Dbqb_Study/吴光慧学习笔记/11.1/study/node_modules/fresh/README.md

# fresh

[![NPM Version][npm-image]][npm-url]
[![NPM Downloads][downloads-image]][downloads-url]
[![Node.js Version][node-version-image]][node-version-url]
[![Build Status][travis-image]][travis-url]
[![Test Coverage][coveralls-image]][coveralls-url]

HTTP response freshness testing

## Installation

This is a [Node.js](https://nodejs.org/en/) module available through the
[npm registry](https://www.npmjs.com/). Installation is done using the
[`npm install` command](https://docs.npmjs.com/getting-started/installing-npm-packages-locally):

```
$ npm install fresh
```

## API

<!-- eslint-disable no-unused-vars -->

```js
var fresh = require('fresh')
```

### fresh(reqHeaders, resHeaders)

Check freshness of the response using request and response headers.

When the response is still "fresh" in the client's cache `true` is
returned, otherwise `false` is returned to indicate that the client
cache is now stale and the full response should be sent.

When a client sends the `Cache-Control: no-cache` request header to
indicate an end-to-end reload request, this module will return `false`
to make handling these requests transparent.

## Known Issues

This module is designed to only follow the HTTP specifications, not
to work-around all kinda of client bugs (especially since this module
typically does not recieve enough information to understand what the
client actually is).

There is a known issue that in certain versions of Safari, Safari
will incorrectly make a request that allows this module to validate
freshness of the resource even when Safari does not have a
representation of the resource in the cache. The module
[jumanji](https://www.npmjs.com/package/jumanji) can be used in
an Express application to work-around this issue and also provides
links to further reading on this Safari bug.

## Example

### API usage

<!-- eslint-disable no-redeclare, no-undef -->

```js
var reqHeaders = { 'if-none-match': '"foo"' }
var resHeaders = { 'etag': '"bar"' }
fresh(reqHeaders, resHeaders)
// => false

var reqHeaders = { 'if-none-match': '"foo"' }
var resHeaders = { 'etag': '"foo"' }
fresh(reqHeaders, resHeaders)
// => true
```

### Using with Node.js http server

```js
var fresh = require('fresh')
var http = require('http')

var server = http.createServer(function (req, res) {
  // perform server logic
  // ... including adding ETag / Last-Modified response headers

  if (isFresh(req, res)) {
    // client has a fresh copy of resource
    res.statusCode = 304
    res.end()
    return
  }

  // send the resource
  res.statusCode = 200
  res.end('hello, world!')
})

function isFresh (req, res) {
  return fresh(req.headers, {
    'etag': res.getHeader('ETag'),
    'last-modified': res.getHeader('Last-Modified')
  })
}

server.listen(3000)
```

## License

[MIT](LICENSE)

[npm-image]: https://img.shields.io/npm/v/fresh.svg
[npm-url]: https://npmjs.org/package/fresh
[node-version-image]: https://img.shields.io/node/v/fresh.svg
[node-version-url]: https://nodejs.org/en/
[travis-image]: https://img.shields.io/travis/jshttp/fresh/master.svg
[travis-url]: https://travis-ci.org/jshttp/fresh
[coveralls-image]: https://img.shields.io/coveralls/jshttp/fresh/master.svg
[coveralls-url]: https://coveralls.io/r/jshttp/fresh?branch=master
[downloads-image]: https://img.shields.io/npm/dm/fresh.svg
[downloads-url]: https://npmjs.org/package/fresh