1. concat
   1.1 源码：

/**
   * Creates a new array concatenating `array` with any additional arrays
   * and/or values.
   *
   * @static
   * @memberOf _
   * @since 4.0.0
   * @category Array
   * @param {Array} array The array to concatenate.
   * @param {...*} [values] The values to concatenate.
   * @returns {Array} Returns the new concatenated array.
   * @example
   *
   * var array = [1];
   * var other = _.concat(array, 2, [3], [[4]]);
   *
   * console.log(other);
   * // => [1, 2, 3, [4]]
   *
   * console.log(array);
   * // => [1]
   */
  function concat() {
    var length = arguments.length;
    if (!length) {
      return [];
    }
    var args = Array(length - 1),
        array = arguments[0],
        index = length;

    while (index--) {
      args[index - 1] = arguments[index];
    }
    return arrayPush(isArray(array) ? copyArray(array) : [array], baseFlatten(args, 1));
  }

1.2 源码分析

var length = arguments.length;
获取参数个数

if (!length) { return []; }
如果没有参数返回空数组

var args = Array(length - 1), array = arguments[0], index = length;
变量声明
args：稍后保存arguments中除0索引位置以外的参数
array:保存arguments中的第一个元素

while (index--) { args[index - 1] = arguments[index]; }
将arguments除0位置以外的元素复制到args中(实际0位置元素也被存储在args中，只不过索引值为-1。Javascript中数组的负值和字符串索引)

return arrayPush(isArray(array) ? copyArray(array) : [array], baseFlatten(args, 1));
首先判断传入的第一个参数arguments[0]是不是一个数组，如果是数组，就复制一个，如果不是，就组成一个。
然后调用baseFlatten对args进行一层深度的展开
最后调用arrayPush，进行数组拼接

1.3 总结
贯彻单一职责原则，一个函数只做一件事。调用isArray, arrayPush, copyArray, baseFlatten完成各自的任务。

1.4 相关调用函数

/**
   * Appends the elements of `values` to `array`.
   *
   * @private
   * @param {Array} array The array to modify.
   * @param {Array} values The values to append.
   * @returns {Array} Returns `array`.
   */
  function arrayPush(array, values) {
    var index = -1,
        length = values.length,
        offset = array.length;

    while (++index < length) {
      array[offset + index] = values[index];
    }
    return array;
  }

/**
     * Copies the values of `source` to `array`.
     *
     * @private
     * @param {Array} source The array to copy values from.
     * @param {Array} [array=[]] The array to copy values to.
     * @returns {Array} Returns `array`.
     */
    function copyArray(source, array) {
      var index = -1,
          length = source.length;

      array || (array = Array(length));
      while (++index < length) {
        array[index] = source[index];
      }
      return array;
    }

/**
     * The base implementation of `_.flatten` with support for restricting flattening.
     *
     * @private
     * @param {Array} array The array to flatten.
     * @param {number} depth The maximum recursion depth.
     * @param {boolean} [predicate=isFlattenable] The function invoked per iteration.
     * @param {boolean} [isStrict] Restrict to values that pass `predicate` checks.
     * @param {Array} [result=[]] The initial result value.
     * @returns {Array} Returns the new flattened array.
     */
    function baseFlatten(array, depth, predicate, isStrict, result) {
      var index = -1,
          length = array.length;

      predicate || (predicate = isFlattenable);
      result || (result = []);

      while (++index < length) {
        var value = array[index];
        if (depth > 0 && predicate(value)) {
          if (depth > 1) {
            // Recursively flatten arrays (susceptible to call stack limits).
            baseFlatten(value, depth - 1, predicate, isStrict, result);
          } else {
            arrayPush(result, value);
          }
        } else if (!isStrict) {
          result[result.length] = value;
        }
      }
      return result;
    }

/**
     * Checks if `value` is classified as an `Array` object.
     *
     * @static
     * @memberOf _
     * @since 0.1.0
     * @category Lang
     * @param {*} value The value to check.
     * @returns {boolean} Returns `true` if `value` is an array, else `false`.
     * @example
     *
     * _.isArray([1, 2, 3]);
     * // => true
     *
     * _.isArray(document.body.children);
     * // => false
     *
     * _.isArray('abc');
     * // => false
     *
     * _.isArray(_.noop);
     * // => false
     */
    var isArray = Array.isArray;