text: 'use strict';
  
  var bind = require('./helpers/bind');
  
  /*global toString:true*/
  
  // utils is a library of generic helper functions non-specific to axios
  
  var toString = Object.prototype.toString;
  
  /**
   * Determine if a value is an Array
   *
   * @param {Object} val The value to test
   * @returns {boolean} True if value is an Array, otherwise false
   */
  function isArray(val) {
    return toString.call(val) === '[object Array]';
  }
  
  /**
   * Determine if a value is undefined
   *
   * @param {Object} val The value to test
   * @returns {boolean} True if the value is undefined, otherwise false
   */
  function isUndefined(val) {
    return typeof val === 'undefined';
  }
  
  /**
   * Determine if a value is a Buffer
   *
   * @param {Object} val The value to test
   * @returns {boolean} True if value is a Buffer, otherwise false
   */
  function isBuffer(val) {
    return val !== null && !isUndefined(val) && val.constructor !== null && !isUndefined(val.constructor)
      && typeof val.constructor.isBuffer === 'function' && val.constructor.isBuffer(val);
  }
  
  /**
   * Determine if a value is an ArrayBuffer
   *
   * @param {Object} val The value to test
   * @returns {boolean} True if value is an ArrayBuffer, otherwise false
   */
  function isArrayBuffer(val) {
    return toString.call(val) === '[object ArrayBuffer]';
  }
  
  /**
   * Determine if a value is a FormData
   *
   * @param {Object} val The value to test
   * @returns {boolean} True if value is an FormData, otherwise false
   */
  function isFormData(val) {
    return (typeof FormData !== 'undefined') && (val instanceof FormData);
  }
  
  /**
   * Determine if a value is a view on an ArrayBuffer
   *
   * @param {Object} val The value to test
   * @returns {boolean} True if value is a view on an ArrayBuffer, otherwise false
   */
  function isArrayBufferView(val) {
    var result;
    // ArrayBuffer.isView(val)判断val是否是视图类型的数据，如DataView或TypedArray
    if ((typeof ArrayBuffer !== 'undefined') && (ArrayBuffer.isView)) {
      result = ArrayBuffer.isView(val);
    } else {
      result = (val) && (val.buffer) && (val.buffer instanceof ArrayBuffer);
    }
    return result;
  }
  
  /**
   * Determine if a value is a String
   *
   * @param {Object} val The value to test
   * @returns {boolean} True if value is a String, otherwise false
   */
  function isString(val) {
    return typeof val === 'string';
  }
  
  /**
   * Determine if a value is a Number
   *
   * @param {Object} val The value to test
   * @returns {boolean} True if value is a Number, otherwise false
   */
  function isNumber(val) {
    return typeof val === 'number';
  }
  
  /**
   * Determine if a value is an Object
   *
   * @param {Object} val The value to test
   * @returns {boolean} True if value is an Object, otherwise false
   */
  function isObject(val) {
    return val !== null && typeof val === 'object';
  }
  
  /**
   * Determine if a value is a plain Object
   *
   * @param {Object} val The value to test
   * @return {boolean} True if value is a plain Object, otherwise false
   */
  function isPlainObject(val) {
    if (toString.call(val) !== '[object Object]') {
      return false;
    }
  
    var prototype = Object.getPrototypeOf(val);
    return prototype === null || prototype === Object.prototype;
  }
  
  /**
   * Determine if a value is a Date
   *
   * @param {Object} val The value to test
   * @returns {boolean} True if value is a Date, otherwise false
   */
  function isDate(val) {
    return toString.call(val) === '[object Date]';
  }
  
  /**
   * Determine if a value is a File
   *
   * @param {Object} val The value to test
   * @returns {boolean} True if value is a File, otherwise false
   */
  function isFile(val) {
    return toString.call(val) === '[object File]';
  }
  
  /**
   * Determine if a value is a Blob
   *
   * @param {Object} val The value to test
   * @returns {boolean} True if value is a Blob, otherwise false
   */
  function isBlob(val) {
    return toString.call(val) === '[object Blob]';
  }
  
  /**
   * Determine if a value is a Function
   *
   * @param {Object} val The value to test
   * @returns {boolean} True if value is a Function, otherwise false
   */
  function isFunction(val) {
    return toString.call(val) === '[object Function]';
  }
  
  /**
   * Determine if a value is a Stream
   *
   * @param {Object} val The value to test
   * @returns {boolean} True if value is a Stream, otherwise false
   */
  function isStream(val) {
    return isObject(val) && isFunction(val.pipe);
  }
  
  /**
   * Determine if a value is a URLSearchParams object
   *
   * @param {Object} val The value to test
   * @returns {boolean} True if value is a URLSearchParams object, otherwise false
   */
  function isURLSearchParams(val) {
    return typeof URLSearchParams !== 'undefined' && val instanceof URLSearchParams;
  }
  
  /**
   * Trim excess whitespace off the beginning and end of a string
   *
   * @param {String} str The String to trim
   * @returns {String} The String freed of excess whitespace
   */
  function trim(str) {
    return str.replace(/^\s*/, '').replace(/\s*$/, '');
  }
  
  /**
   * Determine if we're running in a standard browser environment
   *
   * This allows axios to run in a web worker, and react-native.
   * Both environments support XMLHttpRequest, but not fully standard globals.
   *
   * web workers:
   *  typeof window -> undefined
   *  typeof document -> undefined
   *
   * react-native:
   *  navigator.product -> 'ReactNative'
   * nativescript
   *  navigator.product -> 'NativeScript' or 'NS'
   */
  function isStandardBrowserEnv() {
    if (typeof navigator !== 'undefined' && (navigator.product === 'ReactNative' ||
                                             navigator.product === 'NativeScript' ||
                                             navigator.product === 'NS')) {
      return false;
    }
    return (
      typeof window !== 'undefined' &&
      typeof document !== 'undefined'
    );
  }
  
  /**
   * Iterate over an Array or an Object invoking a function for each item.
   *
   * If `obj` is an Array callback will be called passing
   * the value, index, and complete array for each item.
   *
   * If 'obj' is an Object callback will be called passing
   * the value, key, and complete object for each property.
   *
   * @param {Object|Array} obj The object to iterate
   * @param {Function} fn The callback to invoke for each item
   */
  function forEach(obj, fn) {
    // Don't bother if no value provided
    if (obj === null || typeof obj === 'undefined') {
      return;
    }
  
    // Force an array if not already something iterable
    if (typeof obj !== 'object') {
      /*eslint no-param-reassign:0*/
      obj = [obj];
    }
  
    if (isArray(obj)) {
      // Iterate over array values
      for (var i = 0, l = obj.length; i < l; i++) {
        fn.call(null, obj[i], i, obj);
      }
    } else {
      // Iterate over object keys
      for (var key in obj) {
        if (Object.prototype.hasOwnProperty.call(obj, key)) {
          fn.call(null, obj[key], key, obj);
        }
      }
    }
  }
  
  /**
   * Accepts varargs expecting each argument to be an object, then
   * immutably merges the properties of each object and returns result.
   *
   * When multiple objects contain the same key the later object in
   * the arguments list will take precedence.
   *
   * Example:
   *
   * ```js
   * var result = merge({foo: 123}, {foo: 456});
   * console.log(result.foo); // outputs 456
   * ```
   *
   * @param {Object} obj1 Object to merge
   * @returns {Object} Result of all merge properties
   */
  function merge(/* obj1, obj2, obj3, ... */) {
    var result = {};
    function assignValue(val, key) {
      if (isPlainObject(result[key]) && isPlainObject(val)) {
        result[key] = merge(result[key], val);
      } else if (isPlainObject(val)) {
        result[key] = merge({}, val);
      } else if (isArray(val)) {
        result[key] = val.slice();
      } else {
        result[key] = val;
      }
    }
  
    for (var i = 0, l = arguments.length; i < l; i++) {
      forEach(arguments[i], assignValue);
    }
    return result;
  }
  
  /**
   * Extends object a by mutably adding to it the properties of object b.
   *
   * @param {Object} a The object to be extended
   * @param {Object} b The object to copy properties from
   * @param {Object} thisArg The object to bind function to
   * @return {Object} The resulting value of object a
   */
  /*
    1. 将b对象中的函数属性拷贝到a对象中，并绑定函数内部的this指向thisArg参数
    2. 将b对象中的非函数属性拷贝到a对象中
    3. 返回a对象
  */
  function extend(a, b, thisArg) {
    forEach(b, function assignValue(val, key) {
      if (thisArg && typeof val === 'function') {
        a[key] = bind(val, thisArg);
      } else {
        a[key] = val;
      }
    });
    return a;
  }
  
  /**
   * Remove byte order marker. This catches EF BB BF (the UTF-8 BOM)
   *
   * @param {string} content with BOM
   * @return {string} content value without BOM
   */
  function stripBOM(content) {
    if (content.charCodeAt(0) === 0xFEFF) {
      content = content.slice(1);
    }
    return content;
  }
  
  module.exports = {
    isArray: isArray,
    isArrayBuffer: isArrayBuffer,
    isBuffer: isBuffer,
    isFormData: isFormData,
    isArrayBufferView: isArrayBufferView,
    isString: isString,
    isNumber: isNumber,
    isObject: isObject,
    isPlainObject: isPlainObject,
    isUndefined: isUndefined,
    isDate: isDate,
    isFile: isFile,
    isBlob: isBlob,
    isFunction: isFunction,
    isStream: isStream,
    isURLSearchParams: isURLSearchParams,
    isStandardBrowserEnv: isStandardBrowserEnv,
    forEach: forEach,
    merge: merge,
    extend: extend,
    trim: trim,
    stripBOM: stripBOM
  };
  