koa2-cors实现过程以及应用

1、下面以koa2为例

var koa = require('koa');
var app = new koa();
var router = require('koa-router')();
// CORS是一个W3C标准，全称是"跨域资源共享"（Cross-origin resource sharing）。
// 下面以koa2-cors为例，
const cors = require('koa2-cors');
 
// 具体参数我们在后面进行解释
app.use(cors({
    origin: function (ctx) {
        if (ctx.url === '/test') {
            return "*"; // 允许来自所有域名请求
        }
        return 'http://localhost:8080'; / 这样就能只允许 http://localhost:8080 这个域名的请求了
    },
    exposeHeaders: ['WWW-Authenticate', 'Server-Authorization'],
    maxAge: 5,
    credentials: true,
    allowMethods: ['GET', 'POST', 'DELETE'],
    allowHeaders: ['Content-Type', 'Authorization', 'Accept'],
}))
 
router.post('/', async function (ctx) {
    ctx.body = '恭喜 __小简__ 你成功登陆了'
});
 
app
    .use(router.routes())
    .use(router.allowedMethods());
 
app.listen(3000);

2、下面我们在http://localhost:8080发送请求进行测试

this.$axios.post('http://172.16.186.50:3000/', {})
                    .then((response) => {
                        console.log(response)
                    })
                    .catch(function (error) {
                        console.log(error);
                    });

3、下面我们以具体的请求头信息解释上面的代码，也就是cors具体的实现过程

// 我们可以用下面的中间件理解app.use(cors({}))

app.use(async (ctx, next) => {
    // 允许来自所有域名请求
    ctx.set("Access-Control-Allow-Origin", "*");
    // 这样就能只允许 http://localhost:8080 这个域名的请求了
    // ctx.set("Access-Control-Allow-Origin", "http://localhost:8080"); 
 
    // 设置所允许的HTTP请求方法
    ctx.set("Access-Control-Allow-Methods", "OPTIONS, GET, PUT, POST, DELETE");
 
    // 字段是必需的。它也是一个逗号分隔的字符串，表明服务器支持的所有头信息字段.
    ctx.set("Access-Control-Allow-Headers", "x-requested-with, accept, origin, content-type");
 
    // 服务器收到请求以后，检查了Origin、Access-Control-Request-Method和Access-Control-Request-Headers字段以后，确认允许跨源请求，就可以做出回应。
 
    // Content-Type表示具体请求中的媒体类型信息
    ctx.set("Content-Type", "application/json;charset=utf-8");
 
    // 该字段可选。它的值是一个布尔值，表示是否允许发送Cookie。默认情况下，Cookie不包括在CORS请求之中。
    // 当设置成允许请求携带cookie时，需要保证"Access-Control-Allow-Origin"是服务器有的域名，而不能是"*";
    ctx.set("Access-Control-Allow-Credentials", true);
 
    // 该字段可选，用来指定本次预检请求的有效期，单位为秒。
    // 当请求方法是PUT或DELETE等特殊方法或者Content-Type字段的类型是application/json时，服务器会提前发送一次请求进行验证
    // 下面的的设置只本次验证的有效时间，即在该时间段内服务端可以不用进行验证
    ctx.set("Access-Control-Max-Age", 300);
 
    /*
    CORS请求时，XMLHttpRequest对象的getResponseHeader()方法只能拿到6个基本字段：
        Cache-Control、
        Content-Language、
        Content-Type、
        Expires、
        Last-Modified、
        Pragma。
    */
    // 需要获取其他字段时，使用Access-Control-Expose-Headers，
    // getResponseHeader('myData')可以返回我们所需的值
    ctx.set("Access-Control-Expose-Headers", "myData");
 
    await next();
})

源码：

/**
 * CORS middleware for koa2
 *
 * @param {Object} [options]
 *  - {String|Function(ctx)} origin `Access-Control-Allow-Origin`, default is request Origin header
 *  - {Array} exposeHeaders `Access-Control-Expose-Headers`
 *  - {String|Number} maxAge `Access-Control-Max-Age` in seconds
 *  - {Boolean} credentials `Access-Control-Allow-Credentials`
 *  - {Array} allowMethods `Access-Control-Allow-Methods`,
 *    default is ['GET', 'PUT', 'POST', 'PATCH', 'DELETE', 'HEAD', 'OPTIONS']
 *  - {Array} allowHeaders `Access-Control-Allow-Headers`
 * @return {Function}
 * @api public
 */
module.exports = function crossOrigin(options = {}) {
  const defaultOptions = {
    allowMethods: ['GET', 'PUT', 'POST', 'PATCH', 'DELETE', 'HEAD', 'OPTIONS'],
  };
 
  // set defaultOptions to options
  options = Object.assign({}, defaultOptions, options); // eslint-disable-line no-param-reassign
 
  // eslint-disable-next-line consistent-return
  return async function cors(ctx, next) {
    // always set vary Origin Header
    // https://github.com/rs/cors/issues/10
    ctx.vary('Origin');
 
    let origin;
    if (typeof options.origin === 'function') {
      origin = options.origin(ctx);
    } else {
      origin = options.origin || ctx.get('Origin') || '*';
    }
    if (!origin) {
      return await next();
    }
 
    // Access-Control-Allow-Origin
    ctx.set('Access-Control-Allow-Origin', origin);
 
    if (ctx.method === 'OPTIONS') {
      // Preflight Request
      if (!ctx.get('Access-Control-Request-Method')) {
        return await next();
      }
 
      // Access-Control-Max-Age
      if (options.maxAge) {
        ctx.set('Access-Control-Max-Age', String(options.maxAge));
      }
 
      // Access-Control-Allow-Credentials
      if (options.credentials === true) {
        // When used as part of a response to a preflight request,
        // this indicates whether or not the actual request can be made using credentials.
        ctx.set('Access-Control-Allow-Credentials', 'true');
      }
 
      // Access-Control-Allow-Methods
      if (options.allowMethods) {
        ctx.set('Access-Control-Allow-Methods', options.allowMethods.join(','));
      }
 
      // Access-Control-Allow-Headers
      if (options.allowHeaders) {
        ctx.set('Access-Control-Allow-Headers', options.allowHeaders.join(','));
      } else {
        ctx.set('Access-Control-Allow-Headers', ctx.get('Access-Control-Request-Headers'));
      }
 
      ctx.status = 204; // No Content
    } else {
      // Request
      // Access-Control-Allow-Credentials
      if (options.credentials === true) {
        if (origin === '*') {
          // `credentials` can't be true when the `origin` is set to `*`
          ctx.remove('Access-Control-Allow-Credentials');
        } else {
          ctx.set('Access-Control-Allow-Credentials', 'true');
        }
      }
 
      // Access-Control-Expose-Headers
      if (options.exposeHeaders) {
        ctx.set('Access-Control-Expose-Headers', options.exposeHeaders.join(','));
      }
 
      try {
        await next();
      } catch (err) {
        throw err;
      }
    }
  };
};