Functional and local options

README.md

@@ -40,17 +40,19 @@ function (mimeType: string): Boolean {
 An optional function that checks the response content type to decide whether to compress.
 By default, it uses [compressible](https://github.com/jshttp/compressible).
 
-### options.threshold\<String|Number\>
+### options.threshold\<String|Number|Function\>
 
-Minimum response size in bytes to compress.
+Minimum response size in bytes to compress or a function that returns such response (see below).
 Default `1024` bytes or `1kb`.
 
-### options[encoding]\<Object\>
+### options[encoding]\<Object|Function\>
 
 The current encodings are, in order of preference: `br`, `gzip`, `deflate`.
 Setting `options[encoding] = {}` will pass those options to the encoding function.
 Setting `options[encoding] = false` will disable that encoding.
 
+It can be a function that returns options (see below).
+
 #### options<span></span>.br
 
 [Brotli compression](https://en.wikipedia.org/wiki/Brotli) is supported in node v11.7.0+, which includes it natively.
@@ -77,3 +79,33 @@ app.use((ctx, next) => {
   ctx.body = fs.createReadStream(file)
 })
 ```
+
+`ctx.compress` can be an object similar to `options` above, whose properties (`threshold` and encoding options)
+override the global `options` for this response and bypass the filter check.
+
+## Functional properties
+
+Certain properties (`threshold` and encoding options) can be specified as functions. Such functions will be called
+for every response with three arguments:
+
+* `type` &mdash; the same as `ctx.response.type` (provided for convenience)
+* `size` &mdash; the same as `ctx.response.length` (provided for convenience)
+* `ctx` &mdash; the whole context object, if you want to do something unique
+
+It should return a valid value for that property. It is possible to return a function of the same shape,
+which will be used to calculate the actual property.
+
+Example:
+
+```js
+app.use((ctx, next) => {
+  // ...
+  ctx.compress = (type, size, ctx) => ({
+    br:   size && size >= 65536,
+    gzip: size && size <  65536
+  })
+  ctx.body = payload
+})
+```
+
+Read all about `ctx` in https://koajs.com/#context and `ctx.response` in https://koajs.com/#response

__tests__/index.js

@@ -498,4 +498,124 @@ describe('Compress', () => {
         done()
       })
   })
+
+  it('functional threshold: should not compress', (done) => {
+    const app = new Koa()
+
+    app.use(compress({
+      threshold: () => '1mb'
+    }))
+    app.use(sendString)
+    server = app.listen()
+
+    request(server)
+      .get('/')
+      .expect(200)
+      .end((err, res) => {
+        if (err) { return done(err) }
+
+        assert.equal(res.headers['content-length'], '2048')
+        assert.equal(res.headers.vary, 'Accept-Encoding')
+        assert(!res.headers['content-encoding'])
+        assert(!res.headers['transfer-encoding'])
+        assert.equal(res.text, string)
+
+        done()
+      })
+  })
+
+  it('functional compressors: should not compress', (done) => {
+    const app = new Koa()
+
+    app.use(compress({ br: false, gzip: (type, size) => /^text\//i.test(type) && size > 1000000 }))
+    app.use(sendBuffer)
+    server = app.listen()
+
+    request(server)
+      .get('/')
+      .set('Accept-Encoding', 'br, gzip')
+      .expect(200)
+      .end((err, res) => {
+        if (err) { return done(err) }
+
+        assert.equal(res.headers.vary, 'Accept-Encoding')
+        assert(!res.headers['content-encoding'])
+
+        done()
+      })
+  })
+
+  it('functional compressors: should compress with gzip', (done) => {
+    const app = new Koa()
+
+    app.use(compress({ br: false, gzip: false }))
+    app.use((ctx) => {
+      ctx.compress = { gzip: (type, size) => size < 1000000 }
+      ctx.body = string
+    })
+    server = app.listen()
+
+    request(server)
+      .get('/')
+      .set('Accept-Encoding', 'br, gzip')
+      .expect(200)
+      .end((err, res) => {
+        if (err) { return done(err) }
+
+        assert.equal(res.headers.vary, 'Accept-Encoding')
+        assert.equal(res.headers['content-encoding'], 'gzip')
+
+        done()
+      })
+  })
+
+  it('functional compressors: should NOT compress with gzip', (done) => {
+    const app = new Koa()
+
+    app.use(compress({ br: false, gzip: false }))
+    app.use((ctx) => {
+      ctx.compress = { gzip: (type, size) => size < 100 }
+      ctx.body = string
+    })
+    server = app.listen()
+
+    request(server)
+      .get('/')
+      .set('Accept-Encoding', 'br, gzip')
+      .expect(200)
+      .end((err, res) => {
+        if (err) { return done(err) }
+
+        assert.equal(res.headers.vary, 'Accept-Encoding')
+        assert(!res.headers['content-encoding'])
+
+        done()
+      })
+  })
+
+  it('functional compressors: should compress with br', (done) => {
+    if (!process.versions.brotli) return done()
+
+    const app = new Koa()
+
+    app.use(compress({ br: false, gzip: false }))
+    app.use((ctx) => {
+      ctx.compress = { gzip: () => false, br: () => true }
+      ctx.body = string
+    })
+    server = app.listen()
+
+    request(server)
+      .get('/')
+      .set('Accept-Encoding', 'br, gzip')
+      .expect(200)
+      .end((err, res) => {
+        if (err) { return done(err) }
+
+        assert.equal(res.headers.vary, 'Accept-Encoding')
+        assert.equal(res.headers['content-encoding'], 'br')
+
+        done()
+      })
+  })
 })

lib/index.js

@@ -26,11 +26,8 @@ const NO_TRANSFORM_REGEX = /(?:^|,)\s*?no-transform\s*?(?:,|$)/
  */
 
 module.exports = (options = {}) => {
-  let { filter = compressible, threshold = 1024, defaultEncoding = 'identity' } = options
-  if (typeof threshold === 'string') threshold = bytes(threshold)
-
-  // `options.br = false` would remove it as a preferred encoding
-  const preferredEncodings = Encodings.preferredEncodings.filter((encoding) => options[encoding] !== false && options[encoding] !== null)
+  // "global" options
+  const { filter = compressible, defaultEncoding = 'identity' } = options
 
   return async (ctx, next) => {
     ctx.vary('Accept-Encoding')
@@ -39,6 +36,7 @@ module.exports = (options = {}) => {
 
     // early exit if there's no content body or the body is already encoded
     let { body } = ctx
+    const { type, length: size } = ctx.response
     if (!body) return
     if (ctx.res.headersSent || !ctx.writable) return
     if (ctx.compress === false) return
@@ -47,20 +45,32 @@ module.exports = (options = {}) => {
     if (ctx.response.get('Content-Encoding')) return
 
     // forced compression or implied
-    if (!(ctx.compress === true || filter(ctx.response.type))) return
+    if (!(ctx.compress || filter(type))) return
 
     // don't compress for Cache-Control: no-transform
     // https://tools.ietf.org/html/rfc7234#section-5.2.1.6
     const cacheControl = ctx.response.get('Cache-Control')
     if (cacheControl && NO_TRANSFORM_REGEX.test(cacheControl)) return
 
+    // calculate "local" compression options
+    const responseOptions = { ...options, ...ctx.compress }
+    let { threshold = 1024 } = responseOptions
+    while (typeof threshold === 'function') threshold = threshold(type, size, ctx)
+    if (typeof threshold === 'string') threshold = bytes(threshold)
+
     // don't compress if the current response is below the threshold
-    if (threshold && ctx.response.length < threshold) return
+    if (threshold && size < threshold) return
 
     // get the preferred content encoding
-    const encodings = new Encodings({
-      preferredEncodings
+    Encodings.preferredEncodings.forEach((encoding) => {
+      // calculate compressor options, if any
+      if (!(encoding in responseOptions)) return
+      let compressor = responseOptions[encoding]
+      while (typeof compressor === 'function') compressor = compressor(type, size, ctx)
+      responseOptions[encoding] = compressor
     })
+    const preferredEncodings = Encodings.preferredEncodings.filter((encoding) => responseOptions[encoding] !== false && responseOptions[encoding] !== null)
+    const encodings = new Encodings({ preferredEncodings })
     encodings.parseAcceptEncoding(ctx.request.headers['accept-encoding'] || defaultEncoding)
     const encoding = encodings.getPreferredContentEncoding()
 
@@ -76,7 +86,7 @@ module.exports = (options = {}) => {
     ctx.res.removeHeader('Content-Length')
 
     const compress = Encodings.encodingMethods[encoding]
-    const stream = ctx.body = compress(options[encoding])
+    const stream = ctx.body = compress(responseOptions[encoding])
 
     if (body instanceof Stream) {
       body.pipe(stream)