Cannot Read Properdty Height of Undefined Error Rpg Maker Mv Battle
Got an mistake like this in your React component?
Cannot read property `map` of undefined
In this post we'll talk about how to ready this ane specifically, and forth the way you'll acquire how to approach fixing errors in general.
Nosotros'll cover how to read a stack trace, how to interpret the text of the mistake, and ultimately how to fix it.
The Quick Prepare
This error usually ways you're trying to use .map on an array, simply that array isn't defined yet.
That'southward frequently because the array is a piece of undefined state or an undefined prop.
Brand sure to initialize the state properly. That means if it will eventually be an array, utilise useState([]) instead of something like useState() or useState(null).
Let's look at how nosotros tin interpret an error bulletin and track down where it happened and why.
How to Observe the Error
First order of business is to figure out where the error is.
If you're using Create React App, information technology probably threw upwards a screen like this:
TypeError
Cannot read property 'map' of undefined
App
six | return (
7 | < div className = "App" >
8 | < h1 > Listing of Items < / h1 >
> nine | {items . map((item) => (
| ^
10 | < div key = {item . id} >
11 | {particular . proper noun}
12 | < / div > Wait for the file and the line number kickoff.
Here, that's /src/App.js and line 9, taken from the calorie-free gray text above the code block.
btw, when you see something like /src/App.js:ix:13, the style to decode that is filename:lineNumber:columnNumber.
How to Read the Stack Trace
If you lot're looking at the browser console instead, you lot'll demand to read the stack trace to figure out where the error was.
These always wait long and intimidating, but the trick is that usually y'all tin can ignore virtually of it!
The lines are in guild of execution, with the about recent first.
Here's the stack trace for this error, with the but important lines highlighted:
TypeError: Cannot read property 'map' of undefined at App (App.js:9) at renderWithHooks (react-dom.development.js:10021) at mountIndeterminateComponent (react-dom.development.js:12143) at beginWork (react-dom.development.js:12942) at HTMLUnknownElement.callCallback (react-dom.development.js:2746) at Object.invokeGuardedCallbackDev (react-dom.development.js:2770) at invokeGuardedCallback (react-dom.development.js:2804) at beginWork $i (react-dom.evolution.js:16114) at performUnitOfWork (react-dom.evolution.js:15339) at workLoopSync (react-dom.development.js:15293) at renderRootSync (react-dom.development.js:15268) at performSyncWorkOnRoot (react-dom.development.js:15008) at scheduleUpdateOnFiber (react-dom.development.js:14770) at updateContainer (react-dom.development.js:17211) at eval (react-dom.evolution.js:17610) at unbatchedUpdates (react-dom.evolution.js:15104) at legacyRenderSubtreeIntoContainer (react-dom.development.js:17609) at Object.render (react-dom.evolution.js:17672) at evaluate (alphabetize.js:seven) at z (eval.js:42) at Thousand.evaluate (transpiled-module.js:692) at be.evaluateTranspiledModule (manager.js:286) at be.evaluateModule (manager.js:257) at compile.ts:717 at l (runtime.js:45) at Generator._invoke (runtime.js:274) at Generator.forEach.e. < computed > [every bit next] (runtime.js:97) at t (asyncToGenerator.js:three) at i (asyncToGenerator.js:25) I wasn't kidding when I said you could ignore almost of it! The first two lines are all we care about hither.
The starting time line is the fault message, and every line after that spells out the unwound stack of function calls that led to information technology.
Let's decode a couple of these lines:
Here nosotros have:
-
Appis the name of our component role -
App.jsis the file where it appears -
9is the line of that file where the mistake occurred
Let's look at another one:
at performSyncWorkOnRoot (react-dom.development.js:15008) -
performSyncWorkOnRootis the name of the role where this happened -
react-dom.development.jsis the file -
15008is the line number (it'southward a big file!)
Ignore Files That Aren't Yours
I already mentioned this but I wanted to country information technology explictly: when you're looking at a stack trace, you can about always ignore whatsoever lines that refer to files that are exterior your codebase, like ones from a library.
Commonly, that means you'll pay attention to but the commencement few lines.
Scan downwardly the listing until it starts to veer into file names yous don't recognize.
There are some cases where you practice care nigh the full stack, only they're few and far betwixt, in my experience. Things like… if you suspect a bug in the library y'all're using, or if y'all think some erroneous input is making its style into library lawmaking and blowing up.
The vast majority of the time, though, the bug will be in your ain code ;)
Follow the Clues: How to Diagnose the Error
So the stack trace told u.s. where to await: line nine of App.js. Permit'due south open that upward.
Here'due south the total text of that file:
import "./styles.css" ; consign default function App () { let items ; render ( < div className = "App" > < h1 > List of Items </ h1 > { items . map ( item => ( < div key = { item .id } > { item .proper name } </ div > )) } </ div > ) ; } Line nine is this one:
And just for reference, here'due south that fault message again:
TypeError: Cannot read property 'map' of undefined Let'south break this downwardly!
-
TypeErroris the kind of error
In that location are a scattering of born fault types. MDN says TypeError "represents an error that occurs when a variable or parameter is not of a valid blazon." (this part is, IMO, the to the lowest degree useful part of the error message)
-
Cannot read propertyways the lawmaking was trying to read a property.
This is a good clue! At that place are only a few ways to read backdrop in JavaScript.
The most common is probably the . operator.
As in user.name, to access the name property of the user object.
Or items.map, to access the map property of the items object.
At that place'south also brackets (aka foursquare brackets, []) for accessing items in an array, like items[v] or items['map'].
You might wonder why the error isn't more than specific, like "Cannot read function `map` of undefined" – but remember, the JS interpreter has no idea what we meant that blazon to be. It doesn't know it was supposed to be an array, or that map is a role. Information technology didn't get that far, because items is undefined.
-
'map'is the property the lawmaking was trying to read
This one is some other great clue. Combined with the previous bit, you can be pretty sure you should be looking for .map somewhere on this line.
-
of undefinedis a clue about the value of the variable
Information technology would be way more useful if the error could say "Cannot read property `map` of items". Sadly it doesn't say that. It tells you the value of that variable instead.
So now you lot can slice this all together:
- find the line that the fault occurred on (line 9, here)
- scan that line looking for
.map - look at the variable/expression/whatever immediately before the
.mapand exist very suspicious of it.
Once yous know which variable to look at, you lot can read through the function looking for where it comes from, and whether it's initialized.
In our piddling example, the only other occurrence of items is line 4:
This defines the variable but information technology doesn't set information technology to anything, which ways its value is undefined. There'south the trouble. Fix that, and you fix the error!
Fixing This in the Existent World
Of course this example is tiny and contrived, with a simple mistake, and it's colocated very close to the site of the error. These ones are the easiest to ready!
At that place are a ton of potential causes for an error similar this, though.
Mayhap items is a prop passed in from the parent component – and you forgot to pass it downward.
Or possibly you lot did laissez passer that prop, but the value existence passed in is actually undefined or null.
If it's a local state variable, peradventure you lot're initializing the land as undefined – useState(), written like that with no arguments, will do exactly this!
If it's a prop coming from Redux, mayhap your mapStateToProps is missing the value, or has a typo.
Whatever the case, though, the process is the aforementioned: offset where the fault is and work backwards, verifying your assumptions at each point the variable is used. Throw in some console.logs or apply the debugger to inspect the intermediate values and effigy out why information technology'south undefined.
You'll become information technology fixed! Adept luck :)
Success! Now bank check your e-mail.
Learning React tin can exist a struggle — so many libraries and tools!
My advice? Ignore all of them :)
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@lavenderlens
Source: https://daveceddia.com/fix-react-errors/
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