerror: Cannot Read Property 'available' of Undefined
1 of the virtually common type errors in JavaScript is the famous "Cannot read holding of undefined". This fault occurs when you lot try to read or access a holding on an object that is undefined. Another mutual case that is caused by a like issue, is when you lot become the same error message, simply with null instead of undefined.
Cannot read holding of nothing
Why does this happen?
Imagine the following state of affairs. You have a user object that is initially undefined, and it is populated through a fetch request. Your server is down and so it returns with an undefined value, but you didn't handle this failure path, and you lot still endeavour to read properties from the user object:
let user; // The variable is set to undefined user = wait getUser (id) ; // The request fails and returns `undefined`, simply it is not handled console. log (user.proper noun) ; // You attempt to log the `proper name` property of the `user` object, that is still `undefined` Copied to clipboard!
The variable in the code example above is declared, but its value is withal fix to undefined. Here you are substantially trying to do the following:
console. log ( undefined .name) ; // This will throw "Cannot read property 'proper name' of undefined" // Same as if the request returns with a `nothing` and you endeavor to read properties from that console. log ( null .name) ; // This will throw "Cannot read property 'name' of null" Copied to clipboard!
Since undefined is non an object, you will get a TypeError, similar the one below. So how can nosotros avert this?
Avoiding errors
To avoid getting these types of errors, we need to make sure that the variables we are trying to read do have the correct value. This tin can exist done in diverse means. We can practice if checks before dealing with objects whose values are bound to modify:
if (user !== undefined ) { // Here `user` is surely not `undefined` } if ( typeof (user) !== 'undefined' ) { // We tin can also employ the `typeof` operator } Copied to clipboard!
A much cleaner solution however is to utilise the logical OR operator, when yous assign a value to a variable, or even better, when you return the value from the role:
// Assign a fallback during annunciation user = getUser (id) || { } ; // Assign a fallback during return const getUser = id => { ... return userResponse || { } ; } ; Copied to clipboard!
If getUser returns undefined or zip, and so we can fall dorsum to an empty object, and assign that to the user variable. This way, if we try to admission user.name, we will get undefined, every bit nosotros don't have that property on the user object, only we nonetheless have an object to work with, so nosotros don't get an error. We tin can likewise apply TypeScript to easily spot these types of mistakes correct within our IDE.
Resources:
- Logical OR operator
- The
typeofoperator
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