internal/baseFlatten.js

define(['../lang/isArguments', '../lang/isArray', './isLength', './isObjectLike'], function(isArguments, isArray, isLength, isObjectLike) {

  /**
   * The base implementation of `_.flatten` with added support for restricting
   * flattening and specifying the start index.
   *
   * @private
   * @param {Array} array The array to flatten.
   * @param {boolean} [isDeep] Specify a deep flatten.
   * @param {boolean} [isStrict] Restrict flattening to arrays and `arguments` objects.
   * @param {number} [fromIndex=0] The index to start from.
   * @returns {Array} Returns the new flattened array.
   */
  function baseFlatten(array, isDeep, isStrict, fromIndex) {
    var index = (fromIndex || 0) - 1,
        length = array.length,
        resIndex = -1,
        result = [];

    while (++index < length) {
      var value = array[index];

      if (isObjectLike(value) && isLength(value.length) && (isArray(value) || isArguments(value))) {
        if (isDeep) {
          // Recursively flatten arrays (susceptible to call stack limits).
          value = baseFlatten(value, isDeep, isStrict);
        }
        var valIndex = -1,
            valLength = value.length;

        result.length += valLength;
        while (++valIndex < valLength) {
          result[++resIndex] = value[valIndex];
        }
      } else if (!isStrict) {
        result[++resIndex] = value;
      }
    }
    return result;
  }

  return baseFlatten;
});