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197 lines
5.2 KiB
197 lines
5.2 KiB
# gensync
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This module allows for developers to write common code that can share
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implementation details, hiding whether an underlying request happens
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synchronously or asynchronously. This is in contrast with many current Node
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APIs which explicitly implement the same API twice, once with calls to
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synchronous functions, and once with asynchronous functions.
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Take for example `fs.readFile` and `fs.readFileSync`, if you're writing an API
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that loads a file and then performs a synchronous operation on the data, it
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can be frustrating to maintain two parallel functions.
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## Example
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```js
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const fs = require("fs");
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const gensync = require("gensync");
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const readFile = gensync({
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sync: fs.readFileSync,
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errback: fs.readFile,
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});
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const myOperation = gensync(function* (filename) {
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const code = yield* readFile(filename, "utf8");
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return "// some custom prefix\n" + code;
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});
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// Load and add the prefix synchronously:
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const result = myOperation.sync("./some-file.js");
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// Load and add the prefix asynchronously with promises:
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myOperation.async("./some-file.js").then(result => {
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});
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// Load and add the prefix asynchronously with promises:
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myOperation.errback("./some-file.js", (err, result) => {
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});
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```
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This could even be exposed as your official API by doing
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```js
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// Using the common 'Sync' suffix for sync functions, and 'Async' suffix for
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// promise-returning versions.
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exports.myOperationSync = myOperation.sync;
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exports.myOperationAsync = myOperation.async;
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exports.myOperation = myOperation.errback;
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```
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or potentially expose one of the async versions as the default, with a
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`.sync` property on the function to expose the synchronous version.
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```js
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module.exports = myOperation.errback;
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module.exports.sync = myOperation.sync;
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````
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## API
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### gensync(generatorFnOrOptions)
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Returns a function that can be "await"-ed in another `gensync` generator
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function, or executed via
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* `.sync(...args)` - Returns the computed value, or throws.
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* `.async(...args)` - Returns a promise for the computed value.
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* `.errback(...args, (err, result) => {})` - Calls the callback with the computed value, or error.
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#### Passed a generator
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Wraps the generator to populate the `.sync`/`.async`/`.errback` helpers above to
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allow for evaluation of the generator for the final value.
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##### Example
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```js
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const readFile = function* () {
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return 42;
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};
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const readFileAndMore = gensync(function* (){
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const val = yield* readFile();
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return 42 + val;
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});
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// In general cases
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const code = readFileAndMore.sync("./file.js", "utf8");
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readFileAndMore.async("./file.js", "utf8").then(code => {})
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readFileAndMore.errback("./file.js", "utf8", (err, code) => {});
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// In a generator being called indirectly with .sync/.async/.errback
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const code = yield* readFileAndMore("./file.js", "utf8");
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```
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#### Passed an options object
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* `opts.sync`
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Example: `(...args) => 4`
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A function that will be called when `.sync()` is called on the `gensync()`
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result, or when the result is passed to `yield*` in another generator that
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is being run synchronously.
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Also called for `.async()` calls if no async handlers are provided.
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* `opts.async`
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Example: `async (...args) => 4`
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A function that will be called when `.async()` or `.errback()` is called on
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the `gensync()` result, or when the result is passed to `yield*` in another
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generator that is being run asynchronously.
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* `opts.errback`
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Example: `(...args, cb) => cb(null, 4)`
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A function that will be called when `.async()` or `.errback()` is called on
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the `gensync()` result, or when the result is passed to `yield*` in another
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generator that is being run asynchronously.
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This option allows for simpler compatibility with many existing Node APIs,
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and also avoids introducing the extra even loop turns that promises introduce
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to access the result value.
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* `opts.name`
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Example: `"readFile"`
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A string name to apply to the returned function. If no value is provided,
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the name of `errback`/`async`/`sync` functions will be used, with any
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`Sync` or `Async` suffix stripped off. If the callback is simply named
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with ES6 inference (same name as the options property), the name is ignored.
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* `opts.arity`
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Example: `4`
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A number for the length to set on the returned function. If no value
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is provided, the length will be carried over from the `sync` function's
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`length` value.
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##### Example
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```js
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const readFile = gensync({
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sync: fs.readFileSync,
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errback: fs.readFile,
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});
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const code = readFile.sync("./file.js", "utf8");
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readFile.async("./file.js", "utf8").then(code => {})
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readFile.errback("./file.js", "utf8", (err, code) => {});
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```
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### gensync.all(iterable)
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`Promise.all`-like combinator that works with an iterable of generator objects
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that could be passed to `yield*` within a gensync generator.
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#### Example
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```js
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const loadFiles = gensync(function* () {
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return yield* gensync.all([
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readFile("./one.js"),
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readFile("./two.js"),
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readFile("./three.js"),
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]);
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});
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```
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### gensync.race(iterable)
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`Promise.race`-like combinator that works with an iterable of generator objects
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that could be passed to `yield*` within a gensync generator.
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#### Example
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```js
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const loadFiles = gensync(function* () {
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return yield* gensync.race([
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readFile("./one.js"),
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readFile("./two.js"),
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readFile("./three.js"),
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]);
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});
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```
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