什么是 API？它在 Web 开发中有何用处？

API代表Application Programming Interface（所有交互的主要参与者）
它就像一个信使，将我们的请求发送到系统并通过无缝连接将响应返回给我们。
我们在许多情况下使用 API，例如为 Web 应用程序获取数据，或连接到具有不断变化的天气等数据的远程服务器，或使两个应用程序能够相互交换数据。
API 不仅提供代码的可重用性，而且还使用了抽象的概念（通过隐藏复杂性来显示功能）。
API 概念在现实生活中最常见的用途包括：

餐厅的服务员将您的订单请求交给厨师并带回所要求的菜肴
配电盘只需按一下即可关闭管灯
从 MMT（基于网络）等网站在线预订航班
从 Facebook 帐户注册购物网站（基于网络）

不同的 API 将以不同的方式进行通信：

XML-RCP/SOAP：两者都使用 XML
JavaScript：专注于 Javascript
RESTful API：使用 HTTP 协议（超文本传输协议）（最适合 Web API）

API 在 Web 开发中的主要功能
API 可用于混搭，即来自一个站点的信息可以与另一个站点的信息混合。身份验证是需要注意的重要事项之一，因为所有 API 都不公开。为安全使用进行身份验证时需要 API 密钥。例如，请参考 Gmail API 认证
有些 API 不需要任何访问令牌。

示例： Github API
https://api.github.com

输出：

{
  "current_user_url": "https://api.github.com/user",
  "current_user_authorizations_html_url": "https://github.com/settings/connections/applications{/client_id}",
  "authorizations_url": "https://api.github.com/authorizations",
  "code_search_url": "https://api.github.com/search/code?q={query}{&page, per_page, sort, order}",
  "commit_search_url": "https://api.github.com/search/commits?q={query}{&page, per_page, sort, order}",
  "emails_url": "https://api.github.com/user/emails",
  "emojis_url": "https://api.github.com/emojis",
  "events_url": "https://api.github.com/events",
  "feeds_url": "https://api.github.com/feeds",
  "followers_url": "https://api.github.com/user/followers",
  "following_url": "https://api.github.com/user/following{/target}",
  "gists_url": "https://api.github.com/gists{/gist_id}",
  "hub_url": "https://api.github.com/hub",
  "issue_search_url": "https://api.github.com/search/issues?q={query}{&page, per_page, sort, order}",
  "issues_url": "https://api.github.com/issues",
  "keys_url": "https://api.github.com/user/keys",
  "notifications_url": "https://api.github.com/notifications",
  "organization_repositories_url": "https://api.github.com/orgs/{org}/repos{?type, page, per_page, sort}",
  "organization_url": "https://api.github.com/orgs/{org}",
  "public_gists_url": "https://api.github.com/gists/public",
  "rate_limit_url": "https://api.github.com/rate_limit",
  "repository_url": "https://api.github.com/repos/{owner}/{repo}",
  "repository_search_url": "https://api.github.com/search/repositories?q={query}{&page, per_page, sort, order}",
  "current_user_repositories_url": "https://api.github.com/user/repos{?type, page, per_page, sort}",
  "starred_url": "https://api.github.com/user/starred{/owner}{/repo}",
  "starred_gists_url": "https://api.github.com/gists/starred",
  "team_url": "https://api.github.com/teams",
  "user_url": "https://api.github.com/users/{user}",
  "user_organizations_url": "https://api.github.com/user/orgs",
  "user_repositories_url": "https://api.github.com/users/{user}/repos{?type, page, per_page, sort}",
  "user_search_url": "https://api.github.com/search/users?q={query}{&page, per_page, sort, order}"
}

JSON 格式的列表

https://api.github.com/feeds
输出：

{
  "timeline_url": "https://github.com/timeline",
  "user_url": "https://github.com/{user}",
  "security_advisories_url": "https://github.com/security-advisories",
  "_links": {
    "timeline": {
      "href": "https://github.com/timeline",
      "type": "application/atom+xml"
    },
    "user": {
      "href": "https://github.com/{user}",
      "type": "application/atom+xml"
    },
    "security_advisories": {
      "href": "https://github.com/security-advisories",
      "type": "application/atom+xml"
    }
  }
}

https://api.github.com/rate_limit

{
  "resources": {
    "core": {
      "limit": 60,
      "remaining": 56,
      "reset": 1567928119
    },
    "search": {
      "limit": 10,
      "remaining": 10,
      "reset": 1567924666
    },
    "graphql": {
      "limit": 0,
      "remaining": 0,
      "reset": 1567928206
    },
    "integration_manifest": {
      "limit": 5000,
      "remaining": 5000,
      "reset": 1567928206
    }
  },
  "rate": {
    "limit": 60,
    "remaining": 56,
    "reset": 1567928119
  }
}