Type checking examples in JavaScript. Code from AngularJS source. Also a few Array and Object helpers.

/**
 * @ngdoc function
 * @name angular.lowercase
 * @function
 *
 * @description Converts the specified string to lowercase.
 * @param {string} string String to be converted to lowercase.
 * @returns {string} Lowercased string.
 */
var lowercase = function(string){return isString(string) ? string.toLowerCase() : string;};

var /** holds major version number for IE or NaN for real browsers */
    msie              = int((/msie (d+)/.exec(lowercase(navigator.userAgent)) || [])[1]),
    jqLite,           // delay binding since jQuery could be loaded after us.
    jQuery,           // delay binding
    slice             = [].slice,
    push              = [].push,
    toString          = Object.prototype.toString,

    /** @name angular */
    angular           = window.angular || (window.angular = {})
;

/**
 * @ngdoc function
 * @name angular.isUndefined
 * @function
 *
 * @description
 * Determines if a reference is undefined.
 *
 * @param {*} value Reference to check.
 * @returns {boolean} True if `value` is undefined.
 */
function isUndefined(value){return typeof value == 'undefined';}

/**
 * @ngdoc function
 * @name angular.isDefined
 * @function
 *
 * @description
 * Determines if a reference is defined.
 *
 * @param {*} value Reference to check.
 * @returns {boolean} True if `value` is defined.
 */
function isDefined(value){return typeof value != 'undefined';}

/**
 * @ngdoc function
 * @name angular.isObject
 * @function
 *
 * @description
 * Determines if a reference is an `Object`. Unlike `typeof` in JavaScript, `null`s are not
 * considered to be objects.
 *
 * @param {*} value Reference to check.
 * @returns {boolean} True if `value` is an `Object` but not `null`.
 */
function isObject(value){return value != null && typeof value == 'object';}

/**
 * @ngdoc function
 * @name angular.isString
 * @function
 *
 * @description
 * Determines if a reference is a `String`.
 *
 * @param {*} value Reference to check.
 * @returns {boolean} True if `value` is a `String`.
 */
function isString(value){return typeof value == 'string';}

/**
 * @ngdoc function
 * @name angular.isNumber
 * @function
 *
 * @description
 * Determines if a reference is a `Number`.
 *
 * @param {*} value Reference to check.
 * @returns {boolean} True if `value` is a `Number`.
 */
function isNumber(value){return typeof value == 'number';}

/**
 * @ngdoc function
 * @name angular.isDate
 * @function
 *
 * @description
 * Determines if a value is a date.
 *
 * @param {*} value Reference to check.
 * @returns {boolean} True if `value` is a `Date`.
 */
function isDate(value){
  return toString.apply(value) == '[object Date]';
}

/**
 * @ngdoc function
 * @name angular.isArray
 * @function
 *
 * @description
 * Determines if a reference is an `Array`.
 *
 * @param {*} value Reference to check.
 * @returns {boolean} True if `value` is an `Array`.
 */
function isArray(value) {
  return toString.apply(value) == '[object Array]';
}

/**
 * @ngdoc function
 * @name angular.isFunction
 * @function
 *
 * @description
 * Determines if a reference is a `Function`.
 *
 * @param {*} value Reference to check.
 * @returns {boolean} True if `value` is a `Function`.
 */
function isFunction(value){return typeof value == 'function';}

/**
 * Checks if `obj` is a window object.
 *
 * @private
 * @param {*} obj Object to check
 * @returns {boolean} True if `obj` is a window obj.
 */
function isWindow(obj) {
  return obj && obj.document && obj.location && obj.alert && obj.setInterval;
}

function isScope(obj) {
  return obj && obj.$evalAsync && obj.$watch;
}

function isFile(obj) {
  return toString.apply(obj) === '[object File]';
}

function isBoolean(value) {
  return typeof value == 'boolean';
}

function trim(value) {
  return isString(value) ? value.replace(/^s*/, '').replace(/s*$/, '') : value;
}

/**
 * @ngdoc function
 * @name angular.isElement
 * @function
 *
 * @description
 * Determines if a reference is a DOM element (or wrapped jQuery element).
 *
 * @param {*} value Reference to check.
 * @returns {boolean} True if `value` is a DOM element (or wrapped jQuery element).
 */
function isElement(node) {
  return node &&
    (node.nodeName  // we are a direct element
    || (node.bind && node.find));  // we have a bind and find method part of jQuery API
}

/**
 * @ngdoc function
 * @name angular.forEach
 * @function
 *
 * @description
 * Invokes the `iterator` function once for each item in `obj` collection, which can be either an
 * object or an array. The `iterator` function is invoked with `iterator(value, key)`, where `value`
 * is the value of an object property or an array element and `key` is the object property key or
 * array element index. Specifying a `context` for the function is optional.
 *
 * Note: this function was previously known as `angular.foreach`.
 *
   <pre>
     var values = {name: 'misko', gender: 'male'};
     var log = [];
     angular.forEach(values, function(value, key){
       this.push(key + ': ' + value);
     }, log);
     expect(log).toEqual(['name: misko', 'gender:male']);
   </pre>
 *
 * @param {Object|Array} obj Object to iterate over.
 * @param {Function} iterator Iterator function.
 * @param {Object=} context Object to become context (`this`) for the iterator function.
 * @returns {Object|Array} Reference to `obj`.
 */

/**
 * @private
 * @param {*} obj
 * @return {boolean} Returns true if `obj` is an array or array-like object (NodeList, Arguments, ...)
 */
function isArrayLike(obj) {
  if (!obj || (typeof obj.length !== 'number')) return false;

  // We have on object which has length property. Should we treat it as array?
  if (typeof obj.hasOwnProperty != 'function' &&
      typeof obj.constructor != 'function') {
    // This is here for IE8: it is a bogus object treat it as array;
    return true;
  } else  {
    return obj instanceof JQLite ||                      // JQLite
           (jQuery && obj instanceof jQuery) ||          // jQuery
           toString.call(obj) !== '[object Object]' ||   // some browser native object
           typeof obj.callee === 'function';              // arguments (on IE8 looks like regular obj)
  }
}

function forEach(obj, iterator, context) {
  var key;
  if (obj) {
    if (isFunction(obj)){
      for (key in obj) {
        if (key != 'prototype' && key != 'length' && key != 'name' && obj.hasOwnProperty(key)) {
          iterator.call(context, obj[key], key);
        }
      }
    } else if (obj.forEach && obj.forEach !== forEach) {
      obj.forEach(iterator, context);
    } else if (isArrayLike(obj)) {
      for (key = 0; key < obj.length; key++)
        iterator.call(context, obj[key], key);
    } else {
      for (key in obj) {
        if (obj.hasOwnProperty(key)) {
          iterator.call(context, obj[key], key);
        }
      }
    }
  }
  return obj;
}

function sortedKeys(obj) {
  var keys = [];
  for (var key in obj) {
    if (obj.hasOwnProperty(key)) {
      keys.push(key);
    }
  }
  return keys.sort();
}

function forEachSorted(obj, iterator, context) {
  var keys = sortedKeys(obj);
  for ( var i = 0; i < keys.length; i++) {
    iterator.call(context, obj[keys[i]], keys[i]);
  }
  return keys;
}

/**
 * @ngdoc function
 * @name angular.equals
 * @function
 *
 * @description
 * Determines if two objects or two values are equivalent. Supports value types, arrays and
 * objects.
 *
 * Two objects or values are considered equivalent if at least one of the following is true:
 *
 * * Both objects or values pass `===` comparison.
 * * Both objects or values are of the same type and all of their properties pass `===` comparison.
 * * Both values are NaN. (In JavasScript, NaN == NaN => false. But we consider two NaN as equal)
 *
 * During a property comparision, properties of `function` type and properties with names
 * that begin with `$` are ignored.
 *
 * Scope and DOMWindow objects are being compared only be identify (`===`).
 *
 * @param {*} o1 Object or value to compare.
 * @param {*} o2 Object or value to compare.
 * @returns {boolean} True if arguments are equal.
 */
function equals(o1, o2) {
  if (o1 === o2) return true;
  if (o1 === null || o2 === null) return false;
  if (o1 !== o1 && o2 !== o2) return true; // NaN === NaN
  var t1 = typeof o1, t2 = typeof o2, length, key, keySet;
  if (t1 == t2) {
    if (t1 == 'object') {
      if (isArray(o1)) {
        if ((length = o1.length) == o2.length) {
          for(key=0; key<length; key++) {
            if (!equals(o1[key], o2[key])) return false;
          }
          return true;
        }
      } else if (isDate(o1)) {
        return isDate(o2) && o1.getTime() == o2.getTime();
      } else {
        if (isScope(o1) || isScope(o2) || isWindow(o1) || isWindow(o2)) return false;
        keySet = {};
        for(key in o1) {
          if (key.charAt(0) === '$' || isFunction(o1[key])) continue;
          if (!equals(o1[key], o2[key])) return false;
          keySet[key] = true;
        }
        for(key in o2) {
          if (!keySet[key] &&
              key.charAt(0) !== '$' &&
              o2[key] !== undefined &&
              !isFunction(o2[key])) return false;
        }
        return true;
      }
    }
  }
  return false;
}

/**
 * @description
 * Determines the number of elements in an array, the number of properties an object has, or
 * the length of a string.
 *
 * Note: This function is used to augment the Object type in Angular expressions. See
 * {@link angular.Object} for more information about Angular arrays.
 *
 * @param {Object|Array|string} obj Object, array, or string to inspect.
 * @param {boolean} [ownPropsOnly=false] Count only "own" properties in an object
 * @returns {number} The size of `obj` or `0` if `obj` is neither an object nor an array.
 */
function size(obj, ownPropsOnly) {
  var size = 0, key;

  if (isArray(obj) || isString(obj)) {
    return obj.length;
  } else if (isObject(obj)){
    for (key in obj)
      if (!ownPropsOnly || obj.hasOwnProperty(key))
        size++;
  }

  return size;
}

function includes(array, obj) {
  return indexOf(array, obj) != -1;
}

function indexOf(array, obj) {
  if (array.indexOf) return array.indexOf(obj);

  for ( var i = 0; i < array.length; i++) {
    if (obj === array[i]) return i;
  }
  return -1;
}

function arrayRemove(array, value) {
  var index = indexOf(array, value);
  if (index >=0)
    array.splice(index, 1);
  return value;
}