underscore.js

Underscore.js 1.2.0
(c) 2011 Jeremy Ashkenas, DocumentCloud Inc.
Underscore is freely distributable under the MIT license.
Portions of Underscore are inspired or borrowed from Prototype,
Oliver Steele's Functional, and John Resig's Micro-Templating.
For all details and documentation:
http://documentcloud.github.com/underscore
(function() {

Baseline setup

Establish the root object, window in the browser, or global on the server.

  var root = this;

Save the previous value of the _ variable.

  var previousUnderscore = root._;

Establish the object that gets returned to break out of a loop iteration.

  var breaker = {};

Save bytes in the minified (but not gzipped) version:

  var ArrayProto = Array.prototype, ObjProto = Object.prototype, FuncProto = Function.prototype;

Create quick reference variables for speed access to core prototypes.

  var slice            = ArrayProto.slice,
      unshift          = ArrayProto.unshift,
      toString         = ObjProto.toString,
      hasOwnProperty   = ObjProto.hasOwnProperty;

All ECMAScript 5 native function implementations that we hope to use are declared here.

  var
    nativeForEach      = ArrayProto.forEach,
    nativeMap          = ArrayProto.map,
    nativeReduce       = ArrayProto.reduce,
    nativeReduceRight  = ArrayProto.reduceRight,
    nativeFilter       = ArrayProto.filter,
    nativeEvery        = ArrayProto.every,
    nativeSome         = ArrayProto.some,
    nativeIndexOf      = ArrayProto.indexOf,
    nativeLastIndexOf  = ArrayProto.lastIndexOf,
    nativeIsArray      = Array.isArray,
    nativeKeys         = Object.keys,
    nativeBind         = FuncProto.bind;

Create a safe reference to the Underscore object for use below.

  var _ = function(obj) { return new wrapper(obj); };

Export the Underscore object for CommonJS, with backwards-compatibility for the old require() API. If we're not in CommonJS, add _ to the global object.

  if (typeof module !== 'undefined' && module.exports) {
    module.exports = _;
    _._ = _;
  } else {

Exported as a string, for Closure Compiler "advanced" mode.

    root['_'] = _;
  }

Current version.

  _.VERSION = '1.2.0';

Collection Functions

The cornerstone, an each implementation, aka forEach. Handles objects with the built-in forEach, arrays, and raw objects. Delegates to ECMAScript 5's native forEach if available.

  var each = _.each = _.forEach = function(obj, iterator, context) {
    if (obj == null) return;
    if (nativeForEach && obj.forEach === nativeForEach) {
      obj.forEach(iterator, context);
    } else if (obj.length === +obj.length) {
      for (var i = 0, l = obj.length; i < l; i++) {
        if (i in obj && iterator.call(context, obj[i], i, obj) === breaker) return;
      }
    } else {
      for (var key in obj) {
        if (hasOwnProperty.call(obj, key)) {
          if (iterator.call(context, obj[key], key, obj) === breaker) return;
        }
      }
    }
  };

Return the results of applying the iterator to each element. Delegates to ECMAScript 5's native map if available.

  _.map = function(obj, iterator, context) {
    var results = [];
    if (obj == null) return results;
    if (nativeMap && obj.map === nativeMap) return obj.map(iterator, context);
    each(obj, function(value, index, list) {
      results[results.length] = iterator.call(context, value, index, list);
    });
    return results;
  };

Reduce builds up a single result from a list of values, aka inject, or foldl. Delegates to ECMAScript 5's native reduce if available.

  _.reduce = _.foldl = _.inject = function(obj, iterator, memo, context) {
    var initial = memo !== void 0;
    if (obj == null) obj = [];
    if (nativeReduce && obj.reduce === nativeReduce) {
      if (context) iterator = _.bind(iterator, context);
      return initial ? obj.reduce(iterator, memo) : obj.reduce(iterator);
    }
    each(obj, function(value, index, list) {
      if (!initial) {
        memo = value;
        initial = true;
      } else {
        memo = iterator.call(context, memo, value, index, list);
      }
    });
    if (!initial) throw new TypeError("Reduce of empty array with no initial value");
    return memo;
  };

The right-associative version of reduce, also known as foldr. Delegates to ECMAScript 5's native reduceRight if available.

  _.reduceRight = _.foldr = function(obj, iterator, memo, context) {
    if (obj == null) obj = [];
    if (nativeReduceRight && obj.reduceRight === nativeReduceRight) {
      if (context) iterator = _.bind(iterator, context);
      return memo !== void 0 ? obj.reduceRight(iterator, memo) : obj.reduceRight(iterator);
    }
    var reversed = (_.isArray(obj) ? obj.slice() : _.toArray(obj)).reverse();
    return _.reduce(reversed, iterator, memo, context);
  };

Return the first value which passes a truth test. Aliased as detect.

  _.find = _.detect = function(obj, iterator, context) {
    var result;
    any(obj, function(value, index, list) {
      if (iterator.call(context, value, index, list)) {
        result = value;
        return true;
      }
    });
    return result;
  };

Return all the elements that pass a truth test. Delegates to ECMAScript 5's native filter if available. Aliased as select.

  _.filter = _.select = function(obj, iterator, context) {
    var results = [];
    if (obj == null) return results;
    if (nativeFilter && obj.filter === nativeFilter) return obj.filter(iterator, context);
    each(obj, function(value, index, list) {
      if (iterator.call(context, value, index, list)) results[results.length] = value;
    });
    return results;
  };

Return all the elements for which a truth test fails.

  _.reject = function(obj, iterator, context) {
    var results = [];
    if (obj == null) return results;
    each(obj, function(value, index, list) {
      if (!iterator.call(context, value, index, list)) results[results.length] = value;
    });
    return results;
  };

Determine whether all of the elements match a truth test. Delegates to ECMAScript 5's native every if available. Aliased as all.

  _.every = _.all = function(obj, iterator, context) {
    var result = true;
    if (obj == null) return result;
    if (nativeEvery && obj.every === nativeEvery) return obj.every(iterator, context);
    each(obj, function(value, index, list) {
      if (!(result = result && iterator.call(context, value, index, list))) return breaker;
    });
    return result;
  };

Determine if at least one element in the object matches a truth test. Delegates to ECMAScript 5's native some if available. Aliased as any.

  var any = _.some = _.any = function(obj, iterator, context) {
    iterator = iterator || _.identity;
    var result = false;
    if (obj == null) return result;
    if (nativeSome && obj.some === nativeSome) return obj.some(iterator, context);
    each(obj, function(value, index, list) {
      if (result |= iterator.call(context, value, index, list)) return breaker;
    });
    return !!result;
  };

Determine if a given value is included in the array or object using ===. Aliased as contains.

  _.include = _.contains = function(obj, target) {
    var found = false;
    if (obj == null) return found;
    if (nativeIndexOf && obj.indexOf === nativeIndexOf) return obj.indexOf(target) != -1;
    any(obj, function(value) {
      if (found = value === target) return true;
    });
    return found;
  };

Invoke a method (with arguments) on every item in a collection.

  _.invoke = function(obj, method) {
    var args = slice.call(arguments, 2);
    return _.map(obj, function(value) {
      return (method.call ? method || value : value[method]).apply(value, args);
    });
  };

Convenience version of a common use case of map: fetching a property.

  _.pluck = function(obj, key) {
    return _.map(obj, function(value){ return value[key]; });
  };

Return the maximum element or (element-based computation).

  _.max = function(obj, iterator, context) {
    if (!iterator && _.isArray(obj)) return Math.max.apply(Math, obj);
    if (!iterator && _.isEmpty(obj)) return -Infinity;
    var result = {computed : -Infinity};
    each(obj, function(value, index, list) {
      var computed = iterator ? iterator.call(context, value, index, list) : value;
      computed >= result.computed && (result = {value : value, computed : computed});
    });
    return result.value;
  };

Return the minimum element (or element-based computation).

  _.min = function(obj, iterator, context) {
    if (!iterator && _.isArray(obj)) return Math.min.apply(Math, obj);
    if (!iterator && _.isEmpty(obj)) return Infinity;
    var result = {computed : Infinity};
    each(obj, function(value, index, list) {
      var computed = iterator ? iterator.call(context, value, index, list) : value;
      computed < result.computed && (result = {value : value, computed : computed});
    });
    return result.value;
  };

Shuffle an array.

  _.shuffle = function(obj) {
    var shuffled = [], rand;
    each(obj, function(value, index, list) {
      if (index == 0) {
        shuffled[0] = value;
      } else {
        rand = Math.floor(Math.random() * (index + 1));
        shuffled[index] = shuffled[rand];
        shuffled[rand] = value;
      }
    });
    return shuffled;
  };

Sort the object's values by a criterion produced by an iterator.

  _.sortBy = function(obj, iterator, context) {
    return _.pluck(_.map(obj, function(value, index, list) {
      return {
        value : value,
        criteria : iterator.call(context, value, index, list)
      };
    }).sort(function(left, right) {
      var a = left.criteria, b = right.criteria;
      return a < b ? -1 : a > b ? 1 : 0;
    }), 'value');
  };

Groups the object's values by a criterion produced by an iterator

  _.groupBy = function(obj, iterator) {
    var result = {};
    each(obj, function(value, index) {
      var key = iterator(value, index);
      (result[key] || (result[key] = [])).push(value);
    });
    return result;
  };

Use a comparator function to figure out at what index an object should be inserted so as to maintain order. Uses binary search.

  _.sortedIndex = function(array, obj, iterator) {
    iterator || (iterator = _.identity);
    var low = 0, high = array.length;
    while (low < high) {
      var mid = (low + high) >> 1;
      iterator(array[mid]) < iterator(obj) ? low = mid + 1 : high = mid;
    }
    return low;
  };

Safely convert anything iterable into a real, live array.

  _.toArray = function(iterable) {
    if (!iterable)                return [];
    if (iterable.toArray)         return iterable.toArray();
    if (_.isArray(iterable))      return slice.call(iterable);
    if (_.isArguments(iterable))  return slice.call(iterable);
    return _.values(iterable);
  };

Return the number of elements in an object.

  _.size = function(obj) {
    return _.toArray(obj).length;
  };

Array Functions

Get the first element of an array. Passing n will return the first N values in the array. Aliased as head. The guard check allows it to work with _.map.

  _.first = _.head = function(array, n, guard) {
    return (n != null) && !guard ? slice.call(array, 0, n) : array[0];
  };

Returns everything but the last entry of the array. Especcialy useful on the arguments object. Passing n will return all the values in the array, excluding the last N. The guard check allows it to work with _.map.

  _.initial = function(array, n, guard) {
    return slice.call(array, 0, array.length - ((n == null) || guard ? 1 : n));
  };

Get the last element of an array. Passing n will return the last N values in the array. The guard check allows it to work with _.map.

  _.last = function(array, n, guard) {
    return (n != null) && !guard ? slice.call(array, array.length - n) : array[array.length - 1];
  };

Returns everything but the first entry of the array. Aliased as tail. Especially useful on the arguments object. Passing an index will return the rest of the values in the array from that index onward. The guard check allows it to work with _.map.

  _.rest = _.tail = function(array, index, guard) {
    return slice.call(array, (index == null) || guard ? 1 : index);
  };

Trim out all falsy values from an array.

  _.compact = function(array) {
    return _.filter(array, function(value){ return !!value; });
  };

Return a completely flattened version of an array.

  _.flatten = function(array) {
    return _.reduce(array, function(memo, value) {
      if (_.isArray(value)) return memo.concat(_.flatten(value));
      memo[memo.length] = value;
      return memo;
    }, []);
  };

Return a version of the array that does not contain the specified value(s).

  _.without = function(array) {
    return _.difference(array, slice.call(arguments, 1));
  };

Produce a duplicate-free version of the array. If the array has already been sorted, you have the option of using a faster algorithm. Aliased as unique.

  _.uniq = _.unique = function(array, isSorted, iterator) {
    var initial = iterator ? _.map(array, iterator) : array;
    var result = [];
    _.reduce(initial, function(memo, el, i) {
      if (0 == i || (isSorted === true ? _.last(memo) != el : !_.include(memo, el))) {
        memo[memo.length] = el;
        result[result.length] = array[i];
      }
      return memo;
    }, []);
    return result;
  };

Produce an array that contains the union: each distinct element from all of the passed-in arrays.

  _.union = function() {
    return _.uniq(_.flatten(arguments));
  };

Produce an array that contains every item shared between all the passed-in arrays. (Aliased as "intersect" for back-compat.)

  _.intersection = _.intersect = function(array) {
    var rest = slice.call(arguments, 1);
    return _.filter(_.uniq(array), function(item) {
      return _.every(rest, function(other) {
        return _.indexOf(other, item) >= 0;
      });
    });
  };

Take the difference between one array and another. Only the elements present in just the first array will remain.

  _.difference = function(array, other) {
    return _.filter(array, function(value){ return !_.include(other, value); });
  };

Zip together multiple lists into a single array -- elements that share an index go together.

  _.zip = function() {
    var args = slice.call(arguments);
    var length = _.max(_.pluck(args, 'length'));
    var results = new Array(length);
    for (var i = 0; i < length; i++) results[i] = _.pluck(args, "" + i);
    return results;
  };

If the browser doesn't supply us with indexOf (I'm looking at you, MSIE), we need this function. Return the position of the first occurrence of an item in an array, or -1 if the item is not included in the array. Delegates to ECMAScript 5's native indexOf if available. If the array is large and already in sort order, pass true for isSorted to use binary search.

  _.indexOf = function(array, item, isSorted) {
    if (array == null) return -1;
    var i, l;
    if (isSorted) {
      i = _.sortedIndex(array, item);
      return array[i] === item ? i : -1;
    }
    if (nativeIndexOf && array.indexOf === nativeIndexOf) return array.indexOf(item);
    for (i = 0, l = array.length; i < l; i++) if (array[i] === item) return i;
    return -1;
  };

Delegates to ECMAScript 5's native lastIndexOf if available.

  _.lastIndexOf = function(array, item) {
    if (array == null) return -1;
    if (nativeLastIndexOf && array.lastIndexOf === nativeLastIndexOf) return array.lastIndexOf(item);
    var i = array.length;
    while (i--) if (array[i] === item) return i;
    return -1;
  };

Generate an integer Array containing an arithmetic progression. A port of the native Python range() function. See the Python documentation.

  _.range = function(start, stop, step) {
    if (arguments.length <= 1) {
      stop = start || 0;
      start = 0;
    }
    step = arguments[2] || 1;

    var len = Math.max(Math.ceil((stop - start) / step), 0);
    var idx = 0;
    var range = new Array(len);

    while(idx < len) {
      range[idx++] = start;
      start += step;
    }

    return range;
  };

Function (ahem) Functions

Create a function bound to a given object (assigning this, and arguments, optionally). Binding with arguments is also known as curry. Delegates to ECMAScript 5's native Function.bind if available. We check for func.bind first, to fail fast when func is undefined.

  _.bind = function(func, obj) {
    if (func.bind === nativeBind && nativeBind) return nativeBind.apply(func, slice.call(arguments, 1));
    var args = slice.call(arguments, 2);
    return function() {
      return func.apply(obj, args.concat(slice.call(arguments)));
    };
  };

Bind all of an object's methods to that object. Useful for ensuring that all callbacks defined on an object belong to it.

  _.bindAll = function(obj) {
    var funcs = slice.call(arguments, 1);
    if (funcs.length == 0) funcs = _.functions(obj);
    each(funcs, function(f) { obj[f] = _.bind(obj[f], obj); });
    return obj;
  };

Memoize an expensive function by storing its results.

  _.memoize = function(func, hasher) {
    var memo = {};
    hasher || (hasher = _.identity);
    return function() {
      var key = hasher.apply(this, arguments);
      return hasOwnProperty.call(memo, key) ? memo[key] : (memo[key] = func.apply(this, arguments));
    };
  };

Delays a function for the given number of milliseconds, and then calls it with the arguments supplied.

  _.delay = function(func, wait) {
    var args = slice.call(arguments, 2);
    return setTimeout(function(){ return func.apply(func, args); }, wait);
  };

Defers a function, scheduling it to run after the current call stack has cleared.

  _.defer = function(func) {
    return _.delay.apply(_, [func, 1].concat(slice.call(arguments, 1)));
  };

Internal function used to implement _.throttle and _.debounce.

  var limit = function(func, wait, debounce) {
    var timeout;
    return function() {
      var context = this, args = arguments;
      var throttler = function() {
        timeout = null;
        func.apply(context, args);
      };
      if (debounce) clearTimeout(timeout);
      if (debounce || !timeout) timeout = setTimeout(throttler, wait);
    };
  };

Returns a function, that, when invoked, will only be triggered at most once during a given window of time.

  _.throttle = function(func, wait) {
    return limit(func, wait, false);
  };

Returns a function, that, as long as it continues to be invoked, will not be triggered. The function will be called after it stops being called for N milliseconds.

  _.debounce = function(func, wait) {
    return limit(func, wait, true);
  };

Returns a function that will be executed at most one time, no matter how often you call it. Useful for lazy initialization.

  _.once = function(func) {
    var ran = false, memo;
    return function() {
      if (ran) return memo;
      ran = true;
      return memo = func.apply(this, arguments);
    };
  };

Returns the first function passed as an argument to the second, allowing you to adjust arguments, run code before and after, and conditionally execute the original function.

  _.wrap = function(func, wrapper) {
    return function() {
      var args = [func].concat(slice.call(arguments));
      return wrapper.apply(this, args);
    };
  };

Returns a function that is the composition of a list of functions, each consuming the return value of the function that follows.

  _.compose = function() {
    var funcs = slice.call(arguments);
    return function() {
      var args = slice.call(arguments);
      for (var i = funcs.length - 1; i >= 0; i--) {
        args = [funcs[i].apply(this, args)];
      }
      return args[0];
    };
  };

Returns a function that will only be executed after being called N times.

  _.after = function(times, func) {
    return function() {
      if (--times < 1) { return func.apply(this, arguments); }
    };
  };

Object Functions

Retrieve the names of an object's properties. Delegates to ECMAScript 5's native Object.keys

  _.keys = nativeKeys || function(obj) {
    if (obj !== Object(obj)) throw new TypeError('Invalid object');
    var keys = [];
    for (var key in obj) if (hasOwnProperty.call(obj, key)) keys[keys.length] = key;
    return keys;
  };

Retrieve the values of an object's properties.

  _.values = function(obj) {
    return _.map(obj, _.identity);
  };

Return a sorted list of the function names available on the object. Aliased as methods

  _.functions = _.methods = function(obj) {
    var names = [];
    for (var key in obj) {
      if (_.isFunction(obj[key])) names.push(key);
    }
    return names.sort();
  };

Extend a given object with all the properties in passed-in object(s).

  _.extend = function(obj) {
    each(slice.call(arguments, 1), function(source) {
      for (var prop in source) {
        if (source[prop] !== void 0) obj[prop] = source[prop];
      }
    });
    return obj;
  };

Fill in a given object with default properties.

  _.defaults = function(obj) {
    each(slice.call(arguments, 1), function(source) {
      for (var prop in source) {
        if (obj[prop] == null) obj[prop] = source[prop];
      }
    });
    return obj;
  };

Create a (shallow-cloned) duplicate of an object.

  _.clone = function(obj) {
    return _.isArray(obj) ? obj.slice() : _.extend({}, obj);
  };

Invokes interceptor with the obj, and then returns obj. The primary purpose of this method is to "tap into" a method chain, in order to perform operations on intermediate results within the chain.

  _.tap = function(obj, interceptor) {
    interceptor(obj);
    return obj;
  };

Internal recursive comparison function.

  function eq(a, b, stack) {

Identical objects are equal. 0 === -0, but they aren't identical. See the Harmony egal proposal: http://wiki.ecmascript.org/doku.php?id=harmony:egal.

    if (a === b) return a !== 0 || 1 / a == 1 / b;

A strict comparison is necessary because null == undefined.

    if (a == null) return a === b;

Compare object types.

    var typeA = typeof a;
    if (typeA != typeof b) return false;

Optimization; ensure that both values are truthy or falsy.

    if (!a != !b) return false;

NaN values are equal.

    if (_.isNaN(a)) return _.isNaN(b);

Compare string objects by value.

    var isStringA = _.isString(a), isStringB = _.isString(b);
    if (isStringA || isStringB) return isStringA && isStringB && String(a) == String(b);

Compare number objects by value.

    var isNumberA = _.isNumber(a), isNumberB = _.isNumber(b);
    if (isNumberA || isNumberB) return isNumberA && isNumberB && +a == +b;

Compare boolean objects by value. The value of true is 1; the value of false is 0.

    var isBooleanA = _.isBoolean(a), isBooleanB = _.isBoolean(b);
    if (isBooleanA || isBooleanB) return isBooleanA && isBooleanB && +a == +b;

Compare dates by their millisecond values.

    var isDateA = _.isDate(a), isDateB = _.isDate(b);
    if (isDateA || isDateB) return isDateA && isDateB && a.getTime() == b.getTime();

Compare RegExps by their source patterns and flags.

    var isRegExpA = _.isRegExp(a), isRegExpB = _.isRegExp(b);
    if (isRegExpA || isRegExpB) {

Ensure commutative equality for RegExps.

      return isRegExpA && isRegExpB &&
             a.source == b.source &&
             a.global == b.global &&
             a.multiline == b.multiline &&
             a.ignoreCase == b.ignoreCase;
    }

Ensure that both values are objects.

    if (typeA != 'object') return false;

Unwrap any wrapped objects.

    if (a._chain) a = a._wrapped;
    if (b._chain) b = b._wrapped;

Invoke a custom isEqual method if one is provided.

    if (_.isFunction(a.isEqual)) return a.isEqual(b);

Assume equality for cyclic structures. The algorithm for detecting cyclic structures is adapted from ES 5.1 section 15.12.3, abstract operation JO.

    var length = stack.length;
    while (length--) {

Linear search. Performance is inversely proportional to the number of unique nested structures.

      if (stack[length] == a) return true;
    }

Add the first object to the stack of traversed objects.

    stack.push(a);
    var size = 0, result = true;
    if (a.length === +a.length || b.length === +b.length) {

Compare object lengths to determine if a deep comparison is necessary.

      size = a.length;
      result = size == b.length;
      if (result) {

Deep compare array-like object contents, ignoring non-numeric properties.

        while (size--) {

Ensure commutative equality for sparse arrays.

          if (!(result = size in a == size in b && eq(a[size], b[size], stack))) break;
        }
      }
    } else {

Deep compare objects.

      for (var key in a) {
        if (hasOwnProperty.call(a, key)) {

Count the expected number of properties.

          size++;

Deep compare each member.

          if (!(result = hasOwnProperty.call(b, key) && eq(a[key], b[key], stack))) break;
        }
      }

Ensure that both objects contain the same number of properties.

      if (result) {
        for (key in b) {
          if (hasOwnProperty.call(b, key) && !size--) break;
        }
        result = !size;
      }
    }

Remove the first object from the stack of traversed objects.

    stack.pop();
    return result;
  }

Perform a deep comparison to check if two objects are equal.

  _.isEqual = function(a, b) {
    return eq(a, b, []);
  };

Is a given array or object empty?

  _.isEmpty = function(obj) {
    if (_.isArray(obj) || _.isString(obj)) return obj.length === 0;
    for (var key in obj) if (hasOwnProperty.call(obj, key)) return false;
    return true;
  };

Is a given value a DOM element?

  _.isElement = function(obj) {
    return !!(obj && obj.nodeType == 1);
  };

Is a given value an array? Delegates to ECMA5's native Array.isArray

  _.isArray = nativeIsArray || function(obj) {
    return toString.call(obj) === '[object Array]';
  };

Is a given variable an object?

  _.isObject = function(obj) {
    return obj === Object(obj);
  };

Is a given variable an arguments object?

  _.isArguments = function(obj) {
    return !!(obj && hasOwnProperty.call(obj, 'callee'));
  };

Is a given value a function?

  _.isFunction = function(obj) {
    return !!(obj && obj.constructor && obj.call && obj.apply);
  };

Is a given value a string?

  _.isString = function(obj) {
    return !!(obj === '' || (obj && obj.charCodeAt && obj.substr));
  };

Is a given value a number?

  _.isNumber = function(obj) {
    return !!(obj === 0 || (obj && obj.toExponential && obj.toFixed));
  };

Is the given value NaN? NaN happens to be the only value in JavaScript that does not equal itself.

  _.isNaN = function(obj) {
    return obj !== obj;
  };

Is a given value a boolean?

  _.isBoolean = function(obj) {
    return obj === true || obj === false || toString.call(obj) == '[object Boolean]';
  };

Is a given value a date?

  _.isDate = function(obj) {
    return !!(obj && obj.getTimezoneOffset && obj.setUTCFullYear);
  };

Is the given value a regular expression?

  _.isRegExp = function(obj) {
    return !!(obj && obj.test && obj.exec && (obj.ignoreCase || obj.ignoreCase === false));
  };

Is a given value equal to null?

  _.isNull = function(obj) {
    return obj === null;
  };

Is a given variable undefined?

  _.isUndefined = function(obj) {
    return obj === void 0;
  };

Utility Functions

Run Underscore.js in noConflict mode, returning the _ variable to its previous owner. Returns a reference to the Underscore object.

  _.noConflict = function() {
    root._ = previousUnderscore;
    return this;
  };

Keep the identity function around for default iterators.

  _.identity = function(value) {
    return value;
  };

Run a function n times.

  _.times = function (n, iterator, context) {
    for (var i = 0; i < n; i++) iterator.call(context, i);
  };

Escape a string for HTML interpolation.

  _.escape = function(string) {
    return (''+string).replace(/&(?!\w+;|#\d+;|#x[\da-f]+;)/gi, '&amp;').replace(/</g, '&lt;').replace(/>/g, '&gt;').replace(/"/g, '&quot;').replace(/'/g, '&#x27;').replace(/\//g,'&#x2F;');
  };

Add your own custom functions to the Underscore object, ensuring that they're correctly added to the OOP wrapper as well.

  _.mixin = function(obj) {
    each(_.functions(obj), function(name){
      addToWrapper(name, _[name] = obj[name]);
    });
  };

Generate a unique integer id (unique within the entire client session). Useful for temporary DOM ids.

  var idCounter = 0;
  _.uniqueId = function(prefix) {
    var id = idCounter++;
    return prefix ? prefix + id : id;
  };

By default, Underscore uses ERB-style template delimiters, change the following template settings to use alternative delimiters.

  _.templateSettings = {
    evaluate    : /<%([\s\S]+?)%>/g,
    interpolate : /<%=([\s\S]+?)%>/g,
    escape      : /<%-([\s\S]+?)%>/g
  };

JavaScript micro-templating, similar to John Resig's implementation. Underscore templating handles arbitrary delimiters, preserves whitespace, and correctly escapes quotes within interpolated code.

  _.template = function(str, data) {
    var c  = _.templateSettings;
    var tmpl = 'var __p=[],print=function(){__p.push.apply(__p,arguments);};' +
      'with(obj||{}){__p.push(\'' +
      str.replace(/\\/g, '\\\\')
         .replace(/'/g, "\\'")
         .replace(c.escape, function(match, code) {
           return "',_.escape(" + code.replace(/\\'/g, "'") + "),'";
         })
         .replace(c.interpolate, function(match, code) {
           return "'," + code.replace(/\\'/g, "'") + ",'";
         })
         .replace(c.evaluate || null, function(match, code) {
           return "');" + code.replace(/\\'/g, "'")
                              .replace(/[\r\n\t]/g, ' ') + "__p.push('";
         })
         .replace(/\r/g, '\\r')
         .replace(/\n/g, '\\n')
         .replace(/\t/g, '\\t')
         + "');}return __p.join('');";
    var func = new Function('obj', tmpl);
    return data ? func(data) : func;
  };

The OOP Wrapper

If Underscore is called as a function, it returns a wrapped object that can be used OO-style. This wrapper holds altered versions of all the underscore functions. Wrapped objects may be chained.

  var wrapper = function(obj) { this._wrapped = obj; };

Expose wrapper.prototype as _.prototype

  _.prototype = wrapper.prototype;

Helper function to continue chaining intermediate results.

  var result = function(obj, chain) {
    return chain ? _(obj).chain() : obj;
  };

A method to easily add functions to the OOP wrapper.

  var addToWrapper = function(name, func) {
    wrapper.prototype[name] = function() {
      var args = slice.call(arguments);
      unshift.call(args, this._wrapped);
      return result(func.apply(_, args), this._chain);
    };
  };

Add all of the Underscore functions to the wrapper object.

  _.mixin(_);

Add all mutator Array functions to the wrapper.

  each(['pop', 'push', 'reverse', 'shift', 'sort', 'splice', 'unshift'], function(name) {
    var method = ArrayProto[name];
    wrapper.prototype[name] = function() {
      method.apply(this._wrapped, arguments);
      return result(this._wrapped, this._chain);
    };
  });

Add all accessor Array functions to the wrapper.

  each(['concat', 'join', 'slice'], function(name) {
    var method = ArrayProto[name];
    wrapper.prototype[name] = function() {
      return result(method.apply(this._wrapped, arguments), this._chain);
    };
  });

Start chaining a wrapped Underscore object.

  wrapper.prototype.chain = function() {
    this._chain = true;
    return this;
  };

Extracts the result from a wrapped and chained object.

  wrapper.prototype.value = function() {
    return this._wrapped;
  };

})();