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data-page="{"component":"web/qna/questions/show","props":{"errors":{},"locale":"ru","language":"ru","httpsHost":"https://ru.hexlet.io","host":"ru.hexlet.io","colorScheme":"light","auth":{"user":{"id":null,"last_viewed_notification_id":null,"email":null,"state":null,"first_name":"","last_name":"","created_at":"2026-02-26T20:36:57.230Z","current_program":null,"current_team":null,"full_name":"","guest":true,"can_use_paid_features":false,"is_hexlet_employee":false,"sanitized_phone_number":"","can_subscribe":true,"can_renew_education":false}},"cloudflareTurnstileSiteKey":"0x4AAAAAAA15KmeFXzd2H0Xo","vkIdClientId":"51586979","yandexIdClientId":"88d071f1d3384eb4bd1deb37910235c7","formAuthToken":"cBMexzQNU77U_Uorjy58YlSrho-8Qi4R6jkg1u2jV1ifwtXwxnP-3mK-brODIYwVlKKrJbR10LNX2bqCv6SwNg","category":{"id":2,"title":"JavaScript","slug":"javascript","questions_count":852,"locale":"ru"},"mainStackCategory":{"id":2,"name":"Курсы по веб-разработке","slug":"web_development","short_name":"Веб-разработка","order":190,"state":"published","category_slug":"courses_web_development"},"answerDto":{"id":null,"body":"","meta":{"model":"question_answer","relations":{}}},"question":{"creator":{"id":104929,"email":"feycot@gmail.com","first_name":"Nikolai","last_name":"Gagarinov","telegram":"","full_name":"Nikolai Gagarinov","removed":false},"category":{"id":2,"title":"JavaScript","slug":"javascript","questions_count":852,"locale":"ru"},"tags":[{"id":633,"slug":"javascript","name":"JavaScript"},{"id":571,"slug":"js","name":"JS"}],"id":3970,"answers_count":1,"slug":"v-chem-raznitsa-mezhdu-typeof-undefined-i-null-v-javascript","state":"published","title":"в чем разница между 'typeof !== undefined' и '!= null' в javascript\r","created_at":"2024-09-25T11:44:33.565Z","details":null,"best_answer_id":4513,"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":104929,"email":"feycot@gmail.com","first_name":"Nikolai","last_name":"Gagarinov","telegram":"","full_name":"Nikolai Gagarinov","removed":false},"category":{"id":2,"title":"JavaScript","slug":"javascript","questions_count":852,"locale":"ru"},"tags":[{"id":633,"slug":"javascript","name":"JavaScript"},{"id":571,"slug":"js","name":"JS"}],"id":3970,"answers_count":1,"slug":"v-chem-raznitsa-mezhdu-typeof-undefined-i-null-v-javascript","state":"published","title":"в чем разница между 'typeof !== undefined' и '!= null' в javascript\r","created_at":"2024-09-25T11:44:33.565Z","details":null,"best_answer_id":4513,"related_stacks_count":5},"id":4513,"state":"active","body":"Оба варианта служат для проверки состояния переменной, но они имеют отличия:\r\n\r\nПроверка `typeof !== 'undefined'` проверяет, была ли переменная объявлена и инициализирована, и не равна ли она значению `undefined`.\r\n\r\n ```javascript\r\n\t if (typeof variable !== 'undefined') {\r\n\t // переменная определена\r\n\t }\r\n ```\r\n\r\n`typeof variable` возвращает строку `'undefined'`, если переменная не была объявлена или была объявлена, но не инициализирована. Использование `typeof` не вызывает ошибки, если переменная не существует. Это важно, если вы не уверены, объявлена ли переменная.\r\n\r\n ```javascript\r\n\t let a;\r\n\t console.log(typeof a !== 'undefined'); // true, a объявлена, но не инициализирована\r\n\r\n\t console.log(typeof b !== 'undefined'); // false, b не объявлена\r\n ```\r\n\r\nПроверка `!= null` проверяет, равно ли значение переменной `null` или `undefined` (из-за нестрогого сравнения).\r\n\r\n ```javascript\r\n\t if (variable != null) {\r\n\t // переменная не равна ни null, ни undefined\r\n\t }\r\n ```\r\nПри использовании нестрогого сравнения (`!=`), JavaScript приводит оба операнда к одному типу, и проверка вернет `false`, если значение переменной — `null` или `undefined`.\r\n\r\n ```javascript\r\n\t const c = null;\r\n\t console.log(c != null); // false\r\n\r\n\t const d;\r\n\t console.log(d != null); // false, d равно undefined\r\n\r\n\t const e = 0;\r\n\t console.log(e != null); // true, e установлено в 0\r\n ```\r\n\r\n","votes_up_count":0,"votes_down_count":0,"created_at":"2024-11-27T08:13:41.367Z","user_id":75907,"category_slug":"javascript"}],"relatedQuestions":[{"creator":{"id":104929,"email":"feycot@gmail.com","first_name":"Nikolai","last_name":"Gagarinov","telegram":"","full_name":"Nikolai Gagarinov","removed":false},"category":{"id":2,"title":"JavaScript","slug":"javascript","questions_count":852,"locale":"ru"},"tags":[{"id":633,"slug":"javascript","name":"JavaScript"},{"id":571,"slug":"js","name":"JS"}],"id":3969,"answers_count":1,"slug":"v-chem-raznitsa-mezhdu-throw-new-error-i-throw-someobject-v-javascript","state":"published","title":"в чем разница между throw new error и throw someobject в javascript\r","created_at":"2024-09-25T11:44:33.455Z","details":null,"best_answer_id":4512,"related_stacks_count":5},{"creator":{"id":104929,"email":"feycot@gmail.com","first_name":"Nikolai","last_name":"Gagarinov","telegram":"","full_name":"Nikolai 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Использование `typeof` не вызывает ошибки, если переменная не существует. Это важно, если вы не уверены, объявлена ли переменная.\r\n\r\n ```javascript\r\n\t let a;\r\n\t console.log(typeof a !== 'undefined'); // true, a объявлена, но не инициализирована\r\n\r\n\t console.log(typeof b !== 'undefined'); // false, b не объявлена\r\n ```\r\n\r\nПроверка `!= null` проверяет, равно ли значение переменной `null` или `undefined` (из-за нестрогого сравнения).\r\n\r\n ```javascript\r\n\t if (variable != null) {\r\n\t // переменная не равна ни null, ни undefined\r\n\t }\r\n ```\r\nПри использовании нестрогого сравнения (`!=`), JavaScript приводит оба операнда к одному типу, и проверка вернет `false`, если значение переменной — `null` или `undefined`.\r\n\r\n ```javascript\r\n\t const c = null;\r\n\t console.log(c != null); // false\r\n\r\n\t const d;\r\n\t console.log(d != null); // false, d равно undefined\r\n\r\n\t const e = 0;\r\n\t console.log(e != null); // true, e установлено в 0\r\n ```\r\n\r\n","datePublished":"2024-11-27T08:13:41.367Z","upvoteCount":0,"author":{"@type":"Person","name":"Ivan Gagarinov"},"url":"https://ru.hexlet.io/qna/javascript/questions/v-chem-raznitsa-mezhdu-typeof-undefined-i-null-v-javascript#answer-4513"}}}</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/v-chem-raznitsa-mezhdu-typeof-undefined-i-null-v-javascript","name":"в чем разница между 'typeof !== undefined' и '!= null' в javascript\r"}}]}</script><div style="margin-bottom:var(--mantine-spacing-xs)" class="m_8b3717df mantine-Breadcrumbs-root"><a style="--text-fz:var(--mantine-font-size-sm);--text-lh:var(--mantine-line-height-sm);white-space:normal;color:inherit" class="mantine-focus-auto m_849cf0da m_f678d540 mantine-Breadcrumbs-breadcrumb m_b6d8b162 mantine-Text-root mantine-Anchor-root" data-size="sm" data-underline="hover" href="/"><div style="color:inherit" class="m_4451eb3a mantine-Center-root"><svg xmlns="http://www.w3.org/2000/svg" width="15" height="15" viewBox="0 0 24 24" fill="none" stroke="currentColor" stroke-width="2" stroke-linecap="round" stroke-linejoin="round" class="tabler-icon tabler-icon-home-link "><path d="M20.085 11.085l-8.085 -8.085l-9 9h2v7a2 2 0 0 0 2 2h4.5"></path><path d="M9 21v-6a2 2 0 0 1 2 -2h2a2 2 0 0 1 1.807 1.143"></path><path d="M20 21a1 1 0 1 0 2 0a1 1 0 1 0 -2 0"></path><path d="M20 16a1 1 0 1 0 2 0a1 1 0 1 0 -2 0"></path><path d="M15 19a1 1 0 1 0 2 0a1 1 0 1 0 -2 0"></path><path d="M21 16l-5 3l5 2"></path></svg></div></a><div class="m_3b8f2208 mantine-Breadcrumbs-separator">/</div><a style="--text-fz:var(--mantine-font-size-sm);--text-lh:var(--mantine-line-height-sm);white-space:normal;color:inherit" class="mantine-focus-auto m_849cf0da m_f678d540 mantine-Breadcrumbs-breadcrumb m_b6d8b162 mantine-Text-root mantine-Anchor-root" data-size="sm" data-underline="hover" href="/qna">Вопросы и ответы</a><div class="m_3b8f2208 mantine-Breadcrumbs-separator">/</div><a style="--text-fz:var(--mantine-font-size-sm);--text-lh:var(--mantine-line-height-sm);white-space:normal;color:inherit" class="mantine-focus-auto m_849cf0da m_f678d540 mantine-Breadcrumbs-breadcrumb m_b6d8b162 mantine-Text-root mantine-Anchor-root" data-size="sm" data-underline="hover" href="/qna/javascript/questions">JavaScript</a><div class="m_3b8f2208 mantine-Breadcrumbs-separator">/</div><p style="--text-fz:var(--mantine-font-size-sm);--text-lh:var(--mantine-line-height-sm);white-space:normal;color:var(--mantine-color-dimmed)" class="mantine-focus-auto m_f678d540 mantine-Breadcrumbs-breadcrumb m_b6d8b162 mantine-Text-root" data-size="sm">в чем разница между 'typeof !== undefined' и '!= null' в javascript
</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">в чем разница между 'typeof !== undefined' и '!= null' в javascript
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15v3"></path></svg><p class="mantine-focus-auto m_b6d8b162 mantine-Text-root" data-inherit="true">год 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">Nikolai 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 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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-4513"><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">0<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/v-chem-raznitsa-mezhdu-typeof-undefined-i-null-v-javascript/answers/4513/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>Оба варианта служат для проверки состояния переменной, но они имеют отличия:</p>
<p>Проверка <code style="margin-bottom:var(--mantine-spacing-lg)" class="m_dfe9c588 mantine-InlineCodeHighlight-inlineCodeHighlight m_e597c321 mantine-CodeHighlight-codeHighlight m_dfe9c588 mantine-InlineCodeHighlight-inlineCodeHighlight">typeof !== 'undefined'</code> проверяет, была ли переменная объявлена и инициализирована, и не равна ли она значению <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</code>.</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">if (typeof variable !== 'undefined') {
// переменная определена
}</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><code style="margin-bottom:var(--mantine-spacing-lg)" class="m_dfe9c588 mantine-InlineCodeHighlight-inlineCodeHighlight m_e597c321 mantine-CodeHighlight-codeHighlight m_dfe9c588 mantine-InlineCodeHighlight-inlineCodeHighlight">typeof variable</code> возвращает строку <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'</code>, если переменная не была объявлена или была объявлена, но не инициализирована. Использование <code style="margin-bottom:var(--mantine-spacing-lg)" class="m_dfe9c588 mantine-InlineCodeHighlight-inlineCodeHighlight m_e597c321 mantine-CodeHighlight-codeHighlight m_dfe9c588 mantine-InlineCodeHighlight-inlineCodeHighlight">typeof</code> не вызывает ошибки, если переменная не существует. Это важно, если вы не уверены, объявлена ли переменная.</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">let a;
console.log(typeof a !== 'undefined'); // true, a объявлена, но не инициализирована
console.log(typeof b !== 'undefined'); // false, b не объявлена</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>Проверка <code style="margin-bottom:var(--mantine-spacing-lg)" class="m_dfe9c588 mantine-InlineCodeHighlight-inlineCodeHighlight m_e597c321 mantine-CodeHighlight-codeHighlight m_dfe9c588 mantine-InlineCodeHighlight-inlineCodeHighlight">!= null</code> проверяет, равно ли значение переменной <code style="margin-bottom:var(--mantine-spacing-lg)" class="m_dfe9c588 mantine-InlineCodeHighlight-inlineCodeHighlight m_e597c321 mantine-CodeHighlight-codeHighlight m_dfe9c588 mantine-InlineCodeHighlight-inlineCodeHighlight">null</code> или <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</code> (из-за нестрогого сравнения).</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">if (variable != null) {
// переменная не равна ни null, ни undefined
}</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>При использовании нестрогого сравнения (<code style="margin-bottom:var(--mantine-spacing-lg)" class="m_dfe9c588 mantine-InlineCodeHighlight-inlineCodeHighlight m_e597c321 mantine-CodeHighlight-codeHighlight m_dfe9c588 mantine-InlineCodeHighlight-inlineCodeHighlight">!=</code>), JavaScript приводит оба операнда к одному типу, и проверка вернет <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>, если значение переменной — <code style="margin-bottom:var(--mantine-spacing-lg)" class="m_dfe9c588 mantine-InlineCodeHighlight-inlineCodeHighlight m_e597c321 mantine-CodeHighlight-codeHighlight m_dfe9c588 mantine-InlineCodeHighlight-inlineCodeHighlight">null</code> или <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</code>.</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 c = null;
console.log(c != null); // false
const d;
console.log(d != null); // false, d равно undefined
const e = 0;
console.log(e != null); // true, e установлено в 0</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 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">Ivan Gagarinov</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 style="--carousel-control-size:calc(2.5rem * var(--mantine-scale));--carousel-controls-offset:var(--mantine-spacing-sm);margin-top:var(--mantine-spacing-xl);margin-bottom:var(--mantine-spacing-lg);padding-block:var(--mantine-spacing-sm);background:var(--app-color-surface)" class="m_17884d0f mantine-Carousel-root responsiveClassName" data-orientation="horizontal" data-include-gap-in-size="true"><div class="m_39bc3463 mantine-Carousel-controls" data-orientation="horizontal"><button class="mantine-focus-auto m_64f58e10 mantine-Carousel-control m_87cf2631 mantine-UnstyledButton-root" type="button" data-inactive="true" data-type="previous" tabindex="-1"><svg viewBox="0 0 15 15" fill="none" xmlns="http://www.w3.org/2000/svg" style="transform:rotate(90deg);width:calc(1rem * var(--mantine-scale));height:calc(1rem * var(--mantine-scale));display:block"><path d="M3.13523 6.15803C3.3241 5.95657 3.64052 5.94637 3.84197 6.13523L7.5 9.56464L11.158 6.13523C11.3595 5.94637 11.6759 5.95657 11.8648 6.15803C12.0536 6.35949 12.0434 6.67591 11.842 6.86477L7.84197 10.6148C7.64964 10.7951 7.35036 10.7951 7.15803 10.6148L3.15803 6.86477C2.95657 6.67591 2.94637 6.35949 3.13523 6.15803Z" fill="currentColor" fill-rule="evenodd" clip-rule="evenodd"></path></svg></button><button class="mantine-focus-auto m_64f58e10 mantine-Carousel-control m_87cf2631 mantine-UnstyledButton-root" type="button" data-inactive="true" data-type="next" tabindex="-1"><svg viewBox="0 0 15 15" fill="none" xmlns="http://www.w3.org/2000/svg" style="transform:rotate(-90deg);width:calc(1rem * var(--mantine-scale));height:calc(1rem * var(--mantine-scale));display:block"><path d="M3.13523 6.15803C3.3241 5.95657 3.64052 5.94637 3.84197 6.13523L7.5 9.56464L11.158 6.13523C11.3595 5.94637 11.6759 5.95657 11.8648 6.15803C12.0536 6.35949 12.0434 6.67591 11.842 6.86477L7.84197 10.6148C7.64964 10.7951 7.35036 10.7951 7.15803 10.6148L3.15803 6.86477C2.95657 6.67591 2.94637 6.35949 3.13523 6.15803Z" fill="currentColor" fill-rule="evenodd" clip-rule="evenodd"></path></svg></button></div><div class="m_a2dae653 mantine-Carousel-viewport" data-type="media"><div class="m_fcd81474 mantine-Carousel-container __m__-_R_4bbdiub_" data-orientation="horizontal"><div class="m_d98df724 mantine-Carousel-slide" data-orientation="horizontal"><div tabindex="0" style="cursor:pointer;height:100%"><a style="text-decoration:none" class="mantine-focus-auto m_849cf0da m_b6d8b162 mantine-Text-root mantine-Anchor-root" data-underline="hover" href="/programs/frontend?promo_name=programs_list&promo_position=qna_question&promo_creative=catalog_card&promo_type=card" target="_blank"><div style="height:100%" class="m_e615b15f mantine-Card-root m_1b7284a3 mantine-Paper-root" data-with-border="true"><div style="--group-gap:calc(0.25rem * var(--mantine-scale));--group-align:center;--group-justify:flex-start;--group-wrap:nowrap" class="m_4081bf90 mantine-Group-root"><span style="font-size:var(--mantine-font-size-sm)" class="mantine-focus-auto m_b6d8b162 mantine-Text-root">10 месяцев</span><span class="mantine-focus-auto m_b6d8b162 mantine-Text-root">·</span><span style="font-size:var(--mantine-font-size-sm)" class="mantine-focus-auto m_b6d8b162 mantine-Text-root">С нуля</span></div><p style="margin-bottom:var(--mantine-spacing-sm);font-size:var(--mantine-font-size-h5);font-weight:bold" class="mantine-focus-auto m_b6d8b162 mantine-Text-root">Фронтенд-разработчик</p><p class="mantine-focus-auto m_b6d8b162 mantine-Text-root">Изучите HTML, CSS, JavaScript и React</p><div style="margin-top:auto" class=""><div class="m_4451eb3a mantine-Center-root"><img style="opacity:0.8;width:70%" class="m_9e117634 mantine-Image-root mantine-visible-from-xs" src="https://hexlet.io/rails/active_storage/representations/proxy/eyJfcmFpbHMiOnsiZGF0YSI6MzcyNywicHVyIjoiYmxvYl9pZCJ9fQ==--2d5cbbf5c3b4a73ae4b2c50632305d78f5872e4d/eyJfcmFpbHMiOnsiZGF0YSI6eyJmb3JtYXQiOiJ3ZWJwIiwicmVzaXplX3RvX2xpbWl0IjpbNDAwLDQwMF0sInNhdmVyIjp7InF1YWxpdHkiOjg1fX0sInB1ciI6InZhcmlhdGlvbiJ9fQ==--5b6f46dacd1af664f27558553a58076185091823/Programmer-rafiki.png" alt="Фронтенд-разработчик" loading="eager"/></div><div style="--group-gap:var(--mantine-spacing-md);--group-align:end;--group-justify:space-between;--group-wrap:wrap;margin-top:var(--mantine-spacing-xs)" class="m_4081bf90 mantine-Group-root"><p style="font-size:var(--mantine-font-size-xl)" class="mantine-focus-auto m_b6d8b162 mantine-Text-root">от 6 792 ₽</p><p style="font-size:var(--mantine-font-size-sm)" class="mantine-focus-auto m_b6d8b162 mantine-Text-root">Посмотреть →</p></div></div></div></a></div></div><div class="m_d98df724 mantine-Carousel-slide" data-orientation="horizontal"><div tabindex="0" style="cursor:pointer;height:100%"><a style="text-decoration:none" class="mantine-focus-auto m_849cf0da m_b6d8b162 mantine-Text-root mantine-Anchor-root" data-underline="hover" href="/programs/js-oop?promo_name=programs_list&promo_position=qna_question&promo_creative=catalog_card&promo_type=card" target="_blank"><div style="height:100%" class="m_e615b15f mantine-Card-root m_1b7284a3 mantine-Paper-root" data-with-border="true"><div style="--group-gap:calc(0.25rem * var(--mantine-scale));--group-align:center;--group-justify:flex-start;--group-wrap:nowrap" class="m_4081bf90 mantine-Group-root"><span style="font-size:var(--mantine-font-size-sm)" class="mantine-focus-auto m_b6d8b162 mantine-Text-root">2 месяца</span><span class="mantine-focus-auto m_b6d8b162 mantine-Text-root">·</span><span style="font-size:var(--mantine-font-size-sm)" class="mantine-focus-auto m_b6d8b162 mantine-Text-root">Для продвинутых</span></div><p style="margin-bottom:var(--mantine-spacing-sm);font-size:var(--mantine-font-size-h5);font-weight:bold" class="mantine-focus-auto m_b6d8b162 mantine-Text-root">ООП на Javascript</p><p class="mantine-focus-auto m_b6d8b162 mantine-Text-root">Изучите архитектуру и принципы чистого кода на JS</p><div style="margin-top:auto" class=""><div class="m_4451eb3a mantine-Center-root"><img style="opacity:0.8;width:70%" class="m_9e117634 mantine-Image-root mantine-visible-from-xs" src="https://hexlet.io/rails/active_storage/representations/proxy/eyJfcmFpbHMiOnsiZGF0YSI6NDAxOSwicHVyIjoiYmxvYl9pZCJ9fQ==--84efd2b6854b7000046e9ce06e6be85d38af5ab8/eyJfcmFpbHMiOnsiZGF0YSI6eyJmb3JtYXQiOiJ3ZWJwIiwicmVzaXplX3RvX2xpbWl0IjpbNDAwLDQwMF0sInNhdmVyIjp7InF1YWxpdHkiOjg1fX0sInB1ciI6InZhcmlhdGlvbiJ9fQ==--5b6f46dacd1af664f27558553a58076185091823/JavaScript%20frameworks-cuate.png" alt="ООП на Javascript" loading="eager"/></div><div style="--group-gap:var(--mantine-spacing-md);--group-align:end;--group-justify:space-between;--group-wrap:wrap;margin-top:var(--mantine-spacing-xs)" class="m_4081bf90 mantine-Group-root"><p style="font-size:var(--mantine-font-size-xl)" class="mantine-focus-auto m_b6d8b162 mantine-Text-root">от 3 900 ₽</p><p style="font-size:var(--mantine-font-size-sm)" class="mantine-focus-auto m_b6d8b162 mantine-Text-root">Посмотреть →</p></div></div></div></a></div></div><div class="m_d98df724 mantine-Carousel-slide" data-orientation="horizontal"><div tabindex="0" style="cursor:pointer;height:100%"><a style="text-decoration:none" class="mantine-focus-auto m_849cf0da m_b6d8b162 mantine-Text-root mantine-Anchor-root" data-underline="hover" href="/programs/js-async?promo_name=programs_list&promo_position=qna_question&promo_creative=catalog_card&promo_type=card" target="_blank"><div style="height:100%" class="m_e615b15f mantine-Card-root m_1b7284a3 mantine-Paper-root" data-with-border="true"><div style="--group-gap:calc(0.25rem * var(--mantine-scale));--group-align:center;--group-justify:flex-start;--group-wrap:nowrap" class="m_4081bf90 mantine-Group-root"><span style="font-size:var(--mantine-font-size-sm)" class="mantine-focus-auto m_b6d8b162 mantine-Text-root">1 месяц</span><span class="mantine-focus-auto m_b6d8b162 mantine-Text-root">·</span><span style="font-size:var(--mantine-font-size-sm)" class="mantine-focus-auto m_b6d8b162 mantine-Text-root">Для продвинутых</span></div><p style="margin-bottom:var(--mantine-spacing-sm);font-size:var(--mantine-font-size-h5);font-weight:bold" class="mantine-focus-auto m_b6d8b162 mantine-Text-root">Асинхронное программирование на JS</p><p class="mantine-focus-auto m_b6d8b162 mantine-Text-root">Навык работы с асинхронностью в JS</p><div style="margin-top:auto" class=""><div class="m_4451eb3a mantine-Center-root"><img style="opacity:0.8;width:70%" class="m_9e117634 mantine-Image-root mantine-visible-from-xs" 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style="cursor:pointer;height:100%"><a style="text-decoration:none" class="mantine-focus-auto m_849cf0da m_b6d8b162 mantine-Text-root mantine-Anchor-root" data-underline="hover" href="/programs/typescript?promo_name=programs_list&promo_position=qna_question&promo_creative=catalog_card&promo_type=card" target="_blank"><div style="height:100%" class="m_e615b15f mantine-Card-root m_1b7284a3 mantine-Paper-root" data-with-border="true"><div style="--group-gap:calc(0.25rem * var(--mantine-scale));--group-align:center;--group-justify:flex-start;--group-wrap:nowrap" class="m_4081bf90 mantine-Group-root"><span style="font-size:var(--mantine-font-size-sm)" class="mantine-focus-auto m_b6d8b162 mantine-Text-root">2 месяца</span><span class="mantine-focus-auto m_b6d8b162 mantine-Text-root">·</span><span style="font-size:var(--mantine-font-size-sm)" class="mantine-focus-auto m_b6d8b162 mantine-Text-root">Для продвинутых</span></div><p 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