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<!DOCTYPE html><html lang="en"><head><meta charset="utf-8"><meta name="viewport" content="width=device-width, initial-scale=1.0"><meta name="generator" content="rustdoc"><meta name="description" content="API documentation for the Rust `intern` fn in crate `wasm_bindgen`."><meta name="keywords" content="rust, rustlang, rust-lang, intern"><title>wasm_bindgen::intern - Rust</title><link rel="stylesheet" type="text/css" href="../normalize.css"><link rel="stylesheet" type="text/css" href="../rustdoc.css" id="mainThemeStyle"><link rel="stylesheet" type="text/css" href="../dark.css"><link rel="stylesheet" type="text/css" href="../light.css" id="themeStyle"><script src="../storage.js"></script><noscript><link rel="stylesheet" href="../noscript.css"></noscript><link rel="shortcut icon" href="../favicon.ico"><style type="text/css">#crate-search{background-image:url("../down-arrow.svg");}</style></head><body class="rustdoc fn"><!--[if lte IE 8]><div class="warning">This old browser is unsupported and will most likely display funky things.</div><![endif]--><nav class="sidebar"><div class="sidebar-menu">☰</div><a href='../wasm_bindgen/index.html'><div class='logo-container'><img src='../rust-logo.png' alt='logo'></div></a><div class="sidebar-elems"><p class='location'><a href='index.html'>wasm_bindgen</a></p><script>window.sidebarCurrent = {name: 'intern', ty: 'fn', relpath: ''};</script><script defer src="sidebar-items.js"></script></div></nav><div class="theme-picker"><button id="theme-picker" aria-label="Pick another theme!"><img src="../brush.svg" width="18" alt="Pick another theme!"></button><div id="theme-choices"></div></div><script src="../theme.js"></script><nav class="sub"><form class="search-form"><div class="search-container"><div><select id="crate-search"><option value="All crates">All crates</option></select><input class="search-input" name="search" disabled autocomplete="off" spellcheck="false" placeholder="Click or press ‘S’ to search, ‘?’ for more options…" type="search"></div><a id="settings-menu" href="../settings.html"><img src="../wheel.svg" width="18" alt="Change settings"></a></div></form></nav><section id="main" class="content"><h1 class='fqn'><span class='out-of-band'><span id='render-detail'><a id="toggle-all-docs" href="javascript:void(0)" title="collapse all docs">[<span class='inner'>−</span>]</a></span><a class='srclink' href='../src/wasm_bindgen/cache/intern.rs.html#85-90' title='goto source code'>[src]</a></span><span class='in-band'>Function <a href='index.html'>wasm_bindgen</a>::<wbr><a class="fn" href=''>intern</a></span></h1><pre class='rust fn'>pub fn intern(s: &<a class="primitive" href="https://doc.rust-lang.org/nightly/std/primitive.str.html">str</a>) -> &<a class="primitive" href="https://doc.rust-lang.org/nightly/std/primitive.str.html">str</a></pre><div class='docblock'><p>Interns Rust strings so that it's much faster to send them to JS.</p>
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<p>Sending strings from Rust to JS is slow, because it has to do a full <code>O(n)</code>
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copy and <em>also</em> encode from UTF-8 to UTF-16. This must be done every single
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time a string is sent to JS.</p>
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<p>If you are sending the same string multiple times, you can call this <code>intern</code>
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function, which simply returns its argument unchanged:</p>
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<div class="example-wrap"><pre class="rust rust-example-rendered">
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<span class="ident">intern</span>(<span class="string">"foo"</span>) <span class="comment">// returns "foo"</span></pre></div>
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<p>However, if you enable the <code>"enable-interning"</code> feature for wasm-bindgen,
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then it will add the string into an internal cache.</p>
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<p>When you send that cached string to JS, it will look it up in the cache,
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which completely avoids the <code>O(n)</code> copy and encoding. This has a significant
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speed boost (as high as 783%)!</p>
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<p>However, there is a small cost to this caching, so you shouldn't cache every
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string. Only cache strings which have a high likelihood of being sent
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to JS multiple times.</p>
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<p>Also, keep in mind that this function is a <em>performance hint</em>: it's not
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<em>guaranteed</em> that the string will be cached, and the caching strategy
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might change at any time, so don't rely upon it.</p>
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