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Parameters:\n\n* Source object\n* List of keys to be replaced\n* The data to merge into source object\n\nIn case the key list is empty, you have to merge all the data completely.\n\n```javascript\nimport fill from '../objects.js';\n\nconst company = {\n name: null,\n state: 'moderating',\n};\n\nconst data = {\n name: 'Hexlet',\n state: 'published',\n};\n\n// The calls below should be interpreted as independent\n\nfill(company, ['name'], data);\n// {\n// name: 'Hexlet',\n// state: 'moderating',\n// }\n\nfill(company, [], data);\n// {\n// name: 'Hexlet',\n// state: 'published',\n// }\n```\n\nTry solving this problem by merging object properties.\n\n### Tips\n\n* [_.pick()](https://lodash.com/docs#pick)\n","prepared_readme":"## objects.js\n\nImplement and export as default a function that fills an object with data from another object using the allowed keys list. 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For example, when a user changes their personal data in the account settings, the data that has been changed is sent to the application as an object. Now we need to move this data from the initial object to the `user` object. This is how the `user` is updated:\n\n\n```javascript\n// This user exists in the system\nconst user = { name: 'Tirion', email: 'support@hexlet.io', age: 44 };\n\n// Data from form\nconst data = { name: 'Tirion 2', age: 33 };\n\n// The result should be\n// { name: 'Tirion 2', email: 'support@hexlet.io', age: 33 };\n```\n\nThe solution is to move each property by hand:\n\n```javascript\nuser.name = data.name;\nuser.age = data.age;\n\n// Somewhere here we save the user in the database\n```\n\nDirect assign works well when the data is small and its structure does not change. If there is a lot of data or different data can come in at different points, it can turn into a pile of duplicate code:\n\n```javascript\nif (Object.hasOwn(data, 'name')) {\n user.name = data.name;\n}\n\n// So we need to list all the possible properties\n```\n\nUsing merge, we can reduce this to a single line:\n\n```javascript\nObject.assign(user, data);\nconsole.log(user);\n// => { name: 'Tirion 2', email: 'support@hexlet.io', age: 33 };\n```\n\nThe [Object.assign()](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Object/assign) method takes the object from the first parameter and fills it with the content from the objects in the following parameters. In our case, we have just one source object in the second parameter.\n\nMerging works as follows. If a property was only in the first object, it will stay unchanged. If the property is present in the second (third, fourth etc.) object, it is written to the first object no matter if it was there or not. Therefore, if the property was present in both the first object and the second object, it will be overwritten by the value from the second object:\n\n```javascript\nconst obj1 = { a: 'a', b: 'b' };\nconst obj2 = { c: 'c', b: 'v' };\nObject.assign(obj1, obj2);\nconsole.log(obj1);\n// => { a: 'a', b: 'v', c: 'c' }\n```\n\nThe `Object.assign()` method has one limitation, it only carries out a shallow merge. Nested objects are not compared, but simply replaced:\n\n```javascript\nconst obj1 = { a: { a: 1 } }\nconst obj2 = { a: { b: 1 } }\nObject.assign(obj1, obj2);\nconsole.log(obj1);\n// => { a: { b: 1 } }\n```\n\nLike any other powerful mechanism, merging needs to be used carefully. There are fields in objects that should not be overwritten when merging, such as the amount of money in the user's account. 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It can be used to store information (a set of heterogeneous data) about an object in the real world, such as a Hexlet user. A Hexlet user may be characterized as having an email, name, password, and other parameters. All of this can be \"packed\" into one object.\n\n```javascript\n// The syntax will be explained in the next lesson\nconst user = {\n email: 'inna@example.com',\n name: 'Inna',\n password: 'qwerty',\n};\n```\n\nThe contents of the *package.json* file can also be represented as a JavaScript object. Moreover, the name JSON itself stands for *JavaScript Object Notation*:\n\n```javascript\n// json is the representation of an object as text\nconst data = {\n name: 'MyProjectName',\n description: \"My Projects's Description\",\n dependencies: [\n // here we have dependencies\n ],\n // here we have the other properties\n};\n```\n\nAnd finally, as hard as it may be to believe, functions and arrays in JavaScript are also objects.\n\n```javascript\ntypeof []; // object\n// Functions are a little more complicated\n// Checking if the function is an object\nMath.random instanceof Object; // true\n```\n\nObjects in JavaScript permeate the whole of development from start to finish. They are ubiquitous and used in real-world applications, often for several purposes at once. They are used both as associative arrays (storing key-value pairs) and as objects in the sense of object-oriented programming, which we will definitely get to know later.\n\n\n\nKnowing how to work with objects in JavaScript is a skillset basis that enables novice programmers to solve pretty complex problems. In this course, there will be a gradual introduction to objects and their capabilities. We'll cover the syntax, real-world use cases, and operators and look at the concept of destructuring and ways to traverse object properties in a loop.\n"},"id":278,"slug":"js-objects","challenges_count":7,"name":"JS: Objects","allow_indexing":true,"state":"approved","course_state":"finished","pricing_type":"paid","description":"The most sophisticated data type in JavaScript is an object. Any composite data, including data with multiple values, and actions taken on them are described using this. Nearly everything, including arrays, functions, and the browser, can represent an object in JavaScript. Arrays and functions are also considered to be objects. 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data-size="xs" href="/u/new"><span class="m_80f1301b mantine-Button-inner"><span class="m_811560b9 mantine-Button-label">Sign up</span></span></a></div></div></div></div></div><div class="paywalled m_d08caa0 mantine-Typography-root"><p>Merge is an operation for combining objects allowing you to pass data from one object to another.</p>
<p>Merge is often used when working with web forms. For example, when a user changes their personal data in the account settings, the data that has been changed is sent to the application as an object. Now we need to move this data from the initial object to the <code style="margin-bottom:var(--mantine-spacing-lg)" class="m_dfe9c588 mantine-InlineCodeHighlight-inlineCodeHighlight m_e597c321 mantine-CodeHighlight-codeHighlight m_dfe9c588 mantine-InlineCodeHighlight-inlineCodeHighlight">user</code> object. This is how the <code style="margin-bottom:var(--mantine-spacing-lg)" class="m_dfe9c588 mantine-InlineCodeHighlight-inlineCodeHighlight m_e597c321 mantine-CodeHighlight-codeHighlight m_dfe9c588 mantine-InlineCodeHighlight-inlineCodeHighlight">user</code> is updated:</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">// This user exists in the system
const user = { name: 'Tirion', email: 'support@hexlet.io', age: 44 };
// Data from form
const data = { name: 'Tirion 2', age: 33 };
// The result should be
// { name: 'Tirion 2', email: 'support@hexlet.io', age: 33 };</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>The solution is to move each property by hand:</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">user.name = data.name;
user.age = data.age;
// Somewhere here we save the user in the database</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>Direct assign works well when the data is small and its structure does not change. If there is a lot of data or different data can come in at different points, it can turn into a pile of duplicate 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 (Object.hasOwn(data, 'name')) {
user.name = data.name;
}
// So we need to list all the possible properties</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>Using merge, we can reduce this to a single line:</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">Object.assign(user, data);
console.log(user);
// => { name: 'Tirion 2', email: 'support@hexlet.io', age: 33 };</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>The <a style="text-decoration:underline" class="mantine-focus-auto m_849cf0da m_b6d8b162 mantine-Text-root mantine-Anchor-root" data-underline="hover" href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Object/assign" rel="noopener noreferrer" target="_blank">Object.assign()</a> method takes the object from the first parameter and fills it with the content from the objects in the following parameters. In our case, we have just one source object in the second parameter.</p>
<p>Merging works as follows. If a property was only in the first object, it will stay unchanged. If the property is present in the second (third, fourth etc.) object, it is written to the first object no matter if it was there or not. Therefore, if the property was present in both the first object and the second object, it will be overwritten by the value from the second object:</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 obj1 = { a: 'a', b: 'b' };
const obj2 = { c: 'c', b: 'v' };
Object.assign(obj1, obj2);
console.log(obj1);
// => { a: 'a', b: 'v', c: 'c' }</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>The <code style="margin-bottom:var(--mantine-spacing-lg)" class="m_dfe9c588 mantine-InlineCodeHighlight-inlineCodeHighlight m_e597c321 mantine-CodeHighlight-codeHighlight m_dfe9c588 mantine-InlineCodeHighlight-inlineCodeHighlight">Object.assign()</code> method has one limitation, it only carries out a shallow merge. Nested objects are not compared, but simply replaced:</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 obj1 = { a: { a: 1 } }
const obj2 = { a: { b: 1 } }
Object.assign(obj1, obj2);
console.log(obj1);
// => { a: { b: 1 } }</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>Like any other powerful mechanism, merging needs to be used carefully. There are fields in objects that should not be overwritten when merging, such as the amount of money in the user's account. If you do not control the second object's data structure, important properties may end up being rewritten (accidentally or maliciously) if properties are allowed in that shouldn't be.</p>
<p>When it comes to web forms, it's technically always possible to send more data than the form expects.</p></div><div style="margin-block:var(--mantine-spacing-xl)" class=""><h2 style="--title-fw:var(--mantine-h2-font-weight);--title-lh:var(--mantine-h2-line-height);--title-fz:var(--mantine-h2-font-size);margin-bottom:var(--mantine-spacing-md)" class="m_8a5d1357 mantine-Title-root" data-order="2">Recommended programs</h2><style data-mantine-styles="inline">.__m__-_R_2mremqrdub_{--carousel-slide-gap:var(--mantine-spacing-xs);--carousel-slide-size:70%;}@media(min-width: 36em){.__m__-_R_2mremqrdub_{--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-bottom:var(--mantine-spacing-lg);padding-block:var(--mantine-spacing-sm);background:var(--app-color-surface)" class="m_17884d0f mantine-Carousel-root responsiveClassName" 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