Back in August, I wrote an article in Brazilian-Portuguese explaining how I use async/await to isolate error handling.
Today I’ll translate it to English but with different examples!
I love how Go handle side-effects in a synchronous-like manner. Let’s see this example from the net/http package:
func main() {
res, err := http.Get("http://example.com/")
if err != nil {
// handle `err`
}
// do something with `res`
}
Or perhaps the os package:
func main() {
file, err := os.Open("words.txt")
if err != nil {
// handle `err`
}
// do something with `file`
}
Implementation details aside, I was wondering if there’s a way to write something like this in JavaScript?
Well, as they say, where there’s a will, there’s a way! 😂
Everyday Promise-like functions
Nowadays Promise-like environments are common amongst us.
We can use it to read a file in Node.js:
let util = require("util");
let fs = require("fs");
let read = util.promisify(fs.readFile);
function main() {
read("./test.js", { encoding: "utf8" })
.then(file => {
// do something with `file`
})
.catch(err => {
// handle `err`
});
}
main();
Perhaps fetching some data from an API:
let url = "https://dog.ceo/api/breeds/image/random";
function main() {
fetch(url)
.then(res => res.json())
.then(res => {
// do something with `res`
})
.catch(err => {
// handle `err`
});
}
main();
And being lazy by nature, we create functions to hide some boilerplate for us, so we can write less code across the codebase:
let readFile = require("./readFile");
function main() {
readFile("./test.js")
.then(file => {
// do something with `file`
})
.catch(err => {
// handle `err`
});
}
main();
// readFile.js
let util = require("util");
let fs = require("fs");
let read = util.promisify(fs.readFile);
module.exports = path => {
return read(path, { encoding: "utf8" })
.then(file => {
return file;
})
.catch(err => {
throw err;
});
};
And:
let api = require("./api");
function main() {
api.getRandomDog()
.then(res => {
// do something with `res`
})
.catch(err => {
// handle `err`
});
}
main();
// api.js
let url = "https://dog.ceo/api/breeds/image/random";
let api = {};
api.getRandomDog = () => {
return fetch(url)
.then(res => res.json())
.catch(err => {
throw err;
});
};
module.exports = api;
Still, there’s a lot of repetition here, there’s .then and .catch in both sides of this code snippet.
They say async/await can fix this, so…let’s try it then?
Converting to async/await
Let’s see how our Node.js is doing in async/await:
let readFile = require("./readFile");
async function main() {
try {
let res = await readFile("./test.js");
// do something with `file`
} catch (err) {
// handle `err`
}
}
main();
// readFile.js
let util = require("util");
let fs = require("fs");
let read = util.promisify(fs.readFile);
module.exports = async path => {
try {
let res = await read(path, { encoding: "utf8" });
return res;
} catch (err) {
throw err;
}
};
And how can we fetch our dog with it:
let api = require("./api");
async function main() {
try {
let res = await api.getRandomDog();
// do something with `res`
} catch (err) {
// handle `err`
}
}
main();
// api.js
let url = "https://dog.ceo/api/breeds/image/random";
let api = {};
api.getRandomDog = async () => {
try {
let res = await fetch(url);
let json = await res.json();
return json;
} catch (err) {
throw err;
}
};
module.exports = api;
Phew… I think we changed a problem by another. Now there’s try...catchin both places. Thinking about our current interface between consumer/service, we’ve:
- In our
main()function we’re calling a “service” (readFile and api.) - Our “service” function returns a
Promise - When fulfilled, our service returns a payload
- When rejected, our service throw an error
Hmm… perhaps this is the problem! Our interface between consumer/service are different for fulfilled and rejected scenarios.
Refreshing our memory about our Go example at the top:
func main() {
res, err := http.Get("http://example.com/")
if err != nil {
// handle `err`
}
// do something with `res`
}
Seems we’ve the same interface for both, fulfilled and rejected scenario!
Let’s try that with our last async/await example!
Unified return interface with async/await
In our Node.js example:
let readFile = require("./readFile");
async function main() {
let [err, file] = await readFile("./test.js");
if (err) {
// handle `err`
}
// do something with `file`
}
main();
// readFile.js
let util = require("util");
let fs = require("fs");
let read = util.promisify(fs.readFile);
module.exports = async path => {
try {
let res = await read(path, { encoding: "utf8" });
return [null, res];
} catch (err) {
return [err, null]
}
};
And our Fetch API:
let api = require("./api");
async function main() {
let [err, res] = await api.getRandomDog();
if (err) {
// handle `err`
}
// do something with `res`
}
main();
// api.js
let url = "https://dog.ceo/api/breeds/image/random";
let api = {};
api.getRandomDog = async () => {
try {
let res = await fetch(url);
let json = await res.json();
return [null, json];
} catch (err) {
return [err, null]
}
};
module.exports = api;
Well done!! 🎉🎉🎉
That’s exactly what we were looking for! Our main() function looks like our Go example and now we’ve isolated all try...catch in our “service” functions.
Using this approach you can clean up your Node.js Middlewares/Controllers and in your Front-end, let’s say with React/Redux, clean up redux-thunks or redux-saga functions/generators.
You can also unit test these “service” functions in isolation and guarantee they are returning the expected interface/data.
Source: dev