array/takeWhile.js

define(['../internal/baseCallback', '../internal/baseSlice'], function(baseCallback, baseSlice) {

  /**
   * Creates a slice of `array` with elements taken from the beginning. Elements
   * are taken until `predicate` returns falsey. The predicate is bound to
   * `thisArg` and invoked with three arguments; (value, index, array).
   *
   * If a property name is provided for `predicate` the created "_.property"
   * style callback returns the property value of the given element.
   *
   * If an object is provided for `predicate` the created "_.matches" style
   * callback returns `true` for elements that have the properties of the given
   * object, else `false`.
   *
   * @static
   * @memberOf _
   * @type Function
   * @category Array
   * @param {Array} array The array to query.
   * @param {Function|Object|string} [predicate=_.identity] The function invoked
   *  per element.
   * @param {*} [thisArg] The `this` binding of `predicate`.
   * @returns {Array} Returns the slice of `array`.
   * @example
   *
   * _.takeWhile([1, 2, 3], function(n) { return n < 3; });
   * // => [1, 2]
   *
   * var users = [
   *   { 'user': 'barney',  'status': 'busy', 'active': true },
   *   { 'user': 'fred',    'status': 'busy', 'active': false },
   *   { 'user': 'pebbles', 'status': 'away', 'active': true }
   * ];
   *
   * // using the "_.property" callback shorthand
   * _.pluck(_.takeWhile(users, 'active'), 'user');
   * // => ['barney']
   *
   * // using the "_.matches" callback shorthand
   * _.pluck(_.takeWhile(users, { 'status': 'busy' }), 'user');
   * // => ['barney', 'fred']
   */
  function takeWhile(array, predicate, thisArg) {
    var length = array ? array.length : 0;
    if (!length) {
      return [];
    }
    var index = -1;
    predicate = baseCallback(predicate, thisArg, 3);
    while (++index < length && predicate(array[index], index, array)) {}
    return baseSlice(array, 0, index);
  }

  return takeWhile;
});