Cannot Read Property 'emit' of Undefined Socket.io
Got an error like this in your React component?
Cannot read property `map` of undefined
In this mail service we'll talk about how to prepare this one specifically, and along the manner you'll learn how to approach fixing errors in full general.
We'll comprehend how to read a stack trace, how to interpret the text of the error, and ultimately how to fix it.
The Quick Fix
This fault usually means you lot're trying to utilise .map on an array, merely that array isn't defined yet.
That's frequently considering the array is a piece of undefined state or an undefined prop.
Make certain to initialize the land properly. That means if it will eventually be an array, utilise useState([]) instead of something similar useState() or useState(zilch).
Permit'due south look at how nosotros can translate an error bulletin and rail down where it happened and why.
How to Detect the Fault
First gild of business is to figure out where the error is.
If you lot're using Create React App, it probably threw upward a screen like this:
TypeError
Cannot read property 'map' of undefined
App
6 | return (
7 | < div className = "App" >
8 | < h1 > Listing of Items < / h1 >
> nine | {items . map((item) => (
| ^
x | < div cardinal = {item . id} >
11 | {item . name}
12 | < / div > Expect for the file and the line number commencement.
Hither, that's /src/App.js and line 9, taken from the light gray text higher up the lawmaking block.
btw, when you see something like /src/App.js:nine:thirteen, the way to decode that is filename:lineNumber:columnNumber.
How to Read the Stack Trace
If you're looking at the browser console instead, you'll demand to read the stack trace to figure out where the fault was.
These always look long and intimidating, but the play a joke on is that usually you lot can ignore most of information technology!
The lines are in order of execution, with the virtually recent first.
Hither's the stack trace for this fault, with the only 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 $1 (react-dom.development.js:16114) at performUnitOfWork (react-dom.development.js:15339) at workLoopSync (react-dom.evolution.js:15293) at renderRootSync (react-dom.development.js:15268) at performSyncWorkOnRoot (react-dom.development.js:15008) at scheduleUpdateOnFiber (react-dom.evolution.js:14770) at updateContainer (react-dom.evolution.js:17211) at eval (react-dom.development.js:17610) at unbatchedUpdates (react-dom.development.js:15104) at legacyRenderSubtreeIntoContainer (react-dom.evolution.js:17609) at Object.render (react-dom.development.js:17672) at evaluate (alphabetize.js:vii) at z (eval.js:42) at G.evaluate (transpiled-module.js:692) at be.evaluateTranspiledModule (director.js:286) at exist.evaluateModule (manager.js:257) at compile.ts:717 at 50 (runtime.js:45) at Generator._invoke (runtime.js:274) at Generator.forEach.e. < computed > [as adjacent] (runtime.js:97) at t (asyncToGenerator.js:iii) at i (asyncToGenerator.js:25) I wasn't kidding when I said you could ignore virtually of it! The commencement 2 lines are all we care about hither.
The first line is the error message, and every line after that spells out the unwound stack of function calls that led to it.
Let's decode a couple of these lines:
Here we have:
-
Appis the name of our component function -
App.jsis the file where it appears -
9is the line of that file where the error occurred
Let's look at some other one:
at performSyncWorkOnRoot (react-dom.development.js:15008) -
performSyncWorkOnRootis the proper name of the office where this happened -
react-dom.development.jsis the file -
15008is the line number (it'south a large file!)
Ignore Files That Aren't Yours
I already mentioned this but I wanted to state it explictly: when yous're looking at a stack trace, you tin can about always ignore any lines that refer to files that are exterior your codebase, like ones from a library.
Usually, that means you'll pay attention to only the starting time few lines.
Scan down the list until information technology starts to veer into file names you don't recognize.
There are some cases where y'all do intendance about the full stack, but they're few and far between, in my experience. Things like… if y'all suspect a bug in the library yous're using, or if you think some erroneous input is making its manner into library code and blowing upward.
The vast majority of the time, though, the bug will be in your own lawmaking ;)
Follow the Clues: How to Diagnose the Mistake
And so the stack trace told us where to look: line 9 of App.js. Let'due south open that upwards.
Here's the full text of that file:
import "./styles.css" ; export default function App () { permit items ; return ( < div className = "App" > < h1 > List of Items </ h1 > { items . map ( item => ( < div cardinal = { item .id } > { item .proper name } </ div > )) } </ div > ) ; } Line 9 is this one:
And just for reference, hither's that error message over again:
TypeError: Cannot read holding 'map' of undefined Let'south break this down!
-
TypeErroris the kind of fault
There are a handful of built-in fault types. MDN says TypeError "represents an error that occurs when a variable or parameter is non of a valid blazon." (this part is, IMO, the least useful part of the fault message)
-
Cannot read belongingsmeans the code was trying to read a property.
This is a skillful clue! At that place are only a few ways to read backdrop in JavaScript.
The most common is probably the . operator.
Every bit in user.name, to access the proper name property of the user object.
Or items.map, to access the map property of the items object.
There's likewise brackets (aka square brackets, []) for accessing items in an array, like items[5] or items['map'].
You lot might wonder why the fault isn't more specific, like "Cannot read role `map` of undefined" – but remember, the JS interpreter has no idea what nosotros meant that blazon to exist. Information technology doesn't know it was supposed to be an assortment, or that map is a function. Information technology didn't get that far, because items is undefined.
-
'map'is the holding the code was trying to read
This one is another not bad clue. Combined with the previous bit, you can exist pretty sure you should be looking for .map somewhere on this line.
-
of undefinedis a clue about the value of the variable
It would be way more useful if the error could say "Cannot read holding `map` of items". Sadly information technology doesn't say that. Information technology tells you the value of that variable instead.
So now you tin can piece this all together:
- find the line that the error occurred on (line 9, here)
- scan that line looking for
.map - look at the variable/expression/any immediately before the
.mapand be very suspicious of it.
Once you know which variable to look at, you can read through the part looking for where information technology comes from, and whether it's initialized.
In our little example, the just other occurrence of items is line four:
This defines the variable merely it doesn't ready information technology to anything, which means its value is undefined. At that place's the problem. Ready that, and you fix the fault!
Fixing This in the Existent World
Of class this example is tiny and contrived, with a simple fault, and it's colocated very close to the site of the error. These ones are the easiest to ready!
There are a ton of potential causes for an mistake like this, though.
Mayhap items is a prop passed in from the parent component – and y'all forgot to pass it down.
Or maybe you lot did pass that prop, but the value beingness passed in is actually undefined or null.
If it's a local state variable, possibly you're initializing the state as undefined – useState(), written like that with no arguments, will practise exactly this!
If it's a prop coming from Redux, maybe your mapStateToProps is missing the value, or has a typo.
Any the instance, though, the process is the same: first where the error is and work backwards, verifying your assumptions at each betoken the variable is used. Throw in some panel.logs or apply the debugger to inspect the intermediate values and figure out why it's undefined.
Yous'll get it fixed! Good luck :)
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