Typeerror: Cannot Read Property 'clear' of Undefined
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 set this one specifically, and along the way you lot'll learn how to arroyo fixing errors in general.
We'll comprehend how to read a stack trace, how to interpret the text of the fault, and ultimately how to set up it.
The Quick Fix
This error usually means you lot're trying to use .map on an assortment, just that array isn't divers notwithstanding.
That's often because the array is a slice of undefined state or an undefined prop.
Brand sure to initialize the state properly. That means if information technology will eventually be an assortment, employ useState([]) instead of something like useState() or useState(null).
Let'due south await at how nosotros can interpret an error bulletin and track down where it happened and why.
How to Find the Error
Kickoff guild of business is to figure out where the mistake is.
If you're using Create React App, it probably threw upwards a screen similar this:
TypeError
Cannot read property 'map' of undefined
App
vi | return (
7 | < div className = "App" >
eight | < h1 > List of Items < / h1 >
> ix | {items . map((item) => (
| ^
10 | < div key = {particular . id} >
11 | {item . proper name}
12 | < / div > Look for the file and the line number first.
Here, that'due south /src/App.js and line 9, taken from the light gray text to a higher place the lawmaking block.
btw, when you see something like /src/App.js:9:13, the way to decode that is filename:lineNumber:columnNumber.
How to Read the Stack Trace
If you're looking at the browser console instead, yous'll need to read the stack trace to figure out where the fault was.
These always expect long and intimidating, merely the trick is that normally you can ignore about of it!
The lines are in social club of execution, with the most contempo start.
Here's the stack trace for this error, with the but important lines highlighted:
TypeError: Cannot read belongings 'map' of undefined at App (App.js:nine) 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.evolution.js:2770) at invokeGuardedCallback (react-dom.development.js:2804) at beginWork $one (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.development.js:14770) at updateContainer (react-dom.development.js:17211) at eval (react-dom.development.js:17610) at unbatchedUpdates (react-dom.development.js:15104) at legacyRenderSubtreeIntoContainer (react-dom.development.js:17609) at Object.render (react-dom.development.js:17672) at evaluate (index.js:7) at z (eval.js:42) at K.evaluate (transpiled-module.js:692) at be.evaluateTranspiledModule (manager.js:286) at be.evaluateModule (manager.js:257) at compile.ts:717 at 50 (runtime.js:45) at Generator._invoke (runtime.js:274) at Generator.forEach.e. < computed > [equally next] (runtime.js:97) at t (asyncToGenerator.js:three) at i (asyncToGenerator.js:25) I wasn't kidding when I said you could ignore most of it! The first 2 lines are all we intendance nearly hither.
The first line is the error bulletin, and every line after that spells out the unwound stack of office calls that led to it.
Let's decode a couple of these lines:
Here nosotros have:
-
Appis the name of our component function -
App.jsis the file where information technology appears -
9is the line of that file where the error occurred
Let's wait at some other 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's a big file!)
Ignore Files That Aren't Yours
I already mentioned this but I wanted to land it explictly: when you're looking at a stack trace, yous tin can almost ever ignore whatsoever 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 first few lines.
Browse down the list until it starts to veer into file names you don't recognize.
There are some cases where you do care about the full stack, but they're few and far betwixt, in my feel. Things like… if yous suspect a bug in the library you're using, or if you call back some erroneous input is making its fashion into library code and blowing upwardly.
The vast bulk of the time, though, the bug volition be in your own code ;)
Follow the Clues: How to Diagnose the Fault
So the stack trace told us where to look: line ix of App.js. Let'southward open that upwards.
Here's the full text of that file:
import "./styles.css" ; export default function App () { let items ; return ( < div className = "App" > < h1 > Listing of Items </ h1 > { items . map ( item => ( < div cardinal = { detail .id } > { particular .proper noun } </ div > )) } </ div > ) ; } Line nine is this one:
And but for reference, here's that error message once again:
TypeError: Cannot read holding 'map' of undefined Let'due south break this downwardly!
-
TypeErroris the kind of error
There are a handful of built-in error types. MDN says TypeError "represents an fault that occurs when a variable or parameter is not of a valid type." (this role is, IMO, the least useful part of the error message)
-
Cannot read propertymeans the code was trying to read a belongings.
This is a skillful clue! In that location are only a few means to read properties 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.
In that location's also brackets (aka square brackets, []) for accessing items in an array, like items[5] or items['map'].
You might wonder why the error isn't more than specific, similar "Cannot read function `map` of undefined" – only remember, the JS interpreter has no idea what we meant that type to be. It doesn't know it was supposed to exist an array, or that map is a function. It didn't get that far, because items is undefined.
-
'map'is the property the lawmaking was trying to read
This one is some other dandy clue. Combined with the previous flake, yous can exist pretty sure you should be looking for .map somewhere on this line.
-
of undefinedis a clue near the value of the variable
It 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 at present y'all tin piece this all together:
- find the line that the fault occurred on (line 9, hither)
- scan that line looking for
.map - look at the variable/expression/whatever immediately before the
.mapand exist very suspicious of it.
One time y'all know which variable to await at, y'all can read through the part looking for where it comes from, and whether it'due south initialized.
In our little instance, the only other occurrence of items is line 4:
This defines the variable but it doesn't set it to anything, which means its value is undefined. There'due south the trouble. Prepare that, and you lot gear up the error!
Fixing This in the Real World
Of course this example is tiny and contrived, with a unproblematic mistake, and it's colocated very close to the site of the fault. These ones are the easiest to gear up!
At that place are a ton of potential causes for an error like this, though.
Maybe items is a prop passed in from the parent component – and you lot forgot to pass it down.
Or mayhap you did pass that prop, but the value being passed in is actually undefined or null.
If it'due south a local state variable, maybe y'all're initializing the country equally 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.
Whatever the case, though, the process is the same: get-go where the mistake is and work backwards, verifying your assumptions at each indicate the variable is used. Throw in some console.logdue south or use the debugger to inspect the intermediate values and figure out why it's undefined.
Yous'll become information technology fixed! Good luck :)
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Typeerror: Cannot Read Property 'clear' of Undefined
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