# 一、环境准备

axios 版本 v0.24.0
通过 github1s 网页可以查看 (opens new window) axios 源码
调试需要 clone 到本地

git clone https://github.com/axios/axios.git

cd axios

npm start

http://localhost:3000/

# 二、函数研读

# 1. 辅助函数 helper 总览

# 浏览器适配器 xhr

var utils = require("./../utils");
var settle = require("./../core/settle");
var cookies = require("./../helpers/cookies");
var buildURL = require("./../helpers/buildURL");
var buildFullPath = require("../core/buildFullPath");
var parseHeaders = require("./../helpers/parseHeaders");
var isURLSameOrigin = require("./../helpers/isURLSameOrigin");
var createError = require("../core/createError");
var defaults = require("../defaults");
var Cancel = require("../cancel/Cancel");

# node 适配器 http

var utils = require("./../utils");
var settle = require("./../core/settle");
var buildFullPath = require("../core/buildFullPath");
var buildURL = require("./../helpers/buildURL");
var http = require("http");
var https = require("https");
var httpFollow = require("follow-redirects").http;
var httpsFollow = require("follow-redirects").https;
var url = require("url");
var zlib = require("zlib");
var VERSION = require("./../env/data").version;
var createError = require("../core/createError");
var enhanceError = require("../core/enhanceError");
var defaults = require("../defaults");
var Cancel = require("../cancel/Cancel");

我们找到其中使用到的 core 函数 settle、buildFullPath、createError、enhanceError

# 2. 辅助函数 settle

Resolve or reject a Promise based on response status

依据响应状态来 resolve 或 reject 一个 Promise

'use strict';

var createError = require('./createError');

/**
 * @param {Function} resolve A function that resolves the promise.
 * @param {Function} reject A function that rejects the promise.
 * @param {object} response The response.
 */
module.exports = function settle(resolve, reject, response) {
  var validateStatus = response.config.validateStatus;
  if (!response.status || !validateStatus || validateStatus(response.status)) {
    resolve(response);
  } else {
    reject(createError(
      'Request failed with status code ' + response.status,
      response.config,
      null,
      response.request,
      response
    ));
  }
};

根据 http 响应状态，改变 Promise 的状态
在 reject 时使通过调用自定义的 createError 方法生成 reject 返回内容

# 3. 辅助函数 buildFullPath

Creates a new URL by combining the baseURL with the requestedURL, only when the requestedURL is not already an absolute URL

仅当请求的 URL 不是绝对 URL 时，才通过将 baseURL 与请求的 URL 组合来创建新 URL

If the requestURL is absolute, this function returns the requestedURL untouched

如果请求的 URL 是绝对 URL，此函数会原封不动地返回所请求的 URL

'use strict';

var isAbsoluteURL = require('../helpers/isAbsoluteURL');
var combineURLs = require('../helpers/combineURLs');

/**
 * @param {string} baseURL The base URL
 * @param {string} requestedURL Absolute or relative URL to combine
 * @returns {string} The combined full path
 */
module.exports = function buildFullPath(baseURL, requestedURL) {
  if (baseURL && !isAbsoluteURL(requestedURL)) {
    return combineURLs(baseURL, requestedURL);
  }
  return requestedURL;
};

其中 isAbsoluteURL 和 combineURLs 是两个正则表达式，分别会监测当前 requestedURL 是否为 AbsoluteURL 以及连接 baseURL 与 requestedURL，最终目的让返回的 url 必须为一个绝对地址

Tips：AbsoluteURL 的判断规则是是否以 scheme:// 开头或者是一个 协议相对URL(protocol-relative URL) (opens new window) 即 //开头

# 4. 辅助函数 createError

Create an Error with the specified message, config, error code, request and response

使用指定的 message、config、code、request 和 resopnse 来创建 Error

'use strict';

var enhanceError = require('./enhanceError');

/**
 * @param {string} message The error message.
 * @param {Object} config The config.
 * @param {string} [code] The error code (for example, 'ECONNABORTED').
 * @param {Object} [request] The request.
 * @param {Object} [response] The response.
 * @returns {Error} The created error.
 */
module.exports = function createError(message, config, code, request, response) {
  var error = new Error(message);
  return enhanceError(error, config, code, request, response);
};

createError中使用 new Error 创建了一个错误对象
Error 对象刚生成的时候只有一个 message 属性，需要使用 enhanceError 来增强返回的 error 信息

# 5. 辅助函数 enhanceError

Update an Error with the specified config, error code, and response

使用指定的配置、错误代码和响应来升级 Error

'use strict';

/**
 * @param {Error} error The error to update.
 * @param {Object} config The config.
 * @param {string} [code] The error code (for example, 'ECONNABORTED').
 * @param {Object} [request] The request.
 * @param {Object} [response] The response.
 * @returns {Error} The error.
 */
module.exports = function enhanceError(error, config, code, request, response) {
  error.config = config;
  if (code) {
    error.code = code;
  }

  error.request = request;
  error.response = response;
  error.isAxiosError = true;

  error.toJSON = function toJSON() {
    return {
      // Standard
      message: this.message,
      name: this.name,
      // Microsoft
      description: this.description,
      number: this.number,
      // Mozilla
      fileName: this.fileName,
      lineNumber: this.lineNumber,
      columnNumber: this.columnNumber,
      stack: this.stack,
      // Axios
      config: this.config,
      code: this.code,
      status: this.response && this.response.status ? this.response.status : null
    };
  };
  return error;
};

入参 error 是一个使用 new Error 创建的错误对象
enhanceError 方法就是单纯为了升级 Error 对象，给 Axios 生成的错误对象添加 config、code、request 和 response 属性
另外根据不同的浏览器环境返回了一些对应的补充信息，reject 返回的内容将是一个包含了这些补充信息的 error 类型信息

# 三、参考

1. 林景宜的文章林景宜的记事本 - Axios 源码解析（五）：核心工具方法 (opens new window)

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