{"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":"o-javascript-assincrono-e-de-thread-unico-encontra-o-loop-de-eventos","status":"publish","type":"post","link":"https:\/\/thecodest.co\/pt\/blog\/asynchronous-and-single-threaded-javascript-meet-the-event-loop\/","title":{"rendered":"Ass\u00edncrono e Single-threaded JavaScript?"},"content":{"rendered":"<ul class=\"wp-block-list\">\n<li><strong>Tempo de execu\u00e7\u00e3o<\/strong><\/li>\n<\/ul>\n\n\n\n<p><a href=\"https:\/\/thecodest.co\/pt\/blog\/hire-javascript-developer\/\">JavaScript<\/a> \u00e9 uma linguagem interpretada e n\u00e3o compilada. Isto significa que precisa de um int\u00e9rprete que converta o <a href=\"https:\/\/thecodest.co\/pt\/blog\/hire-vue-js-developers\/\">JS<\/a> <a href=\"https:\/\/thecodest.co\/pt\/dictionary\/what-is-code-refactoring\/\">c\u00f3digo<\/a> para um c\u00f3digo de m\u00e1quina. Existem v\u00e1rios tipos de interpretadores (conhecidos como motores). Os motores de browser mais populares s\u00e3o o V8 (Chrome), o Quantum (Firefox) e o WebKit (Safari). A prop\u00f3sito, o V8 tamb\u00e9m \u00e9 utilizado num runtime popular que n\u00e3o \u00e9 de um navegador, <a href=\"https:\/\/thecodest.co\/pt\/blog\/find-your-node-js-expert-for-hire-today\/\">Node.js<\/a>.<\/p>\n\n\n\n<p>Cada motor cont\u00e9m uma pilha de mem\u00f3ria, uma pilha de chamadas, um ciclo de eventos, uma fila de retorno de chamada e uma WebAPI com pedidos HTTP, temporizadores, eventos, etc., tudo implementado \u00e0 sua maneira para uma interpreta\u00e7\u00e3o mais r\u00e1pida e segura do c\u00f3digo JS.<\/p>\n\n\n\n<figure class=\"wp-block-image\"><img decoding=\"async\" src=\"\/app\/uploads\/2024\/05\/runtime-architecture.png\" alt=\"Desenvolvimento do JavaScript\" title=\"Desenvolvimento do JavaScript\"\/><\/figure>\n\n\n\n<p><em>Arquitetura b\u00e1sica de tempo de execu\u00e7\u00e3o JS. Autor: Alex Zlatkov<\/em><\/p>\n\n\n\n<h2 class=\"wp-block-heading\"><strong>Linha \u00fanica<\/strong><\/h2>\n\n\n\n<p>Uma linguagem single-thread \u00e9 uma linguagem com uma \u00fanica pilha de chamadas e uma \u00fanica pilha de mem\u00f3ria. Isto significa que s\u00f3 executa uma coisa de cada vez.<\/p>\n\n\n\n<p>A&nbsp;<code>pilha<\/code>&nbsp;\u00e9 uma regi\u00e3o cont\u00ednua de mem\u00f3ria, que atribui um contexto local a cada fun\u00e7\u00e3o executada.<\/p>\n\n\n\n<p>A&nbsp;<code>pilha<\/code>&nbsp;\u00e9 uma regi\u00e3o muito maior, que armazena tudo o que \u00e9 atribu\u00eddo dinamicamente.<\/p>\n\n\n\n<p>A&nbsp;<code>pilha de chamadas<\/code>&nbsp;\u00e9 um <a href=\"https:\/\/thecodest.co\/pt\/blog\/app-data-collection-security-risks-value-and-types-explored\/\">dados<\/a> estrutura que basicamente regista o ponto em que nos encontramos no programa.<\/p>\n\n\n\n<h3 class=\"wp-block-heading\"><strong>Pilha de chamadas<\/strong><\/h3>\n\n\n\n<p>Vamos escrever um c\u00f3digo simples e acompanhar o que est\u00e1 a acontecer na pilha de chamadas.<\/p>\n\n\n\n<figure class=\"wp-block-image\"><img decoding=\"async\" src=\"\/app\/uploads\/2024\/05\/stack.gif\" alt=\"Desenvolvimento de software JavaScript\" title=\"Desenvolvimento de software JavaScript\"\/><\/figure>\n\n\n\n<p>Como pode ver, as fun\u00e7\u00f5es s\u00e3o adicionadas \u00e0 pilha, executadas e posteriormente eliminadas. \u00c9 a chamada forma LIFO - Last In, First Out (\u00faltimo a entrar, primeiro a sair). Cada entrada na pilha de chamadas \u00e9 chamada de&nbsp;<code>estrutura da pilha<\/code>.<\/p>\n\n\n\n<p>O conhecimento da pilha de chamadas \u00e9 \u00fatil para ler os tra\u00e7os da pilha de erros. Geralmente, a raz\u00e3o exacta do erro est\u00e1 no topo da primeira linha, embora a ordem de execu\u00e7\u00e3o do c\u00f3digo seja de baixo para cima.<\/p>\n\n\n\n<p>Por vezes, \u00e9 poss\u00edvel lidar com um erro popular, notificado por\u00a0<code>Tamanho m\u00e1ximo da pilha de chamadas excedido<\/code>. \u00c9 f\u00e1cil obter isto utilizando a recurs\u00e3o:<\/p>\n\n\n\n<pre class=\"wp-block-code\"><code lang=\"javascript\" class=\"language-javascript\">fun\u00e7\u00e3o foo() {\n    foo()\n}\nfoo()<\/code><\/pre>\n\n\n\n<p>e o nosso browser ou terminal bloqueia. Cada navegador, mesmo nas suas diferentes vers\u00f5es, tem um limite de tamanho de pilha de chamadas diferente. Na grande maioria dos casos, s\u00e3o suficientes e o problema deve ser procurado noutro local.<\/p>\n\n\n\n<h3 class=\"wp-block-heading\"><strong>Pilha de chamadas bloqueada<\/strong><\/h3>\n\n\n\n<p>Eis um exemplo de bloqueio da thread JS. Vamos tentar ler um&nbsp;<code>bobo<\/code>&nbsp;e um ficheiro&nbsp;<code>bar<\/code>&nbsp;utilizando o <a href=\"https:\/\/thecodest.co\/pt\/dictionary\/what-is-node-js-used-for\/\">N\u00f3<\/a>Fun\u00e7\u00e3o s\u00edncrona .js&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=\"C\u00f3digo JavaScript\" title=\"C\u00f3digo JavaScript\"\/><\/figure>\n\n\n\n<p>Este \u00e9 um GIF em loop. Como pode ver, o motor JS espera at\u00e9 \u00e0 primeira chamada em&nbsp;<code>readFileSync<\/code>&nbsp;est\u00e1 conclu\u00eddo. Mas isso n\u00e3o acontecer\u00e1 porque n\u00e3o h\u00e1&nbsp;<code>bobo<\/code>&nbsp;por isso a segunda fun\u00e7\u00e3o nunca ser\u00e1 chamada.<\/p>\n\n\n\n<h2 class=\"wp-block-heading\"><strong>Comportamento ass\u00edncrono<\/strong><\/h2>\n\n\n\n<p>No entanto, o JS tamb\u00e9m pode ser n\u00e3o-bloqueante e comportar-se como se fosse multi-threaded. Isso significa que ele n\u00e3o espera pela resposta de um <a href=\"https:\/\/thecodest.co\/pt\/blog\/compare-staff-augmentation-firms-that-excel-in-api-team-staffing-for-financial-technology-projects\/\">API<\/a> chamada, eventos de E\/S, etc., e pode continuar a execu\u00e7\u00e3o do c\u00f3digo. Isto \u00e9 poss\u00edvel gra\u00e7as aos motores JS que utilizam (sob o cap\u00f4) linguagens multithreading reais, como C++ (Chrome) ou Rust (Firefox). Estas linguagens fornecem <a href=\"https:\/\/thecodest.co\/pt\/blog\/why-us-companies-are-opting-for-polish-developers\/\">n\u00f3s<\/a> com o <a href=\"https:\/\/thecodest.co\/pt\/blog\/find-your-ideal-stack-for-web-development\/\">Web<\/a> API sob as capas dos navegadores ou ex. API de E\/S no Node.js.<\/p>\n\n\n\n<figure class=\"wp-block-image\"><img decoding=\"async\" src=\"\/app\/uploads\/2024\/05\/callback-queue.gif\" alt=\"Linguagem de programa\u00e7\u00e3o JavaScript\" title=\"Linguagem de programa\u00e7\u00e3o JavaScript\"\/><\/figure>\n\n\n\n<p>No GIF acima, podemos ver que a primeira fun\u00e7\u00e3o \u00e9 empurrada para a pilha de chamadas e&nbsp;<code>Hi<\/code>&nbsp;\u00e9 imediatamente executado na consola.<\/p>\n\n\n\n<p>De seguida, chamamos o&nbsp;<code>setTimeout<\/code>&nbsp;fun\u00e7\u00e3o fornecida pela WebAPI do navegador. Vai para a pilha de chamadas e a sua chamada de retorno ass\u00edncrona&nbsp;<code>bobo<\/code>&nbsp;vai para a fila de espera do WebApi, onde aguarda a chamada, definida para acontecer ap\u00f3s 3 segundos.<\/p>\n\n\n\n<p>Entretanto, o programa continua o c\u00f3digo e vemos&nbsp;<code>Ol\u00e1. N\u00e3o estou bloqueado<\/code>&nbsp;na consola.<\/p>\n\n\n\n<p>Depois de ser invocada, cada fun\u00e7\u00e3o na fila da WebAPI vai para o&nbsp;<code>Fila de retorno de chamada<\/code>. \u00c9 onde as fun\u00e7\u00f5es esperam at\u00e9 que a pilha de chamadas esteja vazia. Quando isso acontece, elas s\u00e3o movidas para l\u00e1 uma a uma.<\/p>\n\n\n\n<p>Assim, quando o nosso&nbsp;<code>setTimeout<\/code>&nbsp;o temporizador termina a contagem decrescente, o nosso&nbsp;<code>bobo<\/code>&nbsp;vai para a fila de callbacks, espera at\u00e9 que a pilha de chamadas fique dispon\u00edvel, vai para l\u00e1, \u00e9 executada e vemos&nbsp;<code>Ol\u00e1 de uma chamada de retorno ass\u00edncrona<\/code>&nbsp;na consola.<\/p>\n\n\n\n<h3 class=\"wp-block-heading\"><strong>Ciclo de eventos<\/strong><\/h3>\n\n\n\n<p>A quest\u00e3o \u00e9: como \u00e9 que o tempo de execu\u00e7\u00e3o sabe que a pilha de chamadas est\u00e1 vazia e como \u00e9 que o evento na fila de retorno de chamada \u00e9 invocado? Conhe\u00e7a o loop de eventos. Ele faz parte do mecanismo JS. Este processo verifica constantemente se a pilha de chamadas est\u00e1 vazia e, se estiver, monitoriza se existe um evento na fila de resposta \u00e0 chamada \u00e0 espera de ser invocado.<\/p>\n\n\n\n<p>\u00c9 toda a magia dos bastidores!<\/p>\n\n\n\n<h2 class=\"wp-block-heading\"><strong>Conclus\u00e3o da teoria<\/strong><\/h2>\n\n\n\n<h3 class=\"wp-block-heading\"><strong>Concorr\u00eancia e paralelismo<\/strong><\/h3>\n\n\n\n<p><code>Concorr\u00eancia<\/code>&nbsp;significa a execu\u00e7\u00e3o de v\u00e1rias tarefas ao mesmo tempo, mas n\u00e3o em simult\u00e2neo. Por exemplo, duas tarefas funcionam em per\u00edodos de tempo sobrepostos.<\/p>\n\n\n\n<p><code>Paralelismo<\/code>&nbsp;significa executar duas ou mais tarefas em simult\u00e2neo, por exemplo, efetuar v\u00e1rios c\u00e1lculos ao mesmo tempo.<\/p>\n\n\n\n<h3 class=\"wp-block-heading\"><strong>T\u00f3picos e processos<\/strong><\/h3>\n\n\n\n<p><code>Fios<\/code>&nbsp;s\u00e3o uma sequ\u00eancia de execu\u00e7\u00e3o de c\u00f3digo que pode ser executada independentemente uma da outra.<\/p>\n\n\n\n<p><code>Processo<\/code>&nbsp;\u00e9 uma inst\u00e2ncia de um programa em execu\u00e7\u00e3o. Um programa pode ter v\u00e1rios processos.<\/p>\n\n\n\n<h3 class=\"wp-block-heading\"><strong>S\u00edncrono e ass\u00edncrono<\/strong><\/h3>\n\n\n\n<p>Em&nbsp;<code>s\u00edncrono<\/code>&nbsp;Na programa\u00e7\u00e3o, as tarefas s\u00e3o executadas uma ap\u00f3s a outra. Cada tarefa aguarda que a tarefa anterior esteja conclu\u00edda e s\u00f3 depois \u00e9 executada.<\/p>\n\n\n\n<p>Em&nbsp;<code>ass\u00edncrono<\/code>&nbsp;programa\u00e7\u00e3o, quando uma tarefa \u00e9 executada, \u00e9 poss\u00edvel mudar para uma tarefa diferente sem esperar que a anterior seja conclu\u00edda.<\/p>\n\n\n\n<h3 class=\"wp-block-heading\"><strong>S\u00edncrono e ass\u00edncrono num ambiente \u00fanico e multi-threaded<\/strong><\/h3>\n\n\n\n<p><code>S\u00edncrono com um \u00fanico segmento<\/code>: As tarefas s\u00e3o executadas uma ap\u00f3s a outra. Cada tarefa espera que a tarefa anterior seja executada.<\/p>\n\n\n\n<p><code>S\u00edncrono com v\u00e1rios threads<\/code>: As tarefas s\u00e3o executadas em diferentes threads mas esperam por outras tarefas em execu\u00e7\u00e3o em qualquer outra thread.<\/p>\n\n\n\n<p><code>Ass\u00edncrono com um \u00fanico thread<\/code>: As tarefas come\u00e7am a ser executadas sem esperar que uma tarefa diferente termine. Num determinado momento, apenas uma \u00fanica tarefa pode ser executada.<\/p>\n\n\n\n<p><code>Ass\u00edncrono com v\u00e1rios threads<\/code>: As tarefas s\u00e3o executadas em diferentes threads sem esperar que outras tarefas sejam conclu\u00eddas e terminam as suas execu\u00e7\u00f5es de forma independente.<\/p>\n\n\n\n<h2 class=\"wp-block-heading\"><strong>Classifica\u00e7\u00e3o JavaScript<\/strong><\/h2>\n\n\n\n<p>Se considerarmos o funcionamento dos motores JS, podemos classificar o JS como uma linguagem interpretada ass\u00edncrona e single-threaded. A palavra \"interpretada\" \u00e9 muito importante porque significa que a linguagem ser\u00e1 sempre dependente do tempo de execu\u00e7\u00e3o e nunca ser\u00e1 t\u00e3o r\u00e1pida como as linguagens compiladas com multi-threading incorporado.<\/p>\n\n\n\n<p>Vale ressaltar que o Node.js pode realizar multi-threading real, desde que cada thread seja iniciada como um processo separado. Existem bibliotecas para isso, mas o Node.js tem um recurso embutido chamado&nbsp;<a href=\"https:\/\/nodejs.org\/api\/worker_threads.html#worker_threads_worker_threads\">Fios de trabalho<\/a>.<\/p>\n\n\n\n<p>Todos os GIFs de loop de evento v\u00eam do&nbsp;<a href=\"https:\/\/github.com\/latentflip\/loupe\">Lupas<\/a>&nbsp;criada por Philip Roberts, onde pode testar os seus cen\u00e1rios ass\u00edncronos.<\/p>\n\n\n\n<p><b>Ler mais:<\/b><\/p>\n\n\n\n<p><a href=\"https:\/\/thecodest.co\/blog\/why-you-should-probably-use-typescript\">Porque \u00e9 que deve (provavelmente) utilizar 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\/\">A qualidade em primeiro lugar! 5 passos f\u00e1ceis para aplicar lint ao seu c\u00f3digo com fluxos de trabalho do GitHub no projeto JavaScript<\/a><\/p>\n\n\n\n<p><a href=\"https:\/\/thecodest.co\/blog\/how-to-improve-vue-js-apps-some-practical-tips\/\">Como melhorar as aplica\u00e7\u00f5es Vue.js? Algumas dicas pr\u00e1ticas<\/a><\/p>","protected":false},"excerpt":{"rendered":"<p>JavaScript \u00e9 uma linguagem single-threaded e, ao mesmo tempo, tamb\u00e9m non-blocking, ass\u00edncrona e concorrente. Este artigo explica-lhe como isso acontece.<\/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\/pt\/blogue\/o-javascript-assincrono-e-de-thread-unico-encontra-o-loop-de-eventos\/\" \/>\n<meta property=\"og:locale\" content=\"pt_PT\" \/>\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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