lodash/internal/baseFlatten.js

import isArguments from '../lang/isArguments';
import isArray from '../lang/isArray';
import isLength from './isLength';
import isObjectLike from './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;
}

export default baseFlatten;