Underscore.js

Portions of Underscore are inspired by or borrowed from Oliver Steele's Functional, Prototype.js and John Resig's Micro-Templating.

Functional programming support is added, but without extending any of the built-in JavaScript objects.

Underscore provides 60-odd functions that support both the usual functional suspects: map, select, invoke — as well as more specialized helpers: function binding, JavaScript templating, deep equality testing, and so on. It delegates to built-in functions, if present, so modern browsers will use the native implementations of forEach, map, reduce, filter, every, some and indexOf.

Ports of Underscore.js are available for:
PHP
Python
Perl

What is new in this release:

• Adds an _.create method, as a slimmed down version of Object.create.
• Works around an iOS bug that can improperly cause isArrayLike to be JIT-ed. Also fixes a bug when passing 0 to isArrayLike.

What is new in version 1.8.2:

• Fixes/changes some old-Internet-Explorer and related edge case behavior.

What is new in version 1.7.0:

• For consistency and speed across browsers, Underscore now ignores native array methods for forEach, map, reduce, reduceRight, filter, every, some, indexOf, and lastIndexOf. "Sparse" arrays are officially dead in Underscore.
• Added _.iteratee to customize the iterators used by collection functions. Many Underscore methods will take a string argument for easier _.property-style lookups, an object for _.where-style filtering, or a function as a custom callback.
• Added _.before as a counterpart to _.after.
• Added _.negate to invert the truth value of a passed-in predicate.
• Added _.noop as a handy empty placeholder function.
• _.isEmpty now works with arguments objects.
• _.has now guards against nullish objects.
• Override base methods like each and some and they'll be used internally by other Underscore functions too.
• The escape functions handle backticks (`), to deal with an IE ≤ 8 bug.
• For consistency, _.union and _.difference now only work with arrays and not variadic args.
• _.memoize exposes the cache of memoized values as a property on the returned function.
• _.pick accepts iteratee and context arguments for a more advanced callback.
• Underscore templates no longer accept an initial data object. _.template always returns a function now.

What is new in version 1.5.0:

• A source map is now supplied for easier debugging of the minified production build of Underscore.
• The defaults function now only overrides undefined values, not null ones.
• Removed the ability to call _.bindAll with no method name arguments. It's pretty much always wiser to white-list the names of the methods you'd like to bind.
• Removed the ability to call _.after with an invocation count of zero. The minimum number of calls is (naturally) now 1.

What is new in version 1.4.4:

• Added _.findWhere, for finding the first element in a list that matches a particular set of keys and values.
• Added _.partial, for partially applying a function without changing its dynamic reference to this.
• Simplified bind by removing some edge cases involving constructor functions. In short: don't _.bind your constructors.
• A minor optimization to invoke.
• Fix bug in the minified version due to the minifier incorrectly optimizing-away isFunction.

What is new in version 1.4.3:

• Added a default _.identity iterator to countBy and groupBy.
• The uniq function can now take array, iterator, context as the argument list.
• The times function now returns the mapped array of iterator results.
• Simplified and fixed bugs in throttle.

What is new in version 1.4.2:

• For backwards compatibility, returned to pre-1.4.0 behavior when passing null to iteration functions. They now become no-ops again.

What is new in version 1.4.0:

• Added a pairs function, for turning a JavaScript object into [key, value] pairs ... as well as an object function, for converting an array of [key, value] pairs into an object.
• Added a countBy function, for counting the number of objects in a list that match a certain criteria.
• Added an invert function, for performing a simple inversion of the keys and values in an object.
• Added a where function, for easy cases of filtering a list for objects with specific values.
• Added an omit function, for filtering an object to remove certain keys.
• Added a random function, to return a random number in a given range.
• _.debounce'd functions now return their last updated value, just like _.throttle'd functions do.
• The sortBy function now runs a stable sort algorithm.
• Added the optional fromIndex option to indexOf and lastIndexOf.
• "Sparse" arrays are no longer supported in Underscore iteration functions. Use a for instead (or better yet, an object).
• The min and max functions may now be called on very large arrays.
• Interpolation in templates now represents null and undefined as the empty string.

What is new in version 1.3.2:

• Added the pick function, which allows you to filter an object literal with a whitelist of allowed property names.
• Added the result function, for convenience when working with APIs that allow either functions or raw properties.
• Added the isFinite function, because sometimes knowing that a value is a number just ain't quite enough.
• The sortBy function may now also be passed the string name of a property to use as the sort order on each object.
• Fixed uniq to work with sparse arrays.
• The difference function now performs a shallow flatten instead of a deep one when computing array differences.
• The debounce function now takes an immediate parameter, which will cause the callback to fire on the leading instead of the trailing edge.

What is new in version 1.3.1:

• Added an _.has function, as a safer way to use hasOwnProperty.
• Added _.collect as an alias for _.map. Smalltalkers, rejoice.
• Reverted an old change so that _.extend will correctly copy over keys with undefined values again.
• Bugfix to stop escaping slashes within interpolations in _.template.

What is new in version 1.3.0:

• Removed AMD (RequireJS) support from Underscore.

What is new in version 1.2.4:

• Can (and probably should) write _.chain(list) instead of _(list).chain().
• Fix for escaped characters in Underscore templates, and for supporting customizations of _.templateSettings that only define one or two of the required regexes.
• Fix for passing an array as the first argument to an _.wrap'd function.
• Improved compatibility with ClojureScript, which adds a call function to String.prototype.

What is new in version 1.2.3:

• Dynamic scope is now preserved for compiled _.template functions, so you can use the value of this if you like.
• Sparse array support of _.indexOf, _.lastIndexOf.
• Both _.reduce and _.reduceRight can now be passed an explicitly undefined value.

What is new in version 1.2.2:

• Continued tweaks to _.isEqual semantics. Now JS primitives are considered equivalent to their wrapped versions, and arrays are compared by their numeric properties only.
• _.escape no longer tries to be smart about not double-escaping already-escaped HTML entities. Now it just escapes regardless.
• In _.template, you may now leave semicolons out of evaluated statements if you wish: <% }) %>.
• _.after(callback, 0) will now trigger the callback immediately, making "after" easier to use with asynchronous APIs.

What is new in version 1.2.1:

• Several important bug fixes for _.isEqual, which should now do better on mutated Arrays, and on non-Array objects with length properties.
• Can now _.groupBy(list, 'property') as a shortcut for grouping values by a particular common property.
• _.throttle'd functions now fire immediately upon invocation, and are rate-limited thereafter.
• Most of the _.is[Type] checks no longer ducktype.
• The _.bind function now also works on constructors, a-la ES5 ... but you would never want to use _.bind on a constructor function.
• _.clone no longer wraps non-object types in Objects.
• _.find and _.filter are now the preferred names for _.detect and _.select.

What is new in version 1.2.0:

• The _.isEqual function now supports true deep equality comparisons, with checks for cyclic structures, thanks to Kit Cambridge.
• Underscore templates now support HTML escaping interpolations, using <%- ... %> syntax.
• Ryan Tenney contributed _.shuffle, which uses a modified Fisher-Yates to give you a shuffled copy of an array.
• _.uniq can now be passed an optional iterator, to determine by what criteria an object should be considered unique.
• _.last now takes an optional argument which will return the last N elements of the list.
• A new _.initial function was added, as a mirror of _.rest, which returns all the initial values of a list (except the last N).

What is new in version 1.1.7:

• Added _.groupBy, which aggregates a collection into groups of like items.
• Added _.union and _.difference, to complement the (re-named) _.intersection.
• Various improvements for support of sparse arrays. _.toArray now returns a clone, if directly passed an array. _.functions now also returns the names of functions that are present in the prototype chain.

What is new in version 1.1.6:

• Added _.after, which will return a function that only runs after first being called a specified number of times. _.invoke can now take a direct function reference. _.every now requires an iterator function to be passed, which mirrors the ECMA5 API. _.extend no longer copies keys when the value is undefined. _.bind now errors when trying to bind an undefined value.

What is new in version 1.1.4:

• Improved compliance with ES5's Array methods when passing null as a value. _.wrap now correctly sets this for the wrapped function. _.indexOf now takes an optional flag for finding the insertion index in an array that is guaranteed to already be sorted.
• Avoiding the use of .callee, to allow _.isArray to work properly in ES5's strict mode.

What is new in version 1.1.3:

• In CommonJS, Underscore may now be required with just:
• var _ = require("underscore"). Added _.throttle and _.debounce functions.
• Removed _.breakLoop, in favor of an ECMA5-style un-break-able each implementation - this removes the try/catch, and you'll now have better stack traces for exceptions that are thrown within an Underscore iterator. Improved the isType family of functions for better interoperability with Internet Explorer host objects. _.template now correctly escapes backslashes in templates. Improved _.reduce compatibility with the ECMA5 version: if you don't pass an initial value, the first item in the collection is used. _.each no longer returns the iterated collection, for improved consistency with ECMA5's forEach.

What is new in version 1.0.4:

• _.memoize function, which can be used to speed up expensive repeated computations by caching the results.

What is new in version 1.0.2:

• Fixed _.isArguments for Opera.