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    <p>Why not hash your objects yourself manually, and use the resulting strings as keys for a regular JavaScript dictionary? After all you are in the best position to know what makes your objects unique. That's what I do.</p> <p>Example:</p> <pre><code>var key = function(obj){ // some unique object-dependent key return obj.totallyUniqueEmployeeIdKey; // just an example }; var dict = {}; dict[key(obj1)] = obj1; dict[key(obj2)] = obj2; </code></pre> <p>This way you can control indexing done by JavaScript without heavy lifting of memory allocation, and overflow handling.</p> <p>Of course, if you truly want the "industrial-grade solution", you can build a class parameterized by the key function, and with all necessary API of the container, but &hellip; we use JavaScript, and trying to be simple and lightweight, so this functional solution is simple and fast.</p> <p>The key function can be as simple as selecting right attributes of the object, e.g., a key, or a set of keys, which are already unique, a combination of keys, which are unique together, or as complex as using some cryptographic hashes like in <a href="http://archive.dojotoolkit.org/nightly/dojotoolkit/dojox/encoding/" rel="noreferrer">DojoX Encoding</a>, or <a href="http://archive.dojotoolkit.org/nightly/dojotoolkit/dojox/uuid/" rel="noreferrer">DojoX UUID</a>. While the latter solutions may produce unique keys, personally I try to avoid them at all costs, especially, if I know what makes my objects unique.</p> <p><strong>Update in 2014:</strong> Answered back in 2008 this simple solution still requires more explanations. Let me clarify the idea in a Q&amp;A form.</p> <p><em>Your solution doesn't have a real hash. Where is it???</em></p> <p>JavaScript is a high-level language. Its basic primitive (<a href="http://en.wikipedia.org/wiki/JavaScript_syntax#Objects" rel="noreferrer">Object</a>) includes a hash table to keep properties. This hash table is usually written in a low-level language for efficiency. Using a simple object with string keys we use an efficiently implemented hash table with no efforts on our part.</p> <p><em>How do you know they use hash?</em></p> <p>There are three major ways to keep a collection of objects addressable by a key:</p> <ul> <li>Unordered. In this case to retrieve an object by its key we have to go over all keys stopping when we find it. On average it will take n/2 comparisons.</li> <li>Ordered. <ul> <li>Example #1: a sorted array &mdash; doing a binary search we will find our key after ~log2(n) comparisons on average. Much better.</li> <li>Example #2: a tree. Again it'll be ~log(n) attempts.</li> </ul></li> <li>Hash table. On average it requires a constant time. Compare: O(n) vs. O(log n) vs. O(1). Boom.</li> </ul> <p>Obviously JavaScript objects use hash tables in some form to handle general cases.</p> <p><em>Do browser vendors really use hash tables???</em></p> <p>Really.</p> <ul> <li>Chrome/node.js/<a href="https://github.com/v8/v8/" rel="noreferrer">V8</a>: <a href="https://github.com/v8/v8/blob/master/src/objects.h#L1680" rel="noreferrer">JSObject</a>. Look for <a href="https://github.com/v8/v8/blob/master/src/objects.h#L3618" rel="noreferrer">NameDictionary</a> and <a href="https://github.com/v8/v8/blob/master/src/objects.h#L3606" rel="noreferrer">NameDictionaryShape</a> with pertinent details in <a href="https://github.com/v8/v8/blob/master/src/objects.cc" rel="noreferrer">objects.cc</a> and <a href="https://github.com/v8/v8/blob/master/src/objects-inl.h" rel="noreferrer">objects-inl.h</a>.</li> <li>Firefox/<a href="https://github.com/mozilla/gecko-dev" rel="noreferrer">Gecko</a>: <a href="https://github.com/mozilla/gecko-dev/blob/master/js/src/jsobj.h#L99" rel="noreferrer">JSObject</a>, <a href="https://github.com/mozilla/gecko-dev/blob/master/js/src/vm/NativeObject.h#L349" rel="noreferrer">NativeObject</a>, and <a href="https://github.com/mozilla/gecko-dev/blob/master/js/src/vm/NativeObject.h#L1238" rel="noreferrer">PlainObject</a> with pertinent details in <a href="https://github.com/mozilla/gecko-dev/blob/master/js/src/jsobj.cpp" rel="noreferrer">jsobj.cpp</a> and <a href="https://github.com/mozilla/gecko-dev/blob/master/js/src/vm/NativeObject.cpp" rel="noreferrer">vm/NativeObject.cpp</a>.</li> </ul> <p><em>Do they handle collisions?</em></p> <p>Yes. See above. If you found a collision on unequal strings, please do not hesitate to file a bug with a vendor.</p> <p><em>So what is your idea?</em></p> <p>If you want to hash an object, find what makes it unique and use it as a key. Do not try to calculate real hash or emulate hash tables &mdash; it is already efficiently handled by the underlying JavaScript object.</p> <p>Use this key with JavaScript <code>Object</code> to leverage its built-in hash table while steering clear of possible clashes with default properties.</p> <p>Examples to get you started:</p> <ul> <li>If your objects include a unique user name &mdash; use it as a key.</li> <li>If it includes a unique customer number &mdash; use it as a key. <ul> <li>If it includes unique government-issued numbers like an SSN, or a passport number, and your system doesn't allow duplicates &mdash; use it as a key.</li> </ul></li> <li>If a combination of fields is unique &mdash; use it as a key. <ul> <li>State abbreviation + driver license number makes an excellent key.</li> <li>Country abbreviation + passport number is an excellent key too.</li> </ul></li> <li>Some function on fields, or a whole object, can return a unique value &mdash; use it as a key.</li> </ul> <p><em>I used your suggestion and cached all objects using a user name. But some wise guy is named "toString", which is a built-in property! What should I do now?</em></p> <p>Obviously, if it is even remotely possible that the resulting key will exclusively consists of Latin characters, you should do something about it. For example, add any non-Latin Unicode character you like at the beginning or at the end to un-clash with default properties: "#toString", "#MarySmith". If a composite key is used, separate key components using some kind of non-Latin delimiter: "name,city,state".</p> <p>In general this is the place, where we have to be creative, and select the easiest keys with given limitations (uniqueness, potential clashes with default properties).</p> <p>Note: unique keys do not clash by definition, while potential hash clashes will be handled by the underlying <code>Object</code>.</p> <p><em>Why don't you like industrial solutions?</em></p> <p>IMHO, the best code is no code at all: it has no errors, requires no maintenance, easy to understand, and executes instantaneously. All "hash tables in JavaScript" I saw were >100 lines of code, and involved multiple objects. Compare it with: <code>dict[key] = value</code>.</p> <p>Another point: is it even possible to beat a performance of a primordial object written in a low-level language, using JavaScript and the very same primordial objects to implement what is already implemented?</p> <p><em>I still want to hash my objects without any keys!</em></p> <p>We are in luck: ECMAScript 6 (scheduled for mid 2015 release, give or take 1-2 years after that to become widespread) defines <a href="https://people.mozilla.org/~jorendorff/es6-draft.html#sec-map-objects" rel="noreferrer">map</a> and <a href="https://people.mozilla.org/~jorendorff/es6-draft.html#sec-set-objects" rel="noreferrer">set</a>.</p> <p>Judging by the definition they can use object's address as a key, which makes objects instantly distinct without artificial keys. OTOH, two different, yet identical objects, will be mapped as distinct.</p>
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    1. COThe use of arbitrary strings as JavaScript object keys must be handled with care; were the `hash` function to return `__proto__` for example, your program would fail. See http://www.2ality.com/2012/01/objects-as-maps.html
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    3. COThis doesn't look like a proper map, because you don't handle collisions. If this happens to be true: hash(obj1) == hash(obj2), you is going to lose your data.
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