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Например:\r\n```javascript\r\nconst isTrue = (content) => {\r\n return !!content;\r\n};\r\n```","best_answer_id":323,"related_stacks_count":5},"answers":[{"user":{"id":75907,"email":"dzencot@gmail.com","first_name":"Ivan","last_name":"Gagarinov","telegram":"igagarinov","full_name":"Ivan Gagarinov","removed":false},"question":{"creator":{"id":75907,"email":"dzencot@gmail.com","first_name":"Ivan","last_name":"Gagarinov","telegram":"igagarinov","full_name":"Ivan Gagarinov","removed":false},"category":{"id":2,"title":"JavaScript","slug":"javascript","questions_count":852,"locale":"ru"},"tags":[{"id":663,"slug":"privedenie-tipov","name":"приведение типов"},{"id":633,"slug":"javascript","name":"JavaScript"}],"id":1889,"answers_count":2,"slug":"chto-oznachaet-dva-vosklitsatelnyh-znaka-pered-vyrazheniem","state":"published","title":"Что означает два восклицательных знака \"!!\" перед выражением?","created_at":"2022-04-26T12:31:16.131Z","details":"Что означает, когда пишут два восклицательных знака перед переменной или любым другим выражением? Например:\r\n```javascript\r\nconst isTrue = (content) => {\r\n return !!content;\r\n};\r\n```","best_answer_id":323,"related_stacks_count":5},"id":323,"state":"active","body":"Прежде всего давайте вспомним что вообще делает восклицательный знак в обычных ситуациях. Он является логическим оператором \"не\". Простой пример, проверяем что выражения между собой равны:\r\n\r\n```javascript\r\nconsole.log('test' === 'test'); // => true\r\n```\r\n\r\nЕсли хотим проверить что они не равны, то добавляем восклицательный знак:\r\n\r\n```javascript\r\nconsole.log('test' !== 'test'); // => false\r\n```\r\n\r\nТо есть мы \"перевернули\" (инвертировали) проверку. Примерно то же самое происходит, когда мы добавляем восклицательный знак перед любым выражением:\r\n\r\n```javascript\r\nconst myVar = 'test';\r\nconsole.log(!myVar); // => false\r\n```\r\n\r\nОбратите внимание, перед `myVar` стоит восклицательный знак, это отрицание, то есть можно прочитать это как \"**не** myVar\". Восклицательный знак инвертирует наше выражение, которое является строкой 'test'. Но тут возникает вопрос: а что является не строкой 'test'? Если подойти к вопросу философски, то можно найти бесконечное множество вещей, которые являются не строкой 'test'. Но у интерпретатора JS на этот счёт есть чёткое правило: он вычисляет выражение справа от восклицательного знака так, чтобы получить либо `true`, либо `false`, то есть приводит к булевому типу. Строка 'test' приводится к булевому типу и вычисляется как `true`, а восклицательный знак инвертирует, то есть переворачивает `true` на `false`, так как `false` является противоположностью `true`.\r\n\r\nТеперь гораздо легче понять что делают два восклицательных знака:\r\n```javascript\r\nconst isTrue = (content) => {\r\n return !!content;\r\n};\r\n```\r\nИнтерпретатор пытается вычислить сначала `!content`. Если `content` будет ложным выражением, то вычислится `true`, если истинным, то `false`. А второй восклицательный знак переворачивает `true` на `false`, а `false` на `true`. В итоге, если контент существует, то функция вернёт `true`, а если не существует, то `false`.\r\n\r\nТеперь возникает вопрос: зачем вообще это нужно? Это делается чтобы мы всегда работали с определённым типом. Такой хитрый способ позволяет нам добиться чтобы функция всегда возвращала булевый тип. Любое значение будет преобразовываться в этот булевый тип. Это называется **приведение типов**.","votes_up_count":40,"votes_down_count":0,"created_at":"2022-04-26T12:59:16.958Z","user_id":75907,"category_slug":"javascript"},{"user":{"id":838612,"email":"dchuvikovsky@mail.ru","first_name":"Дмитрий","last_name":"Чувиковский","telegram":"","full_name":"Дмитрий Чувиковский","removed":false},"question":{"creator":{"id":75907,"email":"dzencot@gmail.com","first_name":"Ivan","last_name":"Gagarinov","telegram":"igagarinov","full_name":"Ivan Gagarinov","removed":false},"category":{"id":2,"title":"JavaScript","slug":"javascript","questions_count":852,"locale":"ru"},"tags":[{"id":663,"slug":"privedenie-tipov","name":"приведение типов"},{"id":633,"slug":"javascript","name":"JavaScript"}],"id":1889,"answers_count":2,"slug":"chto-oznachaet-dva-vosklitsatelnyh-znaka-pered-vyrazheniem","state":"published","title":"Что означает два восклицательных знака \"!!\" перед выражением?","created_at":"2022-04-26T12:31:16.131Z","details":"Что означает, когда пишут два восклицательных знака перед переменной или любым другим выражением? Например:\r\n```javascript\r\nconst isTrue = (content) => {\r\n return !!content;\r\n};\r\n```","best_answer_id":323,"related_stacks_count":5},"id":4472,"state":"active","body":"Еще можно добавить, что в javascript следующие значения будут приведены к булевому типу false: \r\n\r\n- undefined \r\n- null \r\n- NaN \r\n- 0 (число ноль)\r\n- '' (пустая строка)\r\n\r\nВ английской литературе эти значения называются falsy values.\r\n\r\n```\r\n[undefined, null, NaN, 0, ''].map((val) => !!val);\r\n// => [ false, false, false, false, false ]\r\n\r\n// или можно так: [undefined, null, NaN, 0, ''].map((val) => Boolean(val))\r\n```","votes_up_count":4,"votes_down_count":0,"created_at":"2024-11-13T18:17:53.908Z","user_id":838612,"category_slug":"javascript"}],"relatedQuestions":[{"creator":{"id":363630,"email":"a.agaphonov@ya.ru","first_name":"Человек-Молекула","last_name":"","telegram":null,"full_name":"Человек-Молекула","removed":false},"category":{"id":2,"title":"JavaScript","slug":"javascript","questions_count":852,"locale":"ru"},"tags":[],"id":1359,"answers_count":1,"slug":"kak-sdelat-drag-and-drop-javascript","state":"published","title":"Как сделать drag and drop 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Например:\r\n```javascript\r\nconst isTrue = (content) => {\r\n return !!content;\r\n};\r\n```","answerCount":2,"datePublished":"2022-04-26T12:31:16.131Z","author":{"@type":"Person","name":"Ivan Gagarinov"},"acceptedAnswer":{"@type":"Answer","text":"Прежде всего давайте вспомним что вообще делает восклицательный знак в обычных ситуациях. Он является логическим оператором "не". Простой пример, проверяем что выражения между собой равны:\r\n\r\n```javascript\r\nconsole.log('test' === 'test'); // => true\r\n```\r\n\r\nЕсли хотим проверить что они не равны, то добавляем восклицательный знак:\r\n\r\n```javascript\r\nconsole.log('test' !== 'test'); // => false\r\n```\r\n\r\nТо есть мы "перевернули" (инвертировали) проверку. Примерно то же самое происходит, когда мы добавляем восклицательный знак перед любым выражением:\r\n\r\n```javascript\r\nconst myVar = 'test';\r\nconsole.log(!myVar); // => false\r\n```\r\n\r\nОбратите внимание, перед `myVar` стоит восклицательный знак, это отрицание, то есть можно прочитать это как "**не** myVar". Восклицательный знак инвертирует наше выражение, которое является строкой 'test'. Но тут возникает вопрос: а что является не строкой 'test'? Если подойти к вопросу философски, то можно найти бесконечное множество вещей, которые являются не строкой 'test'. Но у интерпретатора JS на этот счёт есть чёткое правило: он вычисляет выражение справа от восклицательного знака так, чтобы получить либо `true`, либо `false`, то есть приводит к булевому типу. Строка 'test' приводится к булевому типу и вычисляется как `true`, а восклицательный знак инвертирует, то есть переворачивает `true` на `false`, так как `false` является противоположностью `true`.\r\n\r\nТеперь гораздо легче понять что делают два восклицательных знака:\r\n```javascript\r\nconst isTrue = (content) => {\r\n return !!content;\r\n};\r\n```\r\nИнтерпретатор пытается вычислить сначала `!content`. Если `content` будет ложным выражением, то вычислится `true`, если истинным, то `false`. А второй восклицательный знак переворачивает `true` на `false`, а `false` на `true`. В итоге, если контент существует, то функция вернёт `true`, а если не существует, то `false`.\r\n\r\nТеперь возникает вопрос: зачем вообще это нужно? Это делается чтобы мы всегда работали с определённым типом. Такой хитрый способ позволяет нам добиться чтобы функция всегда возвращала булевый тип. Любое значение будет преобразовываться в этот булевый тип. Это называется **приведение типов**.","datePublished":"2022-04-26T12:59:16.958Z","upvoteCount":40,"author":{"@type":"Person","name":"Ivan Gagarinov"},"url":"https://ru.hexlet.io/qna/javascript/questions/chto-oznachaet-dva-vosklitsatelnyh-znaka-pered-vyrazheniem#answer-323"},"suggestedAnswer":[{"@type":"Answer","text":"Еще можно добавить, что в javascript следующие значения будут приведены к булевому типу false: \r\n\r\n- undefined \r\n- null \r\n- NaN \r\n- 0 (число ноль)\r\n- '' (пустая строка)\r\n\r\nВ английской литературе эти значения называются falsy values.\r\n\r\n```\r\n[undefined, null, NaN, 0, ''].map((val) => !!val);\r\n// => [ false, false, false, false, false ]\r\n\r\n// или можно так: [undefined, null, NaN, 0, ''].map((val) => Boolean(val))\r\n```","datePublished":"2024-11-13T18:17:53.908Z","upvoteCount":4,"author":{"@type":"Person","name":"Дмитрий Чувиковский"},"url":"https://ru.hexlet.io/qna/javascript/questions/chto-oznachaet-dva-vosklitsatelnyh-znaka-pered-vyrazheniem#answer-4472"}]}}</script><div style="--container-size:var(--container-size-lg);margin-top:var(--mantine-spacing-xl);height:100%" class="m_7485cace mantine-Container-root" data-size="lg" data-strategy="block"><script type="application/ld+json">{"@context":"https://schema.org","@type":"BreadcrumbList","itemListElement":[{"position":1,"@type":"ListItem","item":{"@id":"/qna","name":"Вопросы и ответы"}},{"position":2,"@type":"ListItem","item":{"@id":"/qna/javascript/questions","name":"JavaScript"}},{"position":3,"@type":"ListItem","item":{"@id":"/qna/javascript/questions/chto-oznachaet-dva-vosklitsatelnyh-znaka-pered-vyrazheniem","name":"Что означает два восклицательных знака "!!" перед выражением?"}}]}</script><div style="margin-bottom:var(--mantine-spacing-xs)" class="m_8b3717df mantine-Breadcrumbs-root"><a 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mantine-Text-root" data-size="sm">Что означает два восклицательных знака "!!" перед выражением?</p></div><style data-mantine-styles="inline">.__m__-_R_eub_{margin-bottom:var(--mantine-spacing-xs);}@media(min-width: 36em){.__m__-_R_eub_{margin-bottom:var(--mantine-spacing-xs);}}</style><div style="--group-gap:var(--mantine-spacing-md);--group-align:center;--group-justify:space-between;--group-wrap:wrap" class="m_4081bf90 mantine-Group-root __m__-_R_eub_"><style data-mantine-styles="inline">.__m__-_R_deub_{width:100%;}@media(min-width: 36em){.__m__-_R_deub_{width:70%;}}@media(min-width: 75em){.__m__-_R_deub_{width:75%;}}</style><div class="__m__-_R_deub_"><div style="--group-gap:var(--mantine-spacing-md);--group-align:center;--group-justify:start;--group-wrap:nowrap" class="m_4081bf90 mantine-Group-root"><h1 style="--title-fw:var(--mantine-h1-font-weight);--title-lh:var(--mantine-h1-line-height);--title-fz:var(--mantine-h1-font-size)" class="m_8a5d1357 mantine-Title-root" data-order="1">Что означает два восклицательных знака "!!" перед выражением?</h1></div></div></div><style data-mantine-styles="inline">.__m__-_R_iub_{--grid-gutter:var(--mantine-spacing-md);}</style><div class="m_410352e9 mantine-Grid-root __m__-_R_iub_"><div class="m_dee7bd2f mantine-Grid-inner"><style data-mantine-styles="inline">.__m__-_R_3diub_{--col-flex-grow:auto;--col-flex-basis:100%;--col-max-width:100%;}@media(min-width: 48em){.__m__-_R_3diub_{--col-flex-grow:auto;--col-flex-basis:83.33333333333334%;--col-max-width:83.33333333333334%;}}@media(min-width: 62em){.__m__-_R_3diub_{--col-flex-grow:auto;--col-flex-basis:66.66666666666667%;--col-max-width:66.66666666666667%;}}</style><div class="m_96bdd299 mantine-Grid-col __m__-_R_3diub_"><div style="--group-gap:var(--mantine-spacing-md);--group-align:center;--group-justify:flex-start;--group-wrap:wrap;margin-bottom:var(--mantine-spacing-lg)" class="m_4081bf90 mantine-Group-root"><a class="mantine-focus-auto m_849cf0da m_b6d8b162 mantine-Text-root mantine-Anchor-root" data-underline="hover" href="/qna/javascript/tags/privedenie-tipov"><div style="--badge-height:var(--badge-height-sm);--badge-padding-x:var(--badge-padding-x-sm);--badge-fz:var(--badge-fz-sm);--badge-bg:var(--mantine-color-default);--badge-color:var(--mantine-color-default-color);--badge-bd:calc(0.0625rem * var(--mantine-scale)) solid var(--mantine-color-default-border);cursor:pointer" class="m_347db0ec mantine-Badge-root" data-variant="default" data-size="sm"><span class="m_5add502a mantine-Badge-label">приведение типов</span></div></a><a class="mantine-focus-auto m_849cf0da m_b6d8b162 mantine-Text-root mantine-Anchor-root" data-underline="hover" href="/qna/javascript/tags/javascript"><div style="--badge-height:var(--badge-height-sm);--badge-padding-x:var(--badge-padding-x-sm);--badge-fz:var(--badge-fz-sm);--badge-bg:var(--mantine-color-default);--badge-color:var(--mantine-color-default-color);--badge-bd:calc(0.0625rem * var(--mantine-scale)) solid var(--mantine-color-default-border);cursor:pointer" class="m_347db0ec mantine-Badge-root" data-variant="default" data-size="sm"><span class="m_5add502a mantine-Badge-label">JavaScript</span></div></a></div><div style="margin-bottom:var(--mantine-spacing-xs)" class="m_d08caa0 mantine-Typography-root"><p>Что означает, когда пишут два восклицательных знака перед переменной или любым другим выражением? Например:</p>
<div style="margin-bottom:var(--mantine-spacing-lg)" class="m_e597c321 mantine-CodeHighlight-codeHighlight" dir="ltr"><div class="m_be7e9c9c mantine-CodeHighlight-controls"><button style="--ai-bg:transparent;--ai-hover:transparent;--ai-color:inherit;--ai-bd:none" class="mantine-focus-auto mantine-active m_d498bab7 mantine-CodeHighlight-control m_8d3f4000 mantine-ActionIcon-root m_87cf2631 mantine-UnstyledButton-root" data-variant="none" type="button" aria-label="Copy code"><span class="m_8d3afb97 mantine-ActionIcon-icon"><svg xmlns="http://www.w3.org/2000/svg" viewBox="0 0 24 24" stroke-width="2" stroke="currentColor" fill="none" stroke-linecap="round" stroke-linejoin="round"><path stroke="none" d="M0 0h24v24H0z" fill="none"></path><path d="M8 8m0 2a2 2 0 0 1 2 -2h8a2 2 0 0 1 2 2v8a2 2 0 0 1 -2 2h-8a2 2 0 0 1 -2 -2z"></path><path d="M16 8v-2a2 2 0 0 0 -2 -2h-8a2 2 0 0 0 -2 2v8a2 2 0 0 0 2 2h2"></path></svg></span></button></div><div style="--scrollarea-scrollbar-size:calc(0.25rem * var(--mantine-scale));--sa-corner-width:0px;--sa-corner-height:0px" class="m_f744fd40 mantine-CodeHighlight-scrollarea m_d57069b5 mantine-ScrollArea-root" dir="ltr"><div style="overflow-x:hidden;overflow-y:hidden;overscroll-behavior-inline:none" class="m_c0783ff9 mantine-ScrollArea-viewport" data-scrollbars="xy"><div class="m_b1336c6 mantine-ScrollArea-content"><pre class="m_2c47c4fd mantine-CodeHighlight-pre" style="padding:0"><code class="m_5caae6d3 mantine-CodeHighlight-code">const isTrue = (content) => {
return !!content;
};</code></pre></div></div></div><button class="mantine-focus-auto m_c9378bc2 mantine-CodeHighlight-showCodeButton m_87cf2631 mantine-UnstyledButton-root" data-hidden="true" type="button">Expand code</button></div></div><div style="--group-gap:var(--mantine-spacing-md);--group-align:center;--group-justify:flex-start;--group-wrap:wrap;margin-bottom:var(--mantine-spacing-xl);font-size:var(--mantine-font-size-sm)" class="m_4081bf90 mantine-Group-root"><div style="--group-gap:calc(0.25rem * var(--mantine-scale));--group-align:center;--group-justify:flex-start;--group-wrap:wrap;margin-inline-start:auto" class="m_4081bf90 mantine-Group-root"><svg xmlns="http://www.w3.org/2000/svg" width="16" height="16" viewBox="0 0 24 24" fill="none" stroke="currentColor" stroke-width="2" stroke-linecap="round" stroke-linejoin="round" class="tabler-icon tabler-icon-calendar "><path d="M4 7a2 2 0 0 1 2 -2h12a2 2 0 0 1 2 2v12a2 2 0 0 1 -2 2h-12a2 2 0 0 1 -2 -2v-12"></path><path d="M16 3v4"></path><path d="M8 3v4"></path><path d="M4 11h16"></path><path d="M11 15h1"></path><path d="M12 15v3"></path></svg><p class="mantine-focus-auto m_b6d8b162 mantine-Text-root" data-inherit="true">4 года назад</p></div><div style="--group-gap:calc(0.25rem * var(--mantine-scale));--group-align:center;--group-justify:flex-start;--group-wrap:wrap" class="m_4081bf90 mantine-Group-root"><svg xmlns="http://www.w3.org/2000/svg" width="16" height="16" viewBox="0 0 24 24" fill="none" stroke="currentColor" stroke-width="2" stroke-linecap="round" stroke-linejoin="round" class="tabler-icon tabler-icon-user "><path d="M8 7a4 4 0 1 0 8 0a4 4 0 0 0 -8 0"></path><path d="M6 21v-2a4 4 0 0 1 4 -4h4a4 4 0 0 1 4 4v2"></path></svg><p class="mantine-focus-auto m_b6d8b162 mantine-Text-root" data-inherit="true">Ivan Gagarinov</p></div></div><div role="link" tabindex="0" style="cursor:pointer"><button style="display:block;width:100%" class="mantine-focus-auto m_87cf2631 mantine-UnstyledButton-root" type="button" aria-label="Присоединяйтесь к нашему Telegram-сообществу"><div style="background-color:light-dark(var(--mantine-color-gray-1), var(--mantine-color-dark-6));margin-block:var(--mantine-spacing-xs)" class="m_e615b15f mantine-Card-root m_1b7284a3 mantine-Paper-root"><div style="--group-gap:var(--mantine-spacing-md);--group-align:center;--group-justify:flex-start;--group-wrap:wrap" class="m_4081bf90 mantine-Group-root"><div style="--ti-bg:transparent;--ti-color:var(--mantine-color-indigo-light-color);--ti-bd:calc(0.0625rem * var(--mantine-scale)) solid transparent;margin-inline-end:auto;color:inherit" class="m_7341320d mantine-ThemeIcon-root" data-variant="transparent"><svg xmlns="http://www.w3.org/2000/svg" width="24" height="24" viewBox="0 0 24 24" fill="none" stroke="currentColor" stroke-width="1.2" stroke-linecap="round" stroke-linejoin="round" class="tabler-icon tabler-icon-brand-telegram "><path d="M15 10l-4 4l6 6l4 -16l-18 7l4 2l2 6l3 -4"></path></svg></div>Присоединяйтесь к нашему Telegram-сообществу</div></div></button></div><h2 style="--title-fw:var(--mantine-h2-font-weight);--title-lh:var(--mantine-h2-line-height);--title-fz:var(--mantine-h2-font-size);margin-block:var(--mantine-spacing-xl)" class="m_8a5d1357 mantine-Title-root" data-order="2">Ответы</h2><div style="margin-bottom:var(--mantine-spacing-xl);padding:var(--mantine-spacing-lg)" class="m_e615b15f mantine-Card-root m_1b7284a3 mantine-Paper-root" data-with-border="true" id="answer-323"><div style="--group-gap:calc(1.125rem * var(--mantine-scale));--group-align:stretch;--group-justify:flex-start;--group-wrap:nowrap" class="m_4081bf90 mantine-Group-root"><div style="--stack-gap:var(--mantine-spacing-md);--stack-align:stretch;--stack-justify:flex-start;font-size:var(--mantine-font-size-h1);font-weight:lighter;text-align:center" class="m_6d731127 mantine-Stack-root">40<a style="color:inherit" class="mantine-focus-auto m_849cf0da m_b6d8b162 mantine-Text-root mantine-Anchor-root" data-underline="hover" href="/qna/javascript/questions/chto-oznachaet-dva-vosklitsatelnyh-znaka-pered-vyrazheniem/answers/323/vote"><div style="--ti-bg:transparent;--ti-color:var(--mantine-color-indigo-light-color);--ti-bd:calc(0.0625rem * var(--mantine-scale)) solid transparent;color:inherit" class="m_7341320d mantine-ThemeIcon-root" data-variant="transparent"><svg xmlns="http://www.w3.org/2000/svg" width="24" height="24" viewBox="0 0 24 24" fill="none" stroke="currentColor" stroke-width="2" stroke-linecap="round" stroke-linejoin="round" class="tabler-icon tabler-icon-thumb-up "><path d="M7 11v8a1 1 0 0 1 -1 1h-2a1 1 0 0 1 -1 -1v-7a1 1 0 0 1 1 -1h3a4 4 0 0 0 4 -4v-1a2 2 0 0 1 4 0v5h3a2 2 0 0 1 2 2l-1 5a2 3 0 0 1 -2 2h-7a3 3 0 0 1 -3 -3"></path></svg></div></a><div style="--ti-bg:transparent;--ti-color:var(--mantine-color-indigo-light-color);--ti-bd:calc(0.0625rem * var(--mantine-scale)) solid transparent;color:inherit" class="m_7341320d mantine-ThemeIcon-root" data-variant="transparent"><svg xmlns="http://www.w3.org/2000/svg" width="24" height="24" viewBox="0 0 24 24" fill="none" stroke="currentColor" stroke-width="2" stroke-linecap="round" stroke-linejoin="round" class="tabler-icon tabler-icon-check "><path d="M5 12l5 5l10 -10"></path></svg></div></div><div style="--stack-gap:var(--mantine-spacing-md);--stack-align:stretch;--stack-justify:flex-start;width:100%;min-width:0rem" class="m_6d731127 mantine-Stack-root"><div style="margin-bottom:auto" class="m_d08caa0 mantine-Typography-root"><p>Прежде всего давайте вспомним что вообще делает восклицательный знак в обычных ситуациях. Он является логическим оператором "не". Простой пример, проверяем что выражения между собой равны:</p>
<div style="margin-bottom:var(--mantine-spacing-lg)" class="m_e597c321 mantine-CodeHighlight-codeHighlight" dir="ltr"><div class="m_be7e9c9c mantine-CodeHighlight-controls"><button style="--ai-bg:transparent;--ai-hover:transparent;--ai-color:inherit;--ai-bd:none" class="mantine-focus-auto mantine-active m_d498bab7 mantine-CodeHighlight-control m_8d3f4000 mantine-ActionIcon-root m_87cf2631 mantine-UnstyledButton-root" data-variant="none" type="button" aria-label="Copy code"><span class="m_8d3afb97 mantine-ActionIcon-icon"><svg xmlns="http://www.w3.org/2000/svg" viewBox="0 0 24 24" stroke-width="2" stroke="currentColor" fill="none" stroke-linecap="round" stroke-linejoin="round"><path stroke="none" d="M0 0h24v24H0z" fill="none"></path><path d="M8 8m0 2a2 2 0 0 1 2 -2h8a2 2 0 0 1 2 2v8a2 2 0 0 1 -2 2h-8a2 2 0 0 1 -2 -2z"></path><path d="M16 8v-2a2 2 0 0 0 -2 -2h-8a2 2 0 0 0 -2 2v8a2 2 0 0 0 2 2h2"></path></svg></span></button></div><div style="--scrollarea-scrollbar-size:calc(0.25rem * var(--mantine-scale));--sa-corner-width:0px;--sa-corner-height:0px" class="m_f744fd40 mantine-CodeHighlight-scrollarea m_d57069b5 mantine-ScrollArea-root" dir="ltr"><div style="overflow-x:hidden;overflow-y:hidden;overscroll-behavior-inline:none" class="m_c0783ff9 mantine-ScrollArea-viewport" data-scrollbars="xy"><div class="m_b1336c6 mantine-ScrollArea-content"><pre class="m_2c47c4fd mantine-CodeHighlight-pre" style="padding:0"><code class="m_5caae6d3 mantine-CodeHighlight-code">console.log('test' === 'test'); // => true</code></pre></div></div></div><button class="mantine-focus-auto m_c9378bc2 mantine-CodeHighlight-showCodeButton m_87cf2631 mantine-UnstyledButton-root" data-hidden="true" type="button">Expand code</button></div>
<p>Если хотим проверить что они не равны, то добавляем восклицательный знак:</p>
<div style="margin-bottom:var(--mantine-spacing-lg)" class="m_e597c321 mantine-CodeHighlight-codeHighlight" dir="ltr"><div class="m_be7e9c9c mantine-CodeHighlight-controls"><button style="--ai-bg:transparent;--ai-hover:transparent;--ai-color:inherit;--ai-bd:none" class="mantine-focus-auto mantine-active m_d498bab7 mantine-CodeHighlight-control m_8d3f4000 mantine-ActionIcon-root m_87cf2631 mantine-UnstyledButton-root" data-variant="none" type="button" aria-label="Copy code"><span class="m_8d3afb97 mantine-ActionIcon-icon"><svg xmlns="http://www.w3.org/2000/svg" viewBox="0 0 24 24" stroke-width="2" stroke="currentColor" fill="none" stroke-linecap="round" stroke-linejoin="round"><path stroke="none" d="M0 0h24v24H0z" fill="none"></path><path d="M8 8m0 2a2 2 0 0 1 2 -2h8a2 2 0 0 1 2 2v8a2 2 0 0 1 -2 2h-8a2 2 0 0 1 -2 -2z"></path><path d="M16 8v-2a2 2 0 0 0 -2 -2h-8a2 2 0 0 0 -2 2v8a2 2 0 0 0 2 2h2"></path></svg></span></button></div><div style="--scrollarea-scrollbar-size:calc(0.25rem * var(--mantine-scale));--sa-corner-width:0px;--sa-corner-height:0px" class="m_f744fd40 mantine-CodeHighlight-scrollarea m_d57069b5 mantine-ScrollArea-root" dir="ltr"><div style="overflow-x:hidden;overflow-y:hidden;overscroll-behavior-inline:none" class="m_c0783ff9 mantine-ScrollArea-viewport" data-scrollbars="xy"><div class="m_b1336c6 mantine-ScrollArea-content"><pre class="m_2c47c4fd mantine-CodeHighlight-pre" style="padding:0"><code class="m_5caae6d3 mantine-CodeHighlight-code">console.log('test' !== 'test'); // => false</code></pre></div></div></div><button class="mantine-focus-auto m_c9378bc2 mantine-CodeHighlight-showCodeButton m_87cf2631 mantine-UnstyledButton-root" data-hidden="true" type="button">Expand code</button></div>
<p>То есть мы "перевернули" (инвертировали) проверку. Примерно то же самое происходит, когда мы добавляем восклицательный знак перед любым выражением:</p>
<div style="margin-bottom:var(--mantine-spacing-lg)" class="m_e597c321 mantine-CodeHighlight-codeHighlight" dir="ltr"><div class="m_be7e9c9c mantine-CodeHighlight-controls"><button style="--ai-bg:transparent;--ai-hover:transparent;--ai-color:inherit;--ai-bd:none" class="mantine-focus-auto mantine-active m_d498bab7 mantine-CodeHighlight-control m_8d3f4000 mantine-ActionIcon-root m_87cf2631 mantine-UnstyledButton-root" data-variant="none" type="button" aria-label="Copy code"><span class="m_8d3afb97 mantine-ActionIcon-icon"><svg xmlns="http://www.w3.org/2000/svg" viewBox="0 0 24 24" stroke-width="2" stroke="currentColor" fill="none" stroke-linecap="round" stroke-linejoin="round"><path stroke="none" d="M0 0h24v24H0z" fill="none"></path><path d="M8 8m0 2a2 2 0 0 1 2 -2h8a2 2 0 0 1 2 2v8a2 2 0 0 1 -2 2h-8a2 2 0 0 1 -2 -2z"></path><path d="M16 8v-2a2 2 0 0 0 -2 -2h-8a2 2 0 0 0 -2 2v8a2 2 0 0 0 2 2h2"></path></svg></span></button></div><div style="--scrollarea-scrollbar-size:calc(0.25rem * var(--mantine-scale));--sa-corner-width:0px;--sa-corner-height:0px" class="m_f744fd40 mantine-CodeHighlight-scrollarea m_d57069b5 mantine-ScrollArea-root" dir="ltr"><div style="overflow-x:hidden;overflow-y:hidden;overscroll-behavior-inline:none" class="m_c0783ff9 mantine-ScrollArea-viewport" data-scrollbars="xy"><div class="m_b1336c6 mantine-ScrollArea-content"><pre class="m_2c47c4fd mantine-CodeHighlight-pre" style="padding:0"><code class="m_5caae6d3 mantine-CodeHighlight-code">const myVar = 'test';
console.log(!myVar); // => false</code></pre></div></div></div><button class="mantine-focus-auto m_c9378bc2 mantine-CodeHighlight-showCodeButton m_87cf2631 mantine-UnstyledButton-root" data-hidden="true" type="button">Expand code</button></div>
<p>Обратите внимание, перед <code style="margin-bottom:var(--mantine-spacing-lg)" class="m_dfe9c588 mantine-InlineCodeHighlight-inlineCodeHighlight m_e597c321 mantine-CodeHighlight-codeHighlight m_dfe9c588 mantine-InlineCodeHighlight-inlineCodeHighlight">myVar</code> стоит восклицательный знак, это отрицание, то есть можно прочитать это как "<strong>не</strong> myVar". Восклицательный знак инвертирует наше выражение, которое является строкой 'test'. Но тут возникает вопрос: а что является не строкой 'test'? Если подойти к вопросу философски, то можно найти бесконечное множество вещей, которые являются не строкой 'test'. Но у интерпретатора JS на этот счёт есть чёткое правило: он вычисляет выражение справа от восклицательного знака так, чтобы получить либо <code style="margin-bottom:var(--mantine-spacing-lg)" class="m_dfe9c588 mantine-InlineCodeHighlight-inlineCodeHighlight m_e597c321 mantine-CodeHighlight-codeHighlight m_dfe9c588 mantine-InlineCodeHighlight-inlineCodeHighlight">true</code>, либо <code style="margin-bottom:var(--mantine-spacing-lg)" class="m_dfe9c588 mantine-InlineCodeHighlight-inlineCodeHighlight m_e597c321 mantine-CodeHighlight-codeHighlight m_dfe9c588 mantine-InlineCodeHighlight-inlineCodeHighlight">false</code>, то есть приводит к булевому типу. Строка 'test' приводится к булевому типу и вычисляется как <code style="margin-bottom:var(--mantine-spacing-lg)" class="m_dfe9c588 mantine-InlineCodeHighlight-inlineCodeHighlight m_e597c321 mantine-CodeHighlight-codeHighlight m_dfe9c588 mantine-InlineCodeHighlight-inlineCodeHighlight">true</code>, а восклицательный знак инвертирует, то есть переворачивает <code style="margin-bottom:var(--mantine-spacing-lg)" class="m_dfe9c588 mantine-InlineCodeHighlight-inlineCodeHighlight m_e597c321 mantine-CodeHighlight-codeHighlight m_dfe9c588 mantine-InlineCodeHighlight-inlineCodeHighlight">true</code> на <code style="margin-bottom:var(--mantine-spacing-lg)" class="m_dfe9c588 mantine-InlineCodeHighlight-inlineCodeHighlight m_e597c321 mantine-CodeHighlight-codeHighlight m_dfe9c588 mantine-InlineCodeHighlight-inlineCodeHighlight">false</code>, так как <code style="margin-bottom:var(--mantine-spacing-lg)" class="m_dfe9c588 mantine-InlineCodeHighlight-inlineCodeHighlight m_e597c321 mantine-CodeHighlight-codeHighlight m_dfe9c588 mantine-InlineCodeHighlight-inlineCodeHighlight">false</code> является противоположностью <code style="margin-bottom:var(--mantine-spacing-lg)" class="m_dfe9c588 mantine-InlineCodeHighlight-inlineCodeHighlight m_e597c321 mantine-CodeHighlight-codeHighlight m_dfe9c588 mantine-InlineCodeHighlight-inlineCodeHighlight">true</code>.</p>
<p>Теперь гораздо легче понять что делают два восклицательных знака:</p>
<div style="margin-bottom:var(--mantine-spacing-lg)" class="m_e597c321 mantine-CodeHighlight-codeHighlight" dir="ltr"><div class="m_be7e9c9c mantine-CodeHighlight-controls"><button style="--ai-bg:transparent;--ai-hover:transparent;--ai-color:inherit;--ai-bd:none" class="mantine-focus-auto mantine-active m_d498bab7 mantine-CodeHighlight-control m_8d3f4000 mantine-ActionIcon-root m_87cf2631 mantine-UnstyledButton-root" data-variant="none" type="button" aria-label="Copy code"><span class="m_8d3afb97 mantine-ActionIcon-icon"><svg xmlns="http://www.w3.org/2000/svg" viewBox="0 0 24 24" stroke-width="2" stroke="currentColor" fill="none" stroke-linecap="round" stroke-linejoin="round"><path stroke="none" d="M0 0h24v24H0z" fill="none"></path><path d="M8 8m0 2a2 2 0 0 1 2 -2h8a2 2 0 0 1 2 2v8a2 2 0 0 1 -2 2h-8a2 2 0 0 1 -2 -2z"></path><path d="M16 8v-2a2 2 0 0 0 -2 -2h-8a2 2 0 0 0 -2 2v8a2 2 0 0 0 2 2h2"></path></svg></span></button></div><div style="--scrollarea-scrollbar-size:calc(0.25rem * var(--mantine-scale));--sa-corner-width:0px;--sa-corner-height:0px" class="m_f744fd40 mantine-CodeHighlight-scrollarea m_d57069b5 mantine-ScrollArea-root" dir="ltr"><div style="overflow-x:hidden;overflow-y:hidden;overscroll-behavior-inline:none" class="m_c0783ff9 mantine-ScrollArea-viewport" data-scrollbars="xy"><div class="m_b1336c6 mantine-ScrollArea-content"><pre class="m_2c47c4fd mantine-CodeHighlight-pre" style="padding:0"><code class="m_5caae6d3 mantine-CodeHighlight-code">const isTrue = (content) => {
return !!content;
};</code></pre></div></div></div><button class="mantine-focus-auto m_c9378bc2 mantine-CodeHighlight-showCodeButton m_87cf2631 mantine-UnstyledButton-root" data-hidden="true" type="button">Expand code</button></div>
<p>Интерпретатор пытается вычислить сначала <code style="margin-bottom:var(--mantine-spacing-lg)" class="m_dfe9c588 mantine-InlineCodeHighlight-inlineCodeHighlight m_e597c321 mantine-CodeHighlight-codeHighlight m_dfe9c588 mantine-InlineCodeHighlight-inlineCodeHighlight">!content</code>. Если <code style="margin-bottom:var(--mantine-spacing-lg)" class="m_dfe9c588 mantine-InlineCodeHighlight-inlineCodeHighlight m_e597c321 mantine-CodeHighlight-codeHighlight m_dfe9c588 mantine-InlineCodeHighlight-inlineCodeHighlight">content</code> будет ложным выражением, то вычислится <code style="margin-bottom:var(--mantine-spacing-lg)" class="m_dfe9c588 mantine-InlineCodeHighlight-inlineCodeHighlight m_e597c321 mantine-CodeHighlight-codeHighlight m_dfe9c588 mantine-InlineCodeHighlight-inlineCodeHighlight">true</code>, если истинным, то <code style="margin-bottom:var(--mantine-spacing-lg)" class="m_dfe9c588 mantine-InlineCodeHighlight-inlineCodeHighlight m_e597c321 mantine-CodeHighlight-codeHighlight m_dfe9c588 mantine-InlineCodeHighlight-inlineCodeHighlight">false</code>. А второй восклицательный знак переворачивает <code style="margin-bottom:var(--mantine-spacing-lg)" class="m_dfe9c588 mantine-InlineCodeHighlight-inlineCodeHighlight m_e597c321 mantine-CodeHighlight-codeHighlight m_dfe9c588 mantine-InlineCodeHighlight-inlineCodeHighlight">true</code> на <code style="margin-bottom:var(--mantine-spacing-lg)" class="m_dfe9c588 mantine-InlineCodeHighlight-inlineCodeHighlight m_e597c321 mantine-CodeHighlight-codeHighlight m_dfe9c588 mantine-InlineCodeHighlight-inlineCodeHighlight">false</code>, а <code style="margin-bottom:var(--mantine-spacing-lg)" class="m_dfe9c588 mantine-InlineCodeHighlight-inlineCodeHighlight m_e597c321 mantine-CodeHighlight-codeHighlight m_dfe9c588 mantine-InlineCodeHighlight-inlineCodeHighlight">false</code> на <code style="margin-bottom:var(--mantine-spacing-lg)" class="m_dfe9c588 mantine-InlineCodeHighlight-inlineCodeHighlight m_e597c321 mantine-CodeHighlight-codeHighlight m_dfe9c588 mantine-InlineCodeHighlight-inlineCodeHighlight">true</code>. В итоге, если контент существует, то функция вернёт <code style="margin-bottom:var(--mantine-spacing-lg)" class="m_dfe9c588 mantine-InlineCodeHighlight-inlineCodeHighlight m_e597c321 mantine-CodeHighlight-codeHighlight m_dfe9c588 mantine-InlineCodeHighlight-inlineCodeHighlight">true</code>, а если не существует, то <code style="margin-bottom:var(--mantine-spacing-lg)" class="m_dfe9c588 mantine-InlineCodeHighlight-inlineCodeHighlight m_e597c321 mantine-CodeHighlight-codeHighlight m_dfe9c588 mantine-InlineCodeHighlight-inlineCodeHighlight">false</code>.</p>
<p>Теперь возникает вопрос: зачем вообще это нужно? Это делается чтобы мы всегда работали с определённым типом. Такой хитрый способ позволяет нам добиться чтобы функция всегда возвращала булевый тип. Любое значение будет преобразовываться в этот булевый тип. Это называется <strong>приведение типов</strong>.</p></div><div class="m_3eebeb36 mantine-Divider-root" data-orientation="horizontal" role="separator"></div><div style="--group-gap:var(--mantine-spacing-md);--group-align:center;--group-justify:flex-start;--group-wrap:wrap;font-size:var(--mantine-font-size-sm)" class="m_4081bf90 mantine-Group-root"><div style="--group-gap:var(--mantine-spacing-xs);--group-align:center;--group-justify:flex-start;--group-wrap:wrap;margin-inline-start:auto" class="m_4081bf90 mantine-Group-root"><svg xmlns="http://www.w3.org/2000/svg" width="16" height="16" viewBox="0 0 24 24" fill="none" stroke="currentColor" stroke-width="1.2" stroke-linecap="round" stroke-linejoin="round" class="tabler-icon tabler-icon-calendar "><path d="M4 7a2 2 0 0 1 2 -2h12a2 2 0 0 1 2 2v12a2 2 0 0 1 -2 2h-12a2 2 0 0 1 -2 -2v-12"></path><path d="M16 3v4"></path><path d="M8 3v4"></path><path d="M4 11h16"></path><path d="M11 15h1"></path><path d="M12 15v3"></path></svg><p class="mantine-focus-auto m_b6d8b162 mantine-Text-root" data-inherit="true">4 года назад</p></div><div style="--group-gap:var(--mantine-spacing-xs);--group-align:center;--group-justify:flex-start;--group-wrap:wrap" class="m_4081bf90 mantine-Group-root"><svg xmlns="http://www.w3.org/2000/svg" width="16" height="16" viewBox="0 0 24 24" fill="none" stroke="currentColor" stroke-width="1.2" stroke-linecap="round" stroke-linejoin="round" class="tabler-icon tabler-icon-user "><path d="M8 7a4 4 0 1 0 8 0a4 4 0 0 0 -8 0"></path><path d="M6 21v-2a4 4 0 0 1 4 -4h4a4 4 0 0 1 4 4v2"></path></svg><p class="mantine-focus-auto m_b6d8b162 mantine-Text-root" data-inherit="true">Ivan Gagarinov</p></div></div></div></div></div><div style="margin-bottom:var(--mantine-spacing-xl);padding:var(--mantine-spacing-lg)" class="m_e615b15f mantine-Card-root m_1b7284a3 mantine-Paper-root" data-with-border="true" id="answer-4472"><div style="--group-gap:calc(1.125rem * var(--mantine-scale));--group-align:stretch;--group-justify:flex-start;--group-wrap:nowrap" class="m_4081bf90 mantine-Group-root"><div style="--stack-gap:var(--mantine-spacing-md);--stack-align:stretch;--stack-justify:flex-start;font-size:var(--mantine-font-size-h1);font-weight:lighter;text-align:center" class="m_6d731127 mantine-Stack-root">4<a style="color:inherit" class="mantine-focus-auto m_849cf0da m_b6d8b162 mantine-Text-root mantine-Anchor-root" data-underline="hover" href="/qna/javascript/questions/chto-oznachaet-dva-vosklitsatelnyh-znaka-pered-vyrazheniem/answers/4472/vote"><div style="--ti-bg:transparent;--ti-color:var(--mantine-color-indigo-light-color);--ti-bd:calc(0.0625rem * var(--mantine-scale)) solid transparent;color:inherit" class="m_7341320d mantine-ThemeIcon-root" data-variant="transparent"><svg xmlns="http://www.w3.org/2000/svg" width="24" height="24" viewBox="0 0 24 24" fill="none" stroke="currentColor" stroke-width="2" stroke-linecap="round" stroke-linejoin="round" class="tabler-icon tabler-icon-thumb-up "><path d="M7 11v8a1 1 0 0 1 -1 1h-2a1 1 0 0 1 -1 -1v-7a1 1 0 0 1 1 -1h3a4 4 0 0 0 4 -4v-1a2 2 0 0 1 4 0v5h3a2 2 0 0 1 2 2l-1 5a2 3 0 0 1 -2 2h-7a3 3 0 0 1 -3 -3"></path></svg></div></a></div><div style="--stack-gap:var(--mantine-spacing-md);--stack-align:stretch;--stack-justify:flex-start;width:100%;min-width:0rem" class="m_6d731127 mantine-Stack-root"><div style="margin-bottom:auto" class="m_d08caa0 mantine-Typography-root"><p>Еще можно добавить, что в javascript следующие значения будут приведены к булевому типу false:</p>
<ul>
<li>undefined</li>
<li>null</li>
<li>NaN</li>
<li>0 (число ноль)</li>
<li>'' (пустая строка)</li>
</ul>
<p>В английской литературе эти значения называются falsy values.</p>
<code style="margin-bottom:var(--mantine-spacing-lg)" class="m_dfe9c588 mantine-InlineCodeHighlight-inlineCodeHighlight m_e597c321 mantine-CodeHighlight-codeHighlight m_dfe9c588 mantine-InlineCodeHighlight-inlineCodeHighlight">[undefined, null, NaN, 0, ''].map((val) => !!val);
// => [ false, false, false, false, false ]
// или можно так: [undefined, null, NaN, 0, ''].map((val) => Boolean(val))</code></div><div class="m_3eebeb36 mantine-Divider-root" data-orientation="horizontal" role="separator"></div><div style="--group-gap:var(--mantine-spacing-md);--group-align:center;--group-justify:flex-start;--group-wrap:wrap;font-size:var(--mantine-font-size-sm)" class="m_4081bf90 mantine-Group-root"><div style="--group-gap:var(--mantine-spacing-xs);--group-align:center;--group-justify:flex-start;--group-wrap:wrap;margin-inline-start:auto" class="m_4081bf90 mantine-Group-root"><svg xmlns="http://www.w3.org/2000/svg" width="16" height="16" viewBox="0 0 24 24" fill="none" stroke="currentColor" stroke-width="1.2" stroke-linecap="round" stroke-linejoin="round" class="tabler-icon tabler-icon-calendar "><path d="M4 7a2 2 0 0 1 2 -2h12a2 2 0 0 1 2 2v12a2 2 0 0 1 -2 2h-12a2 2 0 0 1 -2 -2v-12"></path><path d="M16 3v4"></path><path d="M8 3v4"></path><path d="M4 11h16"></path><path d="M11 15h1"></path><path d="M12 15v3"></path></svg><p class="mantine-focus-auto m_b6d8b162 mantine-Text-root" data-inherit="true">год назад</p></div><div style="--group-gap:var(--mantine-spacing-xs);--group-align:center;--group-justify:flex-start;--group-wrap:wrap" class="m_4081bf90 mantine-Group-root"><svg xmlns="http://www.w3.org/2000/svg" width="16" height="16" viewBox="0 0 24 24" fill="none" stroke="currentColor" stroke-width="1.2" stroke-linecap="round" stroke-linejoin="round" class="tabler-icon tabler-icon-user "><path d="M8 7a4 4 0 1 0 8 0a4 4 0 0 0 -8 0"></path><path d="M6 21v-2a4 4 0 0 1 4 -4h4a4 4 0 0 1 4 4v2"></path></svg><p class="mantine-focus-auto m_b6d8b162 mantine-Text-root" data-inherit="true">Дмитрий Чувиковский</p></div></div></div></div></div><style data-mantine-styles="inline">.__m__-_R_4bbdiub_{--carousel-slide-gap:var(--mantine-spacing-xs);--carousel-slide-size:70%;}@media(min-width: 36em){.__m__-_R_4bbdiub_{--carousel-slide-gap:var(--mantine-spacing-xl);--carousel-slide-size:50%;}}</style><div 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