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2022</div></div></div><h1 class="main__header-title">Object references and copying</h1></div></header><div class="content"><article class="formatted" itemscope itemtype="http://schema.org/TechArticle"><meta itemprop="name" content="Object references and copying"><div itemprop="author" itemscope itemtype="http://schema.org/Person"><meta itemprop="email" content="iliakan@gmail.com"><meta itemprop="name" content="Ilya Kantor"></div><div itemprop="articleBody"><p>One of the fundamental differences of objects versus primitives is that objects are stored and copied “by reference”, whereas primitive values: strings, numbers, booleans, etc – are always copied “as a whole value”.</p> <p>That’s easy to understand if we look a bit under the hood of what happens when we copy a value.</p> <p>Let’s start with a primitive, such as a string.</p> <p>Here we put a copy of <code>message</code> into <code>phrase</code>:</p> <div id="z4jkrfajgi" data-trusted="1" class="code-example"> <div class="codebox code-example__codebox"> <div class="codebox__code" data-code="1"> <pre class="line-numbers language-javascript"><code>let message = "Hello!"; let phrase = message;</code></pre> </div> </div> </div><p>As a result we have two independent variables, each one storing the string <code>"Hello!"</code>.</p> <figure><div class="image" style="width:359px"> <div class="image__ratio" style="padding-top:39.83286908077994%"></div> <object type="image/svg+xml" data="/article/object-copy/variable-copy-value.svg" width="359" height="143" class="image__image" data-use-theme> <img src="/article/object-copy/variable-copy-value.svg" alt="" width="359" height="143"> </object> </div></figure><p>Quite an obvious result, right?</p> <p>Objects are not like that.</p> <p><strong>A variable assigned to an object stores not the object itself, but its “address in memory” – in other words “a reference” to it.</strong></p> <p>Let’s look at an example of such a variable:</p> <div id="55rxi6drse" data-trusted="1" class="code-example"> <div class="codebox code-example__codebox"> <div class="codebox__code" data-code="1"> <pre class="line-numbers language-javascript"><code>let user = { name: "John" };</code></pre> </div> </div> </div><p>And here’s how it’s actually stored in memory:</p> <figure><div class="image" style="width:370px"> <div class="image__ratio" style="padding-top:67.29729729729729%"></div> <object type="image/svg+xml" data="/article/object-copy/variable-contains-reference.svg" width="370" height="249" class="image__image" data-use-theme> <img src="/article/object-copy/variable-contains-reference.svg" alt="" width="370" height="249"> </object> </div></figure><p>The object is stored somewhere in memory (at the right of the picture), while the <code>user</code> variable (at the left) has a “reference” to it.</p> <p>We may think of an object variable, such as <code>user</code>, like a sheet of paper with the address of the object on it.</p> <p>When we perform actions with the object, e.g. take a property <code>user.name</code>, the JavaScript engine looks at what’s at that address and performs the operation on the actual object.</p> <p>Now here’s why it’s important.</p> <p><strong>When an object variable is copied, the reference is copied, but the object itself is not duplicated.</strong></p> <p>For instance:</p> <div id="xk2bpaunfy" data-trusted="1" class="code-example"> <div class="codebox code-example__codebox"> <div class="codebox__code" data-code="1"> <pre class="line-numbers language-javascript"><code>let user = { name: "John" }; let admin = user; // copy the reference</code></pre> </div> </div> </div><p>Now we have two variables, each storing a reference to the same object:</p> <figure><div class="image" style="width:599px"> <div class="image__ratio" style="padding-top:43.40567612687813%"></div> <object type="image/svg+xml" data="/article/object-copy/variable-copy-reference.svg" width="599" height="260" class="image__image" data-use-theme> <img src="/article/object-copy/variable-copy-reference.svg" alt="" width="599" height="260"> </object> </div></figure><p>As you can see, there’s still one object, but now with two variables that reference it.</p> <p>We can use either variable to access the object and modify its contents:</p> <div id="z6r5adapu8" data-trusted="1" class="code-example" data-highlight="[{"start":6,"cols":[{"start":6,"end":15}]},{"start":4,"end":4}]"> <div class="codebox code-example__codebox"> <div class="toolbar codebox__toolbar"> <div class="toolbar__tool"> <a href="#" title="run" data-action="run" class="toolbar__button toolbar__button_run"></a> </div> <div class="toolbar__tool"> <a href="#" title="open in sandbox" target="_blank" data-action="edit" class="toolbar__button toolbar__button_edit"></a> </div> </div> <div class="codebox__code" data-code="1"> <pre class="line-numbers language-javascript"><code>let user = { name: 'John' }; let admin = user; admin.name = 'Pete'; // changed by the "admin" reference alert(user.name); // 'Pete', changes are seen from the "user" reference</code></pre> </div> </div> </div><p>It’s as if we had a cabinet with two keys and used one of them (<code>admin</code>) to get into it and make changes. Then, if we later use another key (<code>user</code>), we are still opening the same cabinet and can access the changed contents.</p> <h2><a class="main__anchor" name="comparison-by-reference" href="#comparison-by-reference">Comparison by reference</a></h2><p>Two objects are equal only if they are the same object.</p> <p>For instance, here <code>a</code> and <code>b</code> reference the same object, thus they are equal:</p> <div id="9pek74v9zv" data-trusted="1" class="code-example"> <div class="codebox code-example__codebox"> <div class="toolbar codebox__toolbar"> <div class="toolbar__tool"> <a href="#" title="run" data-action="run" class="toolbar__button toolbar__button_run"></a> </div> <div class="toolbar__tool"> <a href="#" title="open in sandbox" target="_blank" data-action="edit" class="toolbar__button toolbar__button_edit"></a> </div> </div> <div class="codebox__code" data-code="1"> <pre class="line-numbers language-javascript"><code>let a = {}; let b = a; // copy the reference alert( a == b ); // true, both variables reference the same object alert( a === b ); // true</code></pre> </div> </div> </div><p>And here two independent objects are not equal, even though they look alike (both are empty):</p> <div id="uf4nnulsvs" data-trusted="1" class="code-example"> <div class="codebox code-example__codebox"> <div class="toolbar codebox__toolbar"> <div class="toolbar__tool"> <a href="#" title="run" data-action="run" class="toolbar__button toolbar__button_run"></a> </div> <div class="toolbar__tool"> <a href="#" title="open in sandbox" target="_blank" data-action="edit" class="toolbar__button toolbar__button_edit"></a> </div> </div> <div class="codebox__code" data-code="1"> <pre class="line-numbers language-javascript"><code>let a = {}; let b = {}; // two independent objects alert( a == b ); // false</code></pre> </div> </div> </div><p>For comparisons like <code>obj1 > obj2</code> or for a comparison against a primitive <code>obj == 5</code>, objects are converted to primitives. We’ll study how object conversions work very soon, but to tell the truth, such comparisons are needed very rarely – usually they appear as a result of a programming mistake.</p> <div class="important important_smart"> <div class="important__header"><span class="important__type">Const objects can be modified</span></div> <div class="important__content"><p>An important side effect of storing objects as references is that an object declared as <code>const</code> <em>can</em> be modified.</p> <p>For instance:</p> <div id="307foalroo" data-trusted="1" class="code-example" data-highlight="[{"start":4,"end":4}]"> <div class="codebox code-example__codebox"> <div class="toolbar codebox__toolbar"> <div class="toolbar__tool"> <a href="#" title="run" data-action="run" class="toolbar__button toolbar__button_run"></a> </div> <div class="toolbar__tool"> <a href="#" title="open in sandbox" target="_blank" data-action="edit" class="toolbar__button toolbar__button_edit"></a> </div> </div> <div class="codebox__code" data-code="1"> <pre class="line-numbers language-javascript"><code>const user = { name: "John" }; user.name = "Pete"; // (*) alert(user.name); // Pete</code></pre> </div> </div> </div><p>It might seem that the line <code>(*)</code> would cause an error, but it does not. The value of <code>user</code> is constant, it must always reference the same object, but properties of that object are free to change.</p> <p>In other words, the <code>const user</code> gives an error only if we try to set <code>user=...</code> as a whole.</p> <p>That said, if we really need to make constant object properties, it’s also possible, but using totally different methods. We’ll mention that in the chapter <a href="/property-descriptors">Property flags and descriptors</a>.</p> </div></div> <h2><a class="main__anchor" name="cloning-and-merging-object-assign" href="#cloning-and-merging-object-assign">Cloning and merging, Object.assign</a></h2><p>So, copying an object variable creates one more reference to the same object.</p> <p>But what if we need to duplicate an object?</p> <p>We can create a new object and replicate the structure of the existing one, by iterating over its properties and copying them on the primitive level.</p> <p>Like this:</p> <div id="jtp9af4jfv" data-trusted="1" class="code-example" data-highlight="[{"start":5,"end":10}]"> <div class="codebox code-example__codebox"> <div class="toolbar codebox__toolbar"> <div class="toolbar__tool"> <a href="#" title="run" data-action="run" class="toolbar__button toolbar__button_run"></a> </div> <div class="toolbar__tool"> <a href="#" title="open in sandbox" target="_blank" data-action="edit" class="toolbar__button toolbar__button_edit"></a> </div> </div> <div class="codebox__code" data-code="1"> <pre class="line-numbers language-javascript"><code>let user = { name: "John", age: 30 }; let clone = {}; // the new empty object // let's copy all user properties into it for (let key in user) { clone[key] = user[key]; } // now clone is a fully independent object with the same content clone.name = "Pete"; // changed the data in it alert( user.name ); // still John in the original object</code></pre> </div> </div> </div><p>We can also use the method <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Object/assign">Object.assign</a>.</p> <p>The syntax is:</p> <div id="80oqwowvmx" data-trusted="1" class="code-example"> <div class="codebox code-example__codebox"> <div class="codebox__code" data-code="1"> <pre class="line-numbers language-javascript"><code>Object.assign(dest, ...sources)</code></pre> </div> </div> </div><ul> <li>The first argument <code>dest</code> is a target object.</li> <li>Further arguments is a list of source objects.</li> </ul> <p>It copies the properties of all source objects into the target <code>dest</code>, and then returns it as the result.</p> <p>For example, we have <code>user</code> object, let’s add a couple of permissions to it:</p> <div id="m6d0f3444p" data-trusted="1" class="code-example" data-highlight="[{"start":5,"end":6}]"> <div class="codebox code-example__codebox"> <div class="toolbar codebox__toolbar"> <div class="toolbar__tool"> <a href="#" title="run" data-action="run" class="toolbar__button toolbar__button_run"></a> </div> <div class="toolbar__tool"> <a href="#" title="open in sandbox" target="_blank" data-action="edit" class="toolbar__button toolbar__button_edit"></a> </div> </div> <div class="codebox__code" data-code="1"> <pre class="line-numbers language-javascript"><code>let user = { name: "John" }; let permissions1 = { canView: true }; let permissions2 = { canEdit: true }; // copies all properties from permissions1 and permissions2 into user Object.assign(user, permissions1, permissions2); // now user = { name: "John", canView: true, canEdit: true } alert(user.name); // John alert(user.canView); // true alert(user.canEdit); // true</code></pre> </div> </div> </div><p>If the copied property name already exists, it gets overwritten:</p> <div id="c7aqxa9bd4" data-trusted="1" class="code-example"> <div class="codebox code-example__codebox"> <div class="toolbar codebox__toolbar"> <div class="toolbar__tool"> <a href="#" title="run" data-action="run" class="toolbar__button toolbar__button_run"></a> </div> <div class="toolbar__tool"> <a href="#" title="open in sandbox" target="_blank" data-action="edit" class="toolbar__button toolbar__button_edit"></a> </div> </div> <div class="codebox__code" data-code="1"> <pre class="line-numbers language-javascript"><code>let user = { name: "John" }; Object.assign(user, { name: "Pete" }); alert(user.name); // now user = { name: "Pete" }</code></pre> </div> </div> </div><p>We also can use <code>Object.assign</code> to perform a simple object cloning:</p> <div id="50h2sb6pfh" data-trusted="1" class="code-example" data-highlight="[{"start":5,"end":5}]"> <div class="codebox code-example__codebox"> <div class="toolbar codebox__toolbar"> <div class="toolbar__tool"> <a href="#" title="run" data-action="run" class="toolbar__button toolbar__button_run"></a> </div> <div class="toolbar__tool"> <a href="#" title="open in sandbox" target="_blank" data-action="edit" class="toolbar__button toolbar__button_edit"></a> </div> </div> <div class="codebox__code" data-code="1"> <pre class="line-numbers language-javascript"><code>let user = { name: "John", age: 30 }; let clone = Object.assign({}, user); alert(clone.name); // John alert(clone.age); // 30</code></pre> </div> </div> </div><p>Here it copies all properties of <code>user</code> into the empty object and returns it.</p> <p>There are also other methods of cloning an object, e.g. using the <a href="/rest-parameters-spread">spread syntax</a> <code>clone = {...user}</code>, covered later in the tutorial.</p> <h2><a class="main__anchor" name="nested-cloning" href="#nested-cloning">Nested cloning</a></h2><p>Until now we assumed that all properties of <code>user</code> are primitive. But properties can be references to other objects.</p> <p>Like this:</p> <div id="hfw2evcyoi" data-trusted="1" class="code-example"> <div class="codebox code-example__codebox"> <div class="toolbar codebox__toolbar"> <div class="toolbar__tool"> <a href="#" title="run" data-action="run" class="toolbar__button toolbar__button_run"></a> </div> <div class="toolbar__tool"> <a href="#" title="open in sandbox" target="_blank" data-action="edit" class="toolbar__button toolbar__button_edit"></a> </div> </div> <div class="codebox__code" data-code="1"> <pre class="line-numbers language-javascript"><code>let user = { name: "John", sizes: { height: 182, width: 50 } }; alert( user.sizes.height ); // 182</code></pre> </div> </div> </div><p>Now it’s not enough to copy <code>clone.sizes = user.sizes</code>, because <code>user.sizes</code> is an object, and will be copied by reference, so <code>clone</code> and <code>user</code> will share the same sizes:</p> <div id="k5hou1mkcc" data-trusted="1" class="code-example"> <div class="codebox code-example__codebox"> <div class="toolbar codebox__toolbar"> <div class="toolbar__tool"> <a href="#" title="run" data-action="run" class="toolbar__button toolbar__button_run"></a> </div> <div class="toolbar__tool"> <a href="#" title="open in sandbox" target="_blank" data-action="edit" class="toolbar__button toolbar__button_edit"></a> </div> </div> <div class="codebox__code" data-code="1"> <pre class="line-numbers language-javascript"><code>let user = { name: "John", sizes: { height: 182, width: 50 } }; let clone = Object.assign({}, user); alert( user.sizes === clone.sizes ); // true, same object // user and clone share sizes user.sizes.width = 60; // change a property from one place alert(clone.sizes.width); // 60, get the result from the other one</code></pre> </div> </div> </div><p>To fix that and make <code>user</code> and <code>clone</code> truly separate objects, we should use a cloning loop that examines each value of <code>user[key]</code> and, if it’s an object, then replicate its structure as well. That is called a “deep cloning” or “structured cloning”. There’s <a href="https://developer.mozilla.org/en-US/docs/Web/API/structuredClone">structuredClone</a> method that implements deep cloning.</p> <h3><a class="main__anchor" name="structuredclone" href="#structuredclone">structuredClone</a></h3><p>The call <code>structuredClone(object)</code> clones the <code>object</code> with all nested properties.</p> <p>Here’s how we can use it in our example:</p> <div id="sw3v03syxb" data-trusted="1" class="code-example" data-highlight="[{"start":8,"end":8}]"> <div class="codebox code-example__codebox"> <div class="toolbar codebox__toolbar"> <div class="toolbar__tool"> <a href="#" title="run" data-action="run" class="toolbar__button toolbar__button_run"></a> </div> <div class="toolbar__tool"> <a href="#" title="open in sandbox" target="_blank" data-action="edit" class="toolbar__button toolbar__button_edit"></a> </div> </div> <div class="codebox__code" data-code="1"> <pre class="line-numbers language-javascript"><code>let user = { name: "John", sizes: { height: 182, width: 50 } }; let clone = structuredClone(user); alert( user.sizes === clone.sizes ); // false, different objects // user and clone are totally unrelated now user.sizes.width = 60; // change a property from one place alert(clone.sizes.width); // 50, not related</code></pre> </div> </div> </div><p>The <code>structuredClone</code> method can clone most data types, such as objects, arrays, primitive values.</p> <p>It also supports circular references, when an object property references the object itself (directly or via a chain or references).</p> <p>For instance:</p> <div id="3oaamuz77r" data-trusted="1" class="code-example"> <div class="codebox code-example__codebox"> <div class="toolbar codebox__toolbar"> <div class="toolbar__tool"> <a href="#" title="run" data-action="run" class="toolbar__button toolbar__button_run"></a> </div> <div class="toolbar__tool"> <a href="#" title="open in sandbox" target="_blank" data-action="edit" class="toolbar__button toolbar__button_edit"></a> </div> </div> <div class="codebox__code" data-code="1"> <pre class="line-numbers language-javascript"><code>let user = {}; // let's create a circular reference: // user.me references the user itself user.me = user; let clone = structuredClone(user); alert(clone.me === clone); // true</code></pre> </div> </div> </div><p>As you can see, <code>clone.me</code> references the <code>clone</code>, not the <code>user</code>! So the circular reference was cloned correctly as well.</p> <p>Although, there are cases when <code>structuredClone</code> fails.</p> <p>For instance, when an object has a function property:</p> <div id="mr0ooc5zan" data-trusted="1" class="code-example"> <div class="codebox code-example__codebox"> <div class="toolbar codebox__toolbar"> <div class="toolbar__tool"> <a href="#" title="run" data-action="run" class="toolbar__button toolbar__button_run"></a> </div> <div class="toolbar__tool"> <a href="#" title="open in sandbox" target="_blank" data-action="edit" class="toolbar__button toolbar__button_edit"></a> </div> </div> <div class="codebox__code" data-code="1"> <pre class="line-numbers language-javascript"><code>// error structuredClone({ f: function() {} });</code></pre> </div> </div> </div><p>Function properties aren’t supported.</p> <p>To handle such complex cases we may need to use a combination of cloning methods, write custom code or, to not reinvent the wheel, take an existing implementation, for instance <a href="https://lodash.com/docs#cloneDeep">_.cloneDeep(obj)</a> from the JavaScript library <a href="https://lodash.com">lodash</a>.</p> <h2><a class="main__anchor" name="summary" href="#summary">Summary</a></h2><p>Objects are assigned and copied by reference. In other words, a variable stores not the “object value”, but a “reference” (address in memory) for the value. So copying such a variable or passing it as a function argument copies that reference, not the object itself.</p> <p>All operations via copied references (like adding/removing properties) are performed on the same single object.</p> <p>To make a “real copy” (a clone) we can use <code>Object.assign</code> for the so-called “shallow copy” (nested objects are copied by reference) or a “deep cloning” function <code>structuredClone</code> or use a custom cloning implementation, such as <a href="https://lodash.com/docs#cloneDeep">_.cloneDeep(obj)</a>.</p> </div></article></div><div class="page__nav-wrap"><a class="page__nav page__nav_prev" href="/object" data-tooltip="Objects"><span class="page__nav-text"><span class="page__nav-text-shortcut"></span></span><span class="page__nav-text-alternate">Previous lesson</span></a><a class="page__nav page__nav_next" href="/garbage-collection" data-tooltip="Garbage collection"><span class="page__nav-text"><span class="page__nav-text-shortcut"></span></span><span class="page__nav-text-alternate">Next lesson</span></a></div><div class="article-tablet-foot tablet-only"><div class="article-tablet-foot__layout"><div class="share-icons"><span class="share-icons__title">Share</span><a class="share share_tw" href="https://twitter.com/share?url=https%3A%2F%2Fjavascript.info%2Fobject-copy" rel="nofollow"></a><a class="share share_fb" href="https://www.facebook.com/sharer/sharer.php?s=100&p%5Burl%5D=https%3A%2F%2Fjavascript.info%2Fobject-copy" rel="nofollow"></a></div><div class="article-tablet-foot__map"><a class="map" href="/tutorial/map" data-action="tutorial-map"><span class="map__text">Tutorial map</span></a></div></div></div><div class="banner-bottom-carbon"><div id="javascriptinfo_iconbar"></div></div><script src="//m.servedby-buysellads.com/monetization.custom.js" id="javascriptinfo_script" async></script><script>document.getElementById("javascriptinfo_script").onload=function(){_bsa.init("custom","CKYDEK3U","placement:javascriptinfo_iconbar",{target:"#javascriptinfo_iconbar",template:'\n <div class="iconBarFlex">\n <a href="##statlink##" class="iconBarLink" rel="sponsored noopener" target="_blank" title="##company## — ##tagline##">\n <div class="iconBarImage" style="background-color: ##backgroundColor##;">\n <img height="30" width="30" src="##image##" alt="##company## logo">\n </div>\n <div class="iconBarText">\n <div class="iconBarTagline">##companyTagline##</div>\n <div class="iconBarDescription">##description##</div>\n </div>\n </a>\n <a href="##ad_via_link##" class="iconBarVia" rel="sponsored noopener" target="_blank">ads via BuySellAds</a>\n </div>\n '})};</script><div class="comments formatted" id="comments"><div class="comments__header"><h2 class="comments__header-title"><a href="#comments" name="comments">Comments</a></h2><div class="comments__read-before"><span class="comments__read-before-link">read this before commenting…</span><div class="comments__read-before-popup"><div class="comments__read-before-popup-i"><ul><li>If you have suggestions what to improve - please <a href="https://github.com/javascript-tutorial/en.javascript.info/issues/new">submit a GitHub issue</a> or a pull request instead of commenting.</li><li>If you can't understand something in the article – please elaborate.</li><li>To insert few words of code, use the <code><code></code> tag, for several lines – wrap them in <code><pre></code> tag, for more than 10 lines – use a sandbox (<a href='https://plnkr.co/edit/?p=preview'>plnkr</a>, <a href='https://jsbin.com'>jsbin</a>, <a href='http://codepen.io'>codepen</a>…)</li></ul></div></div></div></div><div id="disqus_thread"></div><script>var disqus_config = function() { if (!this.page) this.page = {}; 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