{"id":3044,"date":"2020-09-02T08:51:00","date_gmt":"2020-09-02T08:51:00","guid":{"rendered":"http:\/\/the-codest.localhost\/blog\/asynchronous-and-single-threaded-javascript-meet-the-event-loop\/"},"modified":"2026-04-24T11:34:04","modified_gmt":"2026-04-24T11:34:04","slug":"asynkront-og-enkelttradet-javascript-moder-event-loopen","status":"publish","type":"post","link":"https:\/\/thecodest.co\/da\/blog\/asynchronous-and-single-threaded-javascript-meet-the-event-loop\/","title":{"rendered":"Asynkron og enkelttr\u00e5det JavaScript?"},"content":{"rendered":"<ul class=\"wp-block-list\">\n<li><strong>Runtime<\/strong><\/li>\n<\/ul>\n\n\n\n<p><a href=\"https:\/\/thecodest.co\/da\/blog\/hire-javascript-developer\/\">JavaScript<\/a> er et fortolket sprog, ikke et kompileret. Det betyder, at det har brug for en fortolker, som konverterer <a href=\"https:\/\/thecodest.co\/da\/blog\/hire-vue-js-developers\/\">JS<\/a> <a href=\"https:\/\/thecodest.co\/da\/dictionary\/what-is-code-refactoring\/\">Kode<\/a> til en maskinkode. Der findes flere typer fortolkere (kendt som engines). De mest popul\u00e6re browser-engines er V8 (Chrome), Quantum (Firefox) og WebKit (Safari). V8 bruges i \u00f8vrigt ogs\u00e5 i en popul\u00e6r ikke-browser runtime, <a href=\"https:\/\/thecodest.co\/da\/blog\/find-your-node-js-expert-for-hire-today\/\">Node.js<\/a>.<\/p>\n\n\n\n<p>Hver motor indeholder en memory heap, en call stack, en event loop, en callback queue og en WebAPI med HTTP-anmodninger, timere, events osv., alt sammen implementeret p\u00e5 sin egen m\u00e5de for hurtigere og mere sikker fortolkning af JS-koden.<\/p>\n\n\n\n<figure class=\"wp-block-image\"><img decoding=\"async\" src=\"\/app\/uploads\/2024\/05\/runtime-architecture.png\" alt=\"JavaScript udvikling\" title=\"JavaScript udvikling\"\/><\/figure>\n\n\n\n<p><em>Grundl\u00e6ggende JS runtime-arkitektur. Forfatter: Alex Zlatkov<\/em><\/p>\n\n\n\n<h2 class=\"wp-block-heading\"><strong>Enkelt tr\u00e5d<\/strong><\/h2>\n\n\n\n<p>Et single-thread sprog er et sprog med en enkelt call stack og en enkelt memory heap. Det betyder, at det kun k\u00f8rer \u00e9n ting ad gangen.<\/p>\n\n\n\n<p>A&nbsp;<code>stak<\/code>&nbsp;er en kontinuerlig hukommelsesregion, der tildeler lokal kontekst til hver udf\u00f8rt funktion.<\/p>\n\n\n\n<p>A&nbsp;<code>bunke<\/code>&nbsp;er en meget st\u00f8rre region, der gemmer alt, hvad der er allokeret dynamisk.<\/p>\n\n\n\n<p>A&nbsp;<code>Kald stakken<\/code>&nbsp;er en <a href=\"https:\/\/thecodest.co\/da\/blog\/app-data-collection-security-risks-value-and-types-explored\/\">data<\/a> struktur, som grundl\u00e6ggende registrerer, hvor vi er i programmet.<\/p>\n\n\n\n<h3 class=\"wp-block-heading\"><strong>Kald p\u00e5 stakken<\/strong><\/h3>\n\n\n\n<p>Lad os skrive en simpel kode og f\u00f8lge med i, hvad der sker i kaldestakken.<\/p>\n\n\n\n<figure class=\"wp-block-image\"><img decoding=\"async\" src=\"\/app\/uploads\/2024\/05\/stack.gif\" alt=\"JavaScript softwareudvikling\" title=\"JavaScript softwareudvikling\"\/><\/figure>\n\n\n\n<p>Som du kan se, bliver funktionerne f\u00f8jet til stakken, udf\u00f8rt og senere slettet. Det er den s\u00e5kaldte LIFO-m\u00e5de - Last In, First Out. Hver post i opkaldsstakken kaldes en&nbsp;<code>Stakramme<\/code>.<\/p>\n\n\n\n<p>Kendskab til opkaldsstakken er nyttig, n\u00e5r man skal l\u00e6se fejlstakspor. Generelt st\u00e5r den n\u00f8jagtige \u00e5rsag til fejlen \u00f8verst i f\u00f8rste linje, selv om r\u00e6kkef\u00f8lgen af kodeafviklingen er nedefra og op.<\/p>\n\n\n\n<p>Nogle gange kan du h\u00e5ndtere en popul\u00e6r fejl, der meddeles af\u00a0<code>Maksimal st\u00f8rrelse p\u00e5 opkaldsstakken overskredet<\/code>. Det er nemt at f\u00e5 dette ved hj\u00e6lp af rekursion:<\/p>\n\n\n\n<pre class=\"wp-block-code\"><code lang=\"javascript\" class=\"language-javascript\">funktion foo() {\n    foo()\n}\nfoo()<\/code><\/pre>\n\n\n\n<p>og vores browser eller terminal fryser. Hver browser, selv deres forskellige versioner, har en forskellig gr\u00e6nse for st\u00f8rrelsen p\u00e5 opkaldsstakken. I langt de fleste tilf\u00e6lde er de tilstr\u00e6kkelige, og problemet b\u00f8r s\u00f8ges et andet sted.<\/p>\n\n\n\n<h3 class=\"wp-block-heading\"><strong>Blokeret kaldestak<\/strong><\/h3>\n\n\n\n<p>Her er et eksempel p\u00e5 blokering af JS-tr\u00e5den. Lad os pr\u00f8ve at l\u00e6se en&nbsp;<code>Fjols<\/code>&nbsp;fil og en&nbsp;<code>bar<\/code>&nbsp;ved hj\u00e6lp af <a href=\"https:\/\/thecodest.co\/da\/dictionary\/what-is-node-js-used-for\/\">Knudepunkt<\/a>.js synkron funktion&nbsp;<code>readFileSync<\/code>.<\/p>\n\n\n\n<figure class=\"wp-block-image\"><img decoding=\"async\" src=\"\/app\/uploads\/2024\/05\/blocking.gif\" alt=\"JavaScript-kode\" title=\"JavaScript-kode\"\/><\/figure>\n\n\n\n<p>Dette er en loopet GIF. Som du kan se, venter JS-motoren, indtil det f\u00f8rste kald i&nbsp;<code>readFileSync<\/code>&nbsp;er afsluttet. Men det vil ikke ske, fordi der ikke er nogen&nbsp;<code>Fjols<\/code>&nbsp;filen, s\u00e5 den anden funktion bliver aldrig kaldt.<\/p>\n\n\n\n<h2 class=\"wp-block-heading\"><strong>Asynkron adf\u00e6rd<\/strong><\/h2>\n\n\n\n<p>Men JS kan ogs\u00e5 v\u00e6re ikke-blokerende og opf\u00f8re sig, som om den var multi-threaded. Det betyder, at den ikke venter p\u00e5 svaret fra en <a href=\"https:\/\/thecodest.co\/da\/blog\/compare-staff-augmentation-firms-that-excel-in-api-team-staffing-for-financial-technology-projects\/\">API<\/a> kald, I\/O-begivenheder osv. og kan forts\u00e6tte med at udf\u00f8re koden. Det er muligt takket v\u00e6re JS-motorerne, som (under motorhjelmen) bruger \u00e6gte multi-threading-sprog som C++ (Chrome) eller Rust (Firefox). De giver <a href=\"https:\/\/thecodest.co\/da\/blog\/why-us-companies-are-opting-for-polish-developers\/\">os<\/a> med <a href=\"https:\/\/thecodest.co\/da\/blog\/find-your-ideal-stack-for-web-development\/\">Web<\/a> API under browserh\u00e6tterne eller f.eks. I\/O API under Node.js.<\/p>\n\n\n\n<figure class=\"wp-block-image\"><img decoding=\"async\" src=\"\/app\/uploads\/2024\/05\/callback-queue.gif\" alt=\"JavaScript programmeringssprog\" title=\"JavaScript programmeringssprog\"\/><\/figure>\n\n\n\n<p>I GIF'en ovenfor kan vi se, at den f\u00f8rste funktion skubbes til kaldestakken, og&nbsp;<code>Hej<\/code>&nbsp;udf\u00f8res med det samme i konsollen.<\/p>\n\n\n\n<p>Derefter kalder vi&nbsp;<code>setTimeout<\/code>&nbsp;funktion, der leveres af browserens WebAPI. Den g\u00e5r til opkaldsstakken og dens asynkrone tilbagekald&nbsp;<code>Fjols<\/code>&nbsp;funktionen g\u00e5r til WebApi-k\u00f8en, hvor den venter p\u00e5 opkaldet, der er sat til at ske efter 3 sekunder.<\/p>\n\n\n\n<p>I mellemtiden forts\u00e6tter programmet koden, og vi ser&nbsp;<code>Hej, jeg er ikke blokeret<\/code>&nbsp;i konsollen.<\/p>\n\n\n\n<p>N\u00e5r den er p\u00e5kaldt, g\u00e5r hver funktion i WebAPI-k\u00f8en til&nbsp;<code>K\u00f8 til tilbagekaldelse<\/code>. Det er her, funktioner venter, indtil kaldestakken er tom. N\u00e5r det sker, flyttes de derhen en efter en.<\/p>\n\n\n\n<p>S\u00e5 n\u00e5r vores&nbsp;<code>setTimeout<\/code>&nbsp;timeren afslutter nedt\u00e6llingen, vores&nbsp;<code>Fjols<\/code>&nbsp;funktionen g\u00e5r til tilbagekaldsk\u00f8en, venter p\u00e5, at kaldstakken bliver tilg\u00e6ngelig, g\u00e5r derhen, udf\u00f8res, og vi ser&nbsp;<code>Hej fra asynkront tilbagekald<\/code>&nbsp;i konsollen.<\/p>\n\n\n\n<h3 class=\"wp-block-heading\"><strong>Begivenheds-loop<\/strong><\/h3>\n\n\n\n<p>Sp\u00f8rgsm\u00e5let er, hvordan runtime ved, at callstacken er tom, og hvordan h\u00e6ndelsen i callback-k\u00f8en p\u00e5kaldes? M\u00f8d event loop. Det er en del af JS-motoren. Denne proces kontrollerer hele tiden, om opkaldsstakken er tom, og hvis den er, overv\u00e5ger den, om der er en begivenhed i tilbagekaldsk\u00f8en, som venter p\u00e5 at blive aktiveret.<\/p>\n\n\n\n<p>Det er al magien bag kulisserne!<\/p>\n\n\n\n<h2 class=\"wp-block-heading\"><strong>Afrunding af teorien<\/strong><\/h2>\n\n\n\n<h3 class=\"wp-block-heading\"><strong>Samtidighed og parallelisme<\/strong><\/h3>\n\n\n\n<p><code>Samtidighed<\/code>&nbsp;betyder, at man udf\u00f8rer flere opgaver p\u00e5 samme tid, men ikke samtidigt. F.eks. arbejder to opgaver i overlappende tidsperioder.<\/p>\n\n\n\n<p><code>Parallelisme<\/code>&nbsp;betyder at udf\u00f8re to eller flere opgaver samtidigt, f.eks. at udf\u00f8re flere beregninger p\u00e5 samme tid.<\/p>\n\n\n\n<h3 class=\"wp-block-heading\"><strong>Tr\u00e5de og processer<\/strong><\/h3>\n\n\n\n<p><code>Tr\u00e5de<\/code>&nbsp;er en sekvens af kodeudf\u00f8relse, som kan udf\u00f8res uafh\u00e6ngigt af hinanden.<\/p>\n\n\n\n<p><code>Proces<\/code>&nbsp;er en instans af et k\u00f8rende program. Et program kan have flere processer.<\/p>\n\n\n\n<h3 class=\"wp-block-heading\"><strong>Synkron og asynkron<\/strong><\/h3>\n\n\n\n<p>I&nbsp;<code>synkron<\/code>&nbsp;I programmering udf\u00f8res opgaverne en efter en. Hver opgave venter p\u00e5, at en tidligere opgave er afsluttet, og udf\u00f8res f\u00f8rst derefter.<\/p>\n\n\n\n<p>I&nbsp;<code>asynkron<\/code>&nbsp;programmering, n\u00e5r en opgave er udf\u00f8rt, kan du skifte til en anden opgave uden at vente p\u00e5, at den forrige er f\u00e6rdig.<\/p>\n\n\n\n<h3 class=\"wp-block-heading\"><strong>Synkron og asynkron i et enkelt- og flertr\u00e5det milj\u00f8<\/strong><\/h3>\n\n\n\n<p><code>Synkron med en enkelt tr\u00e5d<\/code>: Opgaverne udf\u00f8res en efter en. Hver opgave venter p\u00e5, at den foreg\u00e5ende opgave bliver udf\u00f8rt.<\/p>\n\n\n\n<p><code>Synkron med flere tr\u00e5de<\/code>: Opgaverne udf\u00f8res i forskellige tr\u00e5de, men venter p\u00e5 andre opgaver, der udf\u00f8res i en anden tr\u00e5d.<\/p>\n\n\n\n<p><code>Asynkron med en enkelt tr\u00e5d<\/code>: Opgaver begynder at blive udf\u00f8rt uden at vente p\u00e5, at en anden opgave bliver f\u00e6rdig. P\u00e5 et givet tidspunkt kan der kun udf\u00f8res en enkelt opgave.<\/p>\n\n\n\n<p><code>Asynkron med flere tr\u00e5de<\/code>: Opgaverne udf\u00f8res i forskellige tr\u00e5de uden at vente p\u00e5, at andre opgaver bliver f\u00e6rdige, og de afslutter deres udf\u00f8relse uafh\u00e6ngigt af hinanden.<\/p>\n\n\n\n<h2 class=\"wp-block-heading\"><strong>JavaScript-klassificering<\/strong><\/h2>\n\n\n\n<p>Hvis vi ser p\u00e5, hvordan JS-motorer fungerer under motorhjelmen, kan vi klassificere JS som et asynkront og enkelttr\u00e5det fortolket sprog. Ordet \"fortolket\" er meget vigtigt, fordi det betyder, at sproget altid vil v\u00e6re runtime-afh\u00e6ngigt og aldrig s\u00e5 hurtigt som kompilerede sprog med indbygget multi-threading.<\/p>\n\n\n\n<p>Det er bem\u00e6rkelsesv\u00e6rdigt, at Node.js kan opn\u00e5 \u00e6gte multi-threading, forudsat at hver tr\u00e5d startes som en separat proces. Der findes biblioteker til dette, men Node.js har en indbygget funktion, der hedder&nbsp;<a href=\"https:\/\/nodejs.org\/api\/worker_threads.html#worker_threads_worker_threads\">Arbejdstr\u00e5de<\/a>.<\/p>\n\n\n\n<p>Alle event-loop-GIF'er kommer fra&nbsp;<a href=\"https:\/\/github.com\/latentflip\/loupe\">Lupe<\/a>&nbsp;applikation skabt af Philip Roberts, hvor du kan teste dine asynkrone scenarier.<\/p>\n\n\n\n<p><b>L\u00e6s mere om det:<\/b><\/p>\n\n\n\n<p><a href=\"https:\/\/thecodest.co\/blog\/why-you-should-probably-use-typescript\">Hvorfor b\u00f8r du (sandsynligvis) bruge Typescript?<\/a><\/p>\n\n\n\n<p><a href=\"https:\/\/thecodest.co\/blog\/quality-first-5-easy-steps-to-lint-your-code-with-github-workflows-in-javascript-project\/\">Kvalitet f\u00f8rst! 5 nemme trin til at lint'e din kode med GitHub-workflows i JavaScript-projektet<\/a><\/p>\n\n\n\n<p><a href=\"https:\/\/thecodest.co\/blog\/how-to-improve-vue-js-apps-some-practical-tips\/\">Hvordan forbedrer man Vue.js-apps? Nogle praktiske tips<\/a><\/p>","protected":false},"excerpt":{"rendered":"<p>JavaScript er et single-threaded sprog og samtidig ogs\u00e5 ikke-blokerende, asynkront og concurrent. Denne artikel vil forklare dig, hvordan det sker.<\/p>","protected":false},"author":2,"featured_media":3045,"comment_status":"open","ping_status":"open","sticky":false,"template":"","format":"standard","meta":{"_acf_changed":false,"inline_featured_image":false,"footnotes":""},"categories":[8],"tags":[],"class_list":["post-3044","post","type-post","status-publish","format-standard","has-post-thumbnail","hentry","category-software-development"],"acf":[],"yoast_head":"<!-- This site is optimized with the Yoast SEO Premium plugin v27.3 (Yoast SEO v27.3) - https:\/\/yoast.com\/product\/yoast-seo-premium-wordpress\/ -->\n<title>Asynchronous and Single-threaded JavaScript? - The Codest<\/title>\n<meta name=\"description\" content=\"JavaScript is a single-threaded language and, at the same time, also non-blocking, asynchronous and concurrent. 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