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# koa-convert [](https://npmjs.org/package/koa-convert) [](https://travis-ci.org/gyson/koa-convert) Convert koa legacy ( 0.x & 1.x ) generator middleware to modern promise middleware ( 2.x ). It could also convert modern promise middleware back to legacy generator middleware ( useful to help modern middleware support koa 0.x or 1.x ). ## Note It should be able to convert any legacy generator middleware to modern promise middleware ( or convert it back ). Please let me know ( send a issue ) if you fail to do so. ## Installation ``` $ npm install koa-convert ``` ## Usage ```js const Koa = require('koa') // koa v2.x const convert = require('koa-convert') const app = new Koa() app.use(modernMiddleware) app.use(convert(legacyMiddleware)) app.use(convert.compose(legacyMiddleware, modernMiddleware)) function * legacyMiddleware (next) { // before yield next // after } function modernMiddleware (ctx, next) { // before return next().then(() => { // after }) } ``` ## Distinguish legacy and modern middleware In koa 0.x and 1.x ( without experimental flag ), `app.use` has an assertion that all ( legacy ) middleware must be generator function and it's tested with `fn.constructor.name == 'GeneratorFunction'` at [here](https://github.com/koajs/koa/blob/7fe29d92f1e826d9ce36029e1b9263b94cba8a7c/lib/application.js#L105). Therefore, we can distinguish legacy and modern middleware with `fn.constructor.name == 'GeneratorFunction'`. ## Migration `app.use(legacyMiddleware)` is everywhere in 0.x and 1.x and it would be painful to manually change all of them to `app.use(convert(legacyMiddleware))`. You can use following snippet to make migration easier. ```js const _use = app.use app.use = x => _use.call(app, convert(x)) ``` The above snippet will override `app.use` method and implicitly convert all legacy generator middleware to modern promise middleware. Therefore, you can have both `app.use(modernMiddleware)` and `app.use(legacyMiddleware)` and your 0.x or 1.x should work without modification. Complete example: ```js const Koa = require('koa') // v2.x const convert = require('koa-convert') const app = new Koa() // ---------- override app.use method ---------- const _use = app.use app.use = x => _use.call(app, convert(x)) // ---------- end ---------- app.use(modernMiddleware) // this will be converted to modern promise middleware implicitly app.use(legacyMiddleware) function * legacyMiddleware (next) { // before yield next // after } function modernMiddleware (ctx, next) { // before return next().then(() => { // after }) } ``` ## API #### `convert()` Convert legacy generator middleware to modern promise middleware. ```js modernMiddleware = convert(legacyMiddleware) ``` #### `convert.compose()` Convert and compose multiple middleware (could mix legacy and modern ones) and return modern promise middleware. ```js composedModernMiddleware = convert.compose(legacyMiddleware, modernMiddleware) // or composedModernMiddleware = convert.compose([legacyMiddleware, modernMiddleware]) ``` #### `convert.back()` Convert modern promise middleware back to legacy generator middleware. This is useful to help modern promise middleware support koa 0.x or 1.x. ```js legacyMiddleware = convert.back(modernMiddleware) ``` ## License MIT