{"id":47689,"date":"2023-01-02T13:22:04","date_gmt":"2023-01-02T12:22:04","guid":{"rendered":"https:\/\/kinqsta.com\/dk\/?p=47689&post_type=knowledgebase&preview_id=47689"},"modified":"2025-10-01T20:03:45","modified_gmt":"2025-10-01T19:03:45","slug":"uncaught-typeerror-cannot-read-property","status":"publish","type":"post","link":"https:\/\/kinqsta.com\/dk\/blog\/uncaught-typeerror-cannot-read-property\/","title":{"rendered":"S\u00e5dan l\u00f8ser du fejlen “uncaught typeerror: cannot read property” i JavaScript"},"content":{"rendered":"

Fejl er en central del af det at l\u00e6re og bruge et programmeringssprog<\/a>. Fejlmeddelelser hj\u00e6lper dig med at forst\u00e5, hvad der er g\u00e5et galt et sted i din kode.<\/p>\n

Mens nogle fejlmeddelelser er ligetil og lette at forst\u00e5, kan andre v\u00e6re lidt forvirrende, n\u00e5r du ser dem.<\/p>\n\n

I denne vejledning vil vi tale om en af de mest almindelige JavaScript-fejl – “uncaught typeerror: cannot read property”-fejlen.<\/p>\n

Ved slutningen af denne tutorial b\u00f8r du forst\u00e5, hvad “uncaught typeerror: cannot read property” betyder, de almindelige \u00e5rsager, og hvordan du l\u00f8ser dem.<\/p>\n

<\/div><\/kinsta-auto-toc>\n

Hvad betyder “uncaught typeerror: cannot read property” i JavaScript?<\/h2>\n

Fejlen “uncaught typeerror: cannot read property” opst\u00e5r hovedsageligt, n\u00e5r du fors\u00f8ger at bruge eller f\u00e5 adgang til en undefined<\/code>-variabel. Denne fejl kan opst\u00e5, n\u00e5r du bruger vanilla JavaScript eller en hvilken som helst Javascript-f<\/a>ramework<\/a>.<\/p>\n

Du kan nemt opdage fejl under kodning ved at bruge forskellige udviklerv\u00e6rkt\u00f8jer som Chrome dev tools og Kinsta one-click staging-v\u00e6rkt\u00f8jet<\/a> til at opfange fejl, teste din kode og meget mere.<\/p>\n

S\u00e5dan l\u00f8ser du fejlen “uncaught typeerror: cannot read property” i JavaScript<\/h2>\n

I de f\u00f8lgende afsnit fremh\u00e6ver vi nogle af \u00e5rsagerne til “uncaught typeerror: cannot read property”-fejlen, og hvordan du l\u00f8ser dem ved hj\u00e6lp af kodeeksempler.<\/p>\n

Adgang til en objektegenskab med en v\u00e6rdi p\u00e5 udefineret<\/h3>\n

I dette afsnit vil vi diskutere en af de mulige \u00e5rsager til fejlen “uncaught typeerror: cannot read property” i JavaScript<\/a>. Dette har at g\u00f8re med adgang til en objektegenskab, der endnu ikke er udefineret.<\/p>\n

Her er et eksempel:<\/p>\n

let person;<\/code><\/pre>\n
I koden ovenfor har vi oprettet en variabel kaldet person<\/code>. Hensigten er at g\u00f8re denne variabel til et objekt med egenskaber som name<\/code>, age<\/code>, hobby<\/code>.<\/p>\n
Hvis du glemmer at oprette disse egenskaber, vil du f\u00e5 en fejl<\/a>, n\u00e5r du fors\u00f8ger at f\u00e5 adgang til dem. Det vil sige:<\/p>\n
let person;\nconsole.log(person.name);\n\/\/ Uncaught TypeError: Cannot read properties of undefined (reading 'name')<\/code><\/pre>\n
Som det kan ses ovenfor, fors\u00f8gte vi at f\u00e5 adgang til name<\/code>-egenskaben, som ikke eksisterer, s\u00e5 vi fik en fejl, der siger: Uncaught TypeError: Cannot read properties of undefined (reading ‘name’).<\/p>\n
Dette kan ske for alle, da du m\u00e5ske glemmer at oprette\/tilf\u00f8je egenskaber til dit objekt.<\/p>\n
Lad os g\u00e5 videre og tilf\u00f8je nogle egenskaber til objektet for at rette fejlen:<\/p>\n
let person = {\n  name: \"John\",\n  age: 200,\n  hobby: \"coding\"\n}\nconsole.log(person.name);\n\/\/ John<\/code><\/pre>\n
I koden ovenfor har vi tilf\u00f8jet name<\/code>-egenskaben, s\u00e5 n\u00e5r du fors\u00f8ger at f\u00e5 adgang til den, f\u00e5r du v\u00e6rdien “John” tilbage.<\/p>\n
Adgang til et element, der ikke findes i et array<\/h3>\n
Ligesom vi s\u00e5 i sidste afsnit med objekter, vil du ogs\u00e5 f\u00e5 fejlen “uncaught typeerror: cannot read property” udl\u00f8st, hvis du fors\u00f8ger at f\u00e5 adgang til et element i et array, der endnu ikke er initialiseret.<\/p>\n
Her er et eksempel:<\/p>\n
let arr;\nconsole.log(arr[0]);\n\/\/ Uncaught TypeError: Cannot read properties of undefined (reading '0')<\/code><\/pre>\n
I eksemplet ovenfor fors\u00f8gte vi at f\u00e5 adgang til det f\u00f8rste element i et array kaldet arr<\/code>.<\/p>\n
Arrayet er blevet erkl\u00e6ret, men ikke initialiseret – det betyder, at der endnu ikke er blevet tildelt noget element til det.<\/p>\n
For at rette op p\u00e5 dette, tilf\u00f8jer vi elementer til arrayet, f\u00f8r vi f\u00e5r adgang til dem. Det vil sige:<\/p>\n
let arr = [2,4,6,8]\nconsole.log(arr[0]);\n\/\/ 2<\/code><\/pre>\n
Adgang til et DOM-element, der ikke eksisterer<\/h3>\n
N\u00e5r du arbejder med Document Object Model (DOM) i JavaScript, kan du st\u00f8de p\u00e5 “uncaught typeerror: cannot read property”.<\/p>\n
Dette kan ske af forskellige \u00e5rsager, f.eks. stavefejl, mens du refererer til DOM-elementet, adgang til et element, der ikke findes, eller adgang til et element, f\u00f8r DOM er blevet indl\u00e6st (vi taler om dette i n\u00e6ste afsnit).<\/p>\n
I dette afsnit vil du se et eksempel, der kaster “uncaught typeerror: cannot read property”-fejlen for adgang til et ikke-eksisterende DOM-element.<\/p>\n
Her er HTML-koden:<\/p>\n
<!DOCTYPE html>\n<html>\n  <head>\n    <title>Error Tutorial<\/title>\n  <\/head>\n  <body>\n    <h1 id=\"content\">Hello World!<\/h1>\n    <script src=\"app.js\"><\/script>\n  <\/body>\n<\/html><\/code><\/pre>\n
I ovenst\u00e5ende kode har vi oprettet et grundl\u00e6ggende HTML-dokument med et h1<\/code>-element med ID “content”.<\/p>\n
Her er JavaScript-koden:<\/p>\n
let content = document.getElementById(\"constent\");\nconsole.log(content.textContent);\n\/\/ Uncaught TypeError: Cannot read properties of null (reading 'textContent');<\/code><\/pre>\n
Ovenst\u00e5ende kode giver anledning til fejlen “uncaught typeerror: cannot read property”, selv om det ser ud til, at vi har gjort alt rigtigt.<\/p>\n
Det er ret nemt at overse, hvad der udl\u00f8ser denne fejl. Den opst\u00e5r, fordi vi stavede “constent” i stedet for “content”, mens vi henviste til ID’et for h1<\/code>-elementet.<\/p>\n
Det vil sige document.getElementById(\"constent\");<\/code> i stedet for document.getElementById(\"content\");<\/code>.<\/p>\n
Den samme fejl vil blive udl\u00f8st, hvis du fors\u00f8ger at referere til et ID eller et klassens navn, der endnu ikke er oprettet i DOM.<\/p>\n
For at l\u00f8se dette kan du blot kontrollere for stavefejl eller sikre dig, at det element, der refereres til, faktisk findes i DOM.<\/p>\n
Placering af scriptelementet over andre DOM-elementer inden for body-tagget<\/h3>\n
Placeringen af dit script<\/code>-element bestemmer, hvordan din JavaScript-kode udf\u00f8res i browseren.<\/p>\n
Hvis du placerer script<\/code>-elementet over alle andre elementer i kroppen af dit HTML-dokument, vil JavaScript-koden blive k\u00f8rt, f\u00f8r DOM er blevet indl\u00e6st.<\/p>\n
Det betyder, at JavaScript ikke vil have nogen reference til DOM-elementerne, fordi det ikke har ventet p\u00e5, at de blev indl\u00e6st, f\u00f8r det blev eksekveret.<\/p>\n
Her er et eksempel:<\/p>\n
<!DOCTYPE html>\n<html>\n  <head>\n    <title>Error Tutorial<\/title>\n  <\/head>\n  <body>\n    <script src=\"app.js\"><\/script>\n    <h1 id=\"content\">Hello World!<\/h1>\n  <\/body>\n<\/html><\/code><\/pre>\n
I koden ovenfor placerede vi script<\/code> -elementet over h1<\/code> -elementet.<\/p>\n
Her er JavaScript-koden, hvor vi fors\u00f8ger at henvise til h1<\/code> -elementet ved hj\u00e6lp af dets ID:<\/p>\n
let content = document.getElementById(\"content\");\nconsole.log(content.textContent);\n\/\/ Uncaught TypeError: Cannot read properties of null (reading 'textContent');<\/code><\/pre>\n
Som forventet blev fejlen “uncaught typeerror: cannot read property” kastet p\u00e5 os, fordi JavaScript-koden f\u00f8r h1<\/code>-elementet kunne registreres i DOM’en.<\/p>\n
For at l\u00f8se dette problem skal du altid placere dit script<\/code>-element f\u00f8r det afsluttende body<\/code>-tag, dvs. under alle andre DOM-elementer i dokumentets krop. P\u00e5 denne m\u00e5de vil scriptet begynde at k\u00f8re, efter at hvert element i DOM er blevet indl\u00e6st.<\/p>\n
<!DOCTYPE html>\n<html>\n  <head>\n    <title>Error Tutorial<\/title>\n  <\/head>\n  <body>\n    <h1 id=\"content\">Hello World!<\/h1>\n    <script src=\"app.js\"><\/script>\n  <\/body>\n<\/html><\/code><\/pre>\n
Opsummering<\/h2>\n
Fejl er en uundg\u00e5elig del af det at v\u00e6re udvikler. Selv om de kan g\u00f8re dig frustreret, mens du koder, kan de ogs\u00e5 hj\u00e6lpe dig med at l\u00e6re mere og mestre det programmeringssprog, du bruger.<\/p>\n
N\u00e5r du st\u00e5r over for en fejl, b\u00f8r du altid fors\u00f8ge at forst\u00e5 fejlmeddelelsen, fordi det vil hj\u00e6lpe dig med at vide, hvor du skal begynde, og hvad du skal rette. Du vil ogs\u00e5 v\u00e6re i stand til at rette andre fejl med lignende fejlmeddelelser.<\/p>\n\n
Hvis du skal bygge et websted, er der en r\u00e6kke f\u00e6rdigheder, der skal l\u00e6res, og der kr\u00e6ves en masse \u00f8velse for at bruge disse f\u00e6rdigheder effektivt. DevKinsta g\u00f8r processen lettere fra design, udvikling og implementering, alt sammen p\u00e5 din lokale maskine. DevKinsta bruges af over 25.000 udviklere, webdesignere og freelancere. Tjek det ud gratis<\/a>\u00a0i dag.<\/p>\n","protected":false},"excerpt":{"rendered":"
Fejl er en central del af det at l\u00e6re og bruge et programmeringssprog. Fejlmeddelelser hj\u00e6lper dig med at forst\u00e5, hvad der er g\u00e5et galt et sted …<\/p>\n","protected":false},"author":240,"featured_media":47857,"comment_status":"open","ping_status":"closed","sticky":false,"template":"","format":"standard","meta":{"_kinsta_gated_content":false,"_kinsta_gated_content_redirect":"","footnotes":""},"tags":[],"topic":[704],"class_list":["post-47689","post","type-post","status-publish","format-standard","has-post-thumbnail","hentry","topic-javascript-fejl"],"yoast_head":"\nS\u00e5dan l\u00f8ser du fejlen "uncaught typeerror: cannot read property" i JavaScript<\/title>\n<meta name=\"description\" content=\"Der kan v\u00e6re flere \u00e5rsager til "uncaught typeerror" beskeden i JavaScript. 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