Jest Typeerror Cannot Read Property of Undefined
Got an error similar this in your React component?
Cannot read property `map` of undefined
In this post we'll talk about how to fix this one specifically, and forth the way yous'll learn how to arroyo fixing errors in full general.
Nosotros'll comprehend how to read a stack trace, how to interpret the text of the error, and ultimately how to prepare it.
The Quick Gear up
This error commonly means you lot're trying to use .map on an assortment, but that array isn't defined yet.
That's ofttimes because the assortment is a slice of undefined land or an undefined prop.
Make certain to initialize the state properly. That ways if it will eventually be an assortment, employ useState([]) instead of something like useState() or useState(zippo).
Permit's look at how we can interpret an error message and track downwards where it happened and why.
How to Find the Error
Get-go order of concern is to figure out where the error is.
If you're using Create React App, information technology probably threw up a screen like this:
TypeError
Cannot read property 'map' of undefined
App
6 | return (
7 | < div className = "App" >
8 | < h1 > List of Items < / h1 >
> 9 | {items . map((item) => (
| ^
10 | < div cardinal = {item . id} >
11 | {item . name}
12 | < / div > Look for the file and the line number commencement.
Here, that's /src/App.js and line 9, taken from the light grey text above the code cake.
btw, when you lot come across something similar /src/App.js:9:xiii, the fashion to decode that is filename:lineNumber:columnNumber.
How to Read the Stack Trace
If yous're looking at the browser console instead, you'll need to read the stack trace to figure out where the mistake was.
These always look long and intimidating, only the trick is that ordinarily you can ignore near of information technology!
The lines are in gild of execution, with the most contempo first.
Hither'south the stack trace for this error, 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.evolution.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.evolution.js:2804) at beginWork $1 (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.development.js:15104) at legacyRenderSubtreeIntoContainer (react-dom.development.js:17609) at Object.render (react-dom.development.js:17672) at evaluate (index.js:vii) at z (eval.js:42) at 1000.evaluate (transpiled-module.js:692) at exist.evaluateTranspiledModule (manager.js:286) at exist.evaluateModule (director.js:257) at compile.ts:717 at fifty (runtime.js:45) at Generator._invoke (runtime.js:274) at Generator.forEach.e. < computed > [as adjacent] (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 commencement 2 lines are all nosotros intendance about here.
The first line is the error 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:
Hither we have:
-
Appis the name of our component function -
App.jsis the file where it appears -
9is the line of that file where the mistake occurred
Let'south wait at another ane:
at performSyncWorkOnRoot (react-dom.evolution.js:15008) -
performSyncWorkOnRootis the name of the office where this happened -
react-dom.evolution.jsis the file -
15008is the line number (information technology's a big file!)
Ignore Files That Aren't Yours
I already mentioned this but I wanted to land it explictly: when you lot're looking at a stack trace, yous can most always ignore any lines that refer to files that are outside your codebase, like ones from a library.
Ordinarily, that means y'all'll pay attention to merely the get-go few lines.
Scan downwards the listing until information technology starts to veer into file names y'all don't recognize.
There are some cases where you do care about the full stack, but they're few and far between, in my experience. Things like… if you suspect a bug in the library y'all're using, or if yous recollect some erroneous input is making its way into library code and blowing up.
The vast majority of the fourth dimension, though, the bug will be in your own code ;)
Follow the Clues: How to Diagnose the Error
So the stack trace told us where to look: line ix of App.js. Let's open that up.
Here's the total text of that file:
import "./styles.css" ; export default function App () { let items ; render ( < div className = "App" > < h1 > List of Items </ h1 > { items . map ( item => ( < div key = { particular .id } > { particular .name } </ div > )) } </ div > ) ; } Line nine is this ane:
And just for reference, hither's that error message again:
TypeError: Cannot read holding 'map' of undefined Let'south break this down!
-
TypeErroris the kind of fault
In that location 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 part is, IMO, the to the lowest degree useful office of the error message)
-
Cannot read belongingsmeans the lawmaking was trying to read a property.
This is a practiced clue! There are but a few ways to read backdrop in JavaScript.
The most common is probably the . operator.
As in user.proper name, to access the name property of the user object.
Or items.map, to access the map holding of the items object.
There'south also brackets (aka square brackets, []) for accessing items in an array, similar items[5] or items['map'].
You lot might wonder why the error isn't more than specific, like "Cannot read office `map` of undefined" – but remember, the JS interpreter has no thought what we meant that type to exist. It doesn't know information technology was supposed to be an array, or that map is a function. It didn't go that far, because items is undefined.
-
'map'is the property the code was trying to read
This one is another great inkling. Combined with the previous fleck, you tin be pretty sure y'all should be looking for .map somewhere on this line.
-
of undefinedis a clue about the value of the variable
It would be mode 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 tin can slice this all together:
- observe the line that the error occurred on (line nine, here)
- browse that line looking for
.map - look at the variable/expression/whatever immediately before the
.mapand be very suspicious of it.
Once you 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 little example, the only other occurrence of items is line 4:
This defines the variable only it doesn't set it to annihilation, which ways its value is undefined. In that location's the trouble. Fix that, and you fix the error!
Fixing This in the Real World
Of grade this example is tiny and contrived, with a simple mistake, and it's colocated very shut to the site of the error. These ones are the easiest to fix!
There are a ton of potential causes for an error similar this, though.
Maybe items is a prop passed in from the parent component – and you forgot to laissez passer it down.
Or maybe you did laissez passer that prop, merely the value existence passed in is really undefined or null.
If it'southward a local state variable, maybe you're initializing the state as undefined – useState(), written similar that with no arguments, volition do 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: commencement where the error is and work backwards, verifying your assumptions at each point the variable is used. Throw in some panel.logs or utilize the debugger to inspect the intermediate values and figure out why information technology'south undefined.
You'll become it fixed! Good luck :)
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