{"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":"asynkroninen-ja-yksisaikeinen-javascript-tapaavat-tapahtumasilmukan","status":"publish","type":"post","link":"https:\/\/thecodest.co\/fi\/blog\/asynchronous-and-single-threaded-javascript-meet-the-event-loop\/","title":{"rendered":"Asynkroninen ja yksis\u00e4ikeinen JavaScript?"},"content":{"rendered":"<ul class=\"wp-block-list\">\n<li><strong>Suoritusaika<\/strong><\/li>\n<\/ul>\n\n\n\n<p><a href=\"https:\/\/thecodest.co\/fi\/blog\/hire-javascript-developer\/\">JavaScript<\/a> on tulkattu kieli, ei k\u00e4\u00e4nnetty. T\u00e4m\u00e4 tarkoittaa sit\u00e4, ett\u00e4 se tarvitsee tulkin, joka muuntaa kielikoodin <a href=\"https:\/\/thecodest.co\/fi\/blog\/hire-vue-js-developers\/\">JS<\/a> <a href=\"https:\/\/thecodest.co\/fi\/dictionary\/what-is-code-refactoring\/\">koodi<\/a> konekoodiksi. Tulkkeja (ns. moottoreita) on useita erilaisia. Suosituimmat selainmoottorit ovat V8 (Chrome), Quantum (Firefox) ja WebKit (Safari). V8:aa k\u00e4ytet\u00e4\u00e4n muuten my\u00f6s suositussa muussa kuin selaimen ajoajassa, <a href=\"https:\/\/thecodest.co\/fi\/blog\/find-your-node-js-expert-for-hire-today\/\">Node.js<\/a>.<\/p>\n\n\n\n<p>Jokainen moottori sis\u00e4lt\u00e4\u00e4 muistikasan, kutsupinon, tapahtumasilmukan, takaisinkutsujonon ja WebAPI:n, jossa on HTTP-pyynt\u00f6j\u00e4, ajastimia, tapahtumia jne., jotka on kaikki toteutettu omalla tavallaan JS-koodin nopeamman ja turvallisemman tulkinnan varmistamiseksi.<\/p>\n\n\n\n<figure class=\"wp-block-image\"><img decoding=\"async\" src=\"\/app\/uploads\/2024\/05\/runtime-architecture.png\" alt=\"JavaScript-kehitys\" title=\"JavaScript-kehitys\"\/><\/figure>\n\n\n\n<p><em>JS:n perusarkkitehtuuri. Kirjoittaja: Alex Zlatkov<\/em><\/p>\n\n\n\n<h2 class=\"wp-block-heading\"><strong>Yksitt\u00e4inen lanka<\/strong><\/h2>\n\n\n\n<p>Yksis\u00e4ikeinen kieli on kieli, jossa on yksi kutsupino ja yksi muistikasa. Se tarkoittaa, ett\u00e4 sill\u00e4 suoritetaan vain yht\u00e4 asiaa kerrallaan.<\/p>\n\n\n\n<p>A&nbsp;<code>pino<\/code>&nbsp;on jatkuva muistialue, joka varaa paikallisen kontekstin jokaiselle suoritettavalle funktiolle.<\/p>\n\n\n\n<p>A&nbsp;<code>kasa<\/code>&nbsp;on paljon laajempi alue, johon tallennetaan kaikki dynaamisesti varattu.<\/p>\n\n\n\n<p>A&nbsp;<code>kutsupino<\/code>&nbsp;on <a href=\"https:\/\/thecodest.co\/fi\/blog\/app-data-collection-security-risks-value-and-types-explored\/\">tiedot<\/a> rakenne, joka periaatteessa tallentaa, miss\u00e4 vaiheessa ohjelmaa olemme.<\/p>\n\n\n\n<h3 class=\"wp-block-heading\"><strong>Kutsupino<\/strong><\/h3>\n\n\n\n<p>Kirjoitetaan yksinkertainen koodi ja seurataan, mit\u00e4 kutsupinossa tapahtuu.<\/p>\n\n\n\n<figure class=\"wp-block-image\"><img decoding=\"async\" src=\"\/app\/uploads\/2024\/05\/stack.gif\" alt=\"JavaScript-ohjelmiston kehitt\u00e4minen\" title=\"JavaScript-ohjelmiston kehitt\u00e4minen\"\/><\/figure>\n\n\n\n<p>Kuten n\u00e4et, funktiot lis\u00e4t\u00e4\u00e4n pinoon, suoritetaan ja my\u00f6hemmin poistetaan. T\u00e4m\u00e4 on niin sanottu LIFO-menetelm\u00e4 - Last In, First Out. Jokaista kutsupinon merkint\u00e4\u00e4 kutsutaan&nbsp;<code>pinokehys<\/code>.<\/p>\n\n\n\n<p>Kutsupinon tunteminen on hy\u00f6dyllist\u00e4 virhepinojen j\u00e4lkien lukemisessa. Yleens\u00e4 virheen tarkka syy on ensimm\u00e4isell\u00e4 rivill\u00e4 ylh\u00e4\u00e4ll\u00e4, vaikka koodin suoritusj\u00e4rjestys on alhaalta yl\u00f6sp\u00e4in.<\/p>\n\n\n\n<p>Joskus voit k\u00e4sitell\u00e4 suosittua virhett\u00e4, jonka ilmoittaa\u00a0<code>Kutsupinon enimm\u00e4iskoko ylitetty<\/code>. T\u00e4m\u00e4 on helppo saada aikaan rekursiolla:<\/p>\n\n\n\n<pre class=\"wp-block-code\"><code lang=\"javascript\" class=\"language-javascript\">funktio foo() {\n    foo()\n}\nfoo()<\/code><\/pre>\n\n\n\n<p>ja selaimemme tai p\u00e4\u00e4telaitteemme j\u00e4\u00e4tyy. Jokaisella selaimella, jopa niiden eri versioilla, on erilainen kutsupinon kokorajoitus. Suurimmassa osassa tapauksia ne ovat riitt\u00e4vi\u00e4, ja ongelma on etsitt\u00e4v\u00e4 muualta.<\/p>\n\n\n\n<h3 class=\"wp-block-heading\"><strong>Estetty kutsupino<\/strong><\/h3>\n\n\n\n<p>T\u00e4ss\u00e4 on esimerkki JS-s\u00e4ikeen est\u00e4misest\u00e4. Yritet\u00e4\u00e4n lukea&nbsp;<code>foo<\/code>&nbsp;tiedosto ja&nbsp;<code>baari<\/code>&nbsp;k\u00e4ytt\u00e4m\u00e4ll\u00e4 <a href=\"https:\/\/thecodest.co\/fi\/dictionary\/what-is-node-js-used-for\/\">Solmu<\/a>.js synkroninen toiminto&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-koodi\" title=\"JavaScript-koodi\"\/><\/figure>\n\n\n\n<p>T\u00e4m\u00e4 on silmukoitu GIF. Kuten n\u00e4et, JS-moottori odottaa, kunnes ensimm\u00e4inen kutsu kohdassa&nbsp;<code>readFileSync<\/code>&nbsp;on valmis. Mutta n\u00e4in ei tapahdu, koska ei ole olemassa&nbsp;<code>foo<\/code>&nbsp;tiedostossa, joten toista funktiota ei koskaan kutsuta.<\/p>\n\n\n\n<h2 class=\"wp-block-heading\"><strong>Asynkroninen k\u00e4ytt\u00e4ytyminen<\/strong><\/h2>\n\n\n\n<p>JS voi kuitenkin olla my\u00f6s lukkiutumaton ja k\u00e4ytt\u00e4yty\u00e4 kuin se olisi monis\u00e4ikeinen. Se tarkoittaa sit\u00e4, ett\u00e4 se ei odota vastausta <a href=\"https:\/\/thecodest.co\/fi\/blog\/compare-staff-augmentation-firms-that-excel-in-api-team-staffing-for-financial-technology-projects\/\">API<\/a> kutsu, I\/O-tapahtumat jne. ja voi jatkaa koodin suorittamista. T\u00e4m\u00e4 on mahdollista JS-moottoreiden ansiosta, jotka k\u00e4ytt\u00e4v\u00e4t (konepellin alla) todellisia monis\u00e4ikeistyskieli\u00e4, kuten C++ (Chrome) tai Rust (Firefox). Ne tarjoavat <a href=\"https:\/\/thecodest.co\/fi\/blog\/why-us-companies-are-opting-for-polish-developers\/\">us<\/a> kanssa <a href=\"https:\/\/thecodest.co\/fi\/blog\/find-your-ideal-stack-for-web-development\/\">Web<\/a> API selaimen hupun alla tai esim. I\/O API Node.js:n alla.<\/p>\n\n\n\n<figure class=\"wp-block-image\"><img decoding=\"async\" src=\"\/app\/uploads\/2024\/05\/callback-queue.gif\" alt=\"JavaScript-ohjelmointikieli\" title=\"JavaScript-ohjelmointikieli\"\/><\/figure>\n\n\n\n<p>Yll\u00e4 olevasta GIF-kuvasta n\u00e4emme, ett\u00e4 ensimm\u00e4inen funktio ty\u00f6nnet\u00e4\u00e4n kutsupinoon ja&nbsp;<code>Hei<\/code>&nbsp;suoritetaan v\u00e4litt\u00f6m\u00e4sti konsolissa.<\/p>\n\n\n\n<p>Sitten kutsumme&nbsp;<code>setTimeout<\/code>&nbsp;selaimen WebAPI:n tarjoama toiminto. Se siirtyy kutsupinoon ja sen asynkroniseen takaisinkutsuun.&nbsp;<code>foo<\/code>&nbsp;toiminto siirtyy WebApin jonoon, jossa se odottaa kutsua, joka tapahtuu 3 sekunnin kuluttua.<\/p>\n\n\n\n<p>Sill\u00e4 v\u00e4lin ohjelma jatkaa koodia ja n\u00e4emme, ett\u00e4&nbsp;<code>Hei. Minua ei ole estetty<\/code>&nbsp;konsolissa.<\/p>\n\n\n\n<p>Kun sit\u00e4 kutsutaan, jokainen WebAPI-jonossa oleva toiminto siirtyy komentosarjaan&nbsp;<code>Takaisinkutsujono<\/code>. Siin\u00e4 funktiot odottavat, kunnes kutsupino on tyhj\u00e4. Kun se tapahtuu, ne siirret\u00e4\u00e4n sinne yksi kerrallaan.<\/p>\n\n\n\n<p>Joten, kun meid\u00e4n&nbsp;<code>setTimeout<\/code>&nbsp;ajastin lopettaa l\u00e4ht\u00f6laskennan, meid\u00e4n&nbsp;<code>foo<\/code>&nbsp;funktio menee takaisinkutsujonoon, odottaa kunnes kutsupino tulee saataville, menee sinne, suoritetaan ja n\u00e4emme, ett\u00e4&nbsp;<code>Hei asynkronisesta takaisinkutsusta<\/code>&nbsp;konsolissa.<\/p>\n\n\n\n<h3 class=\"wp-block-heading\"><strong>Tapahtumasilmukka<\/strong><\/h3>\n\n\n\n<p>Kysymys kuuluu, mist\u00e4 runtime tiet\u00e4\u00e4, ett\u00e4 kutsupino on tyhj\u00e4, ja miten kutsujonossa oleva tapahtuma kutsutaan? Tapahtumasilmukka. Se on osa JS-moottoria. T\u00e4m\u00e4 prosessi tarkistaa jatkuvasti, onko kutsupino tyhj\u00e4, ja jos on, se seuraa, onko callback-jonossa tapahtuma, joka odottaa kutsua.<\/p>\n\n\n\n<p>Siin\u00e4 kaikki taika kulissien takana!<\/p>\n\n\n\n<h2 class=\"wp-block-heading\"><strong>Teorian kiteytt\u00e4minen<\/strong><\/h2>\n\n\n\n<h3 class=\"wp-block-heading\"><strong>Rinnakkaisuus ja rinnakkaisuus<\/strong><\/h3>\n\n\n\n<p><code>Samanaikaisuus<\/code>&nbsp;tarkoittaa useiden teht\u00e4vien suorittamista samanaikaisesti mutta ei samanaikaisesti. Esim. kaksi teht\u00e4v\u00e4\u00e4 toimii p\u00e4\u00e4llekk\u00e4isin\u00e4 ajanjaksoina.<\/p>\n\n\n\n<p><code>Rinnakkaisuus<\/code>&nbsp;tarkoittaa kahden tai useamman teht\u00e4v\u00e4n suorittamista samanaikaisesti, esimerkiksi useiden laskutoimitusten suorittamista samanaikaisesti.<\/p>\n\n\n\n<h3 class=\"wp-block-heading\"><strong>Kierteet ja prosessit<\/strong><\/h3>\n\n\n\n<p><code>Kierteet<\/code>&nbsp;ovat koodin suoritusjaksoja, jotka voidaan suorittaa toisistaan riippumatta.<\/p>\n\n\n\n<p><code>Prosessi<\/code>&nbsp;on k\u00e4ynniss\u00e4 olevan ohjelman instanssi. Ohjelmalla voi olla useita prosesseja.<\/p>\n\n\n\n<h3 class=\"wp-block-heading\"><strong>Synkroninen ja asynkroninen<\/strong><\/h3>\n\n\n\n<p>Osoitteessa&nbsp;<code>synkroninen<\/code>&nbsp;ohjelmoinnissa teht\u00e4v\u00e4t suoritetaan per\u00e4kk\u00e4in. Kukin teht\u00e4v\u00e4 odottaa, ett\u00e4 edellinen teht\u00e4v\u00e4 on valmis, ja se suoritetaan vasta sitten.<\/p>\n\n\n\n<p>Osoitteessa&nbsp;<code>asynkroninen<\/code>&nbsp;ohjelmointi, kun yksi teht\u00e4v\u00e4 on suoritettu, voit siirty\u00e4 toiseen teht\u00e4v\u00e4\u00e4n odottamatta edellisen teht\u00e4v\u00e4n valmistumista.<\/p>\n\n\n\n<h3 class=\"wp-block-heading\"><strong>Synkroninen ja asynkroninen yksis\u00e4ikeisess\u00e4 ja monis\u00e4ikeisess\u00e4 ymp\u00e4rist\u00f6ss\u00e4<\/strong><\/h3>\n\n\n\n<p><code>Synkroninen yhden s\u00e4ikeen kanssa<\/code>: Teht\u00e4v\u00e4t suoritetaan yksi toisensa j\u00e4lkeen. Kukin teht\u00e4v\u00e4 odottaa edellisen teht\u00e4v\u00e4n suorittamista.<\/p>\n\n\n\n<p><code>Synkroninen useiden s\u00e4ikeiden kanssa<\/code>: Teht\u00e4v\u00e4t suoritetaan eri s\u00e4ikeiss\u00e4, mutta ne odottavat muita suoritettavia teht\u00e4vi\u00e4 miss\u00e4 tahansa toisessa s\u00e4ikeess\u00e4.<\/p>\n\n\n\n<p><code>Asynkroninen yhdell\u00e4 s\u00e4ikeell\u00e4<\/code>: Teht\u00e4vi\u00e4 aletaan suorittaa odottamatta toisen teht\u00e4v\u00e4n valmistumista. Tiettyn\u00e4 aikana voidaan suorittaa vain yksi teht\u00e4v\u00e4.<\/p>\n\n\n\n<p><code>Asynkroninen useilla s\u00e4ikeill\u00e4<\/code>: Teht\u00e4v\u00e4t suoritetaan eri s\u00e4ikeiss\u00e4 odottamatta muiden teht\u00e4vien valmistumista ja suorittavat suorituksensa itsen\u00e4isesti.<\/p>\n\n\n\n<h2 class=\"wp-block-heading\"><strong>JavaScript-luokitus<\/strong><\/h2>\n\n\n\n<p>Jos tarkastelemme, miten JS-moottorit toimivat konepellin alla, voimme luokitella JS:n asynkroniseksi ja yksis\u00e4ikeiseksi tulkatuksi kieleksi. Sana \"tulkittu\" on eritt\u00e4in t\u00e4rke\u00e4, koska se tarkoittaa, ett\u00e4 kieli on aina ajasta riippuvainen eik\u00e4 koskaan yht\u00e4 nopea kuin k\u00e4\u00e4nnetyt kielet, joissa on sis\u00e4\u00e4nrakennettu monis\u00e4ikeist\u00e4minen.<\/p>\n\n\n\n<p>Huomionarvoista on, ett\u00e4 Node.js:ll\u00e4 voidaan toteuttaa todellista monis\u00e4ikeistyst\u00e4 edellytt\u00e4en, ett\u00e4 jokainen s\u00e4ie k\u00e4ynnistet\u00e4\u00e4n erillisen\u00e4 prosessina. T\u00e4t\u00e4 varten on olemassa kirjastoja, mutta Node.js:ss\u00e4 on sis\u00e4\u00e4nrakennettu ominaisuus nimelt\u00e4&nbsp;<a href=\"https:\/\/nodejs.org\/api\/worker_threads.html#worker_threads_worker_threads\">Ty\u00f6ntekij\u00e4n langat<\/a>.<\/p>\n\n\n\n<p>Kaikki tapahtumasilmukan GIF-kuvat tulevat&nbsp;<a href=\"https:\/\/github.com\/latentflip\/loupe\">Luuppi<\/a>&nbsp;Philip Robertsin luoma sovellus, jossa voit testata asynkronisia skenaarioita.<\/p>\n\n\n\n<p><b>Lue lis\u00e4\u00e4:<\/b><\/p>\n\n\n\n<p><a href=\"https:\/\/thecodest.co\/blog\/why-you-should-probably-use-typescript\">Miksi sinun pit\u00e4isi (luultavasti) k\u00e4ytt\u00e4\u00e4 Typescripti\u00e4?<\/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\/\">Laatu ensin! 5 helppoa vaihetta koodin niputtamiseen GitHub-ty\u00f6nkulkujen avulla JavaScript-projektissa.<\/a><\/p>\n\n\n\n<p><a href=\"https:\/\/thecodest.co\/blog\/how-to-improve-vue-js-apps-some-practical-tips\/\">Miten parantaa Vue.js-sovelluksia? Joitakin k\u00e4yt\u00e4nn\u00f6n vinkkej\u00e4<\/a><\/p>","protected":false},"excerpt":{"rendered":"<p>JavaScript on yksis\u00e4ikeinen kieli, joka on samalla my\u00f6s lukkiutumaton, asynkroninen ja samanaikainen. T\u00e4ss\u00e4 artikkelissa selitet\u00e4\u00e4n sinulle, miten se tapahtuu.<\/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=\"robots\" content=\"index, follow, max-snippet:-1, max-image-preview:large, max-video-preview:-1\" \/>\n<link rel=\"canonical\" href=\"https:\/\/thecodest.co\/fi\/blogi\/asynkroninen-ja-yksisaikeinen-javascript-tapaavat-tapahtumasilmukan\/\" \/>\n<meta property=\"og:locale\" content=\"fi_FI\" \/>\n<meta property=\"og:type\" content=\"article\" \/>\n<meta property=\"og:title\" content=\"Asynchronous and Single-threaded JavaScript?\" \/>\n<meta property=\"og:description\" content=\"JavaScript is a single-threaded language and, at the same time, also non-blocking, asynchronous and concurrent. 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