Node.js since v8 provide a promisify function in "util" package see: https://nodejs.org/api/util.html#util_util_promisify_original
IMPORTANT NOTE: I now beleve that the process of promisifiying functions should be left to the developer and not handled by a module.
TypeScript module to program sequentially in node.js
In javaScript/TypeScrip long running functions does not return the result directly but via a callback function.
The problem with that is we end up having a callback in a callback in a callback...
To deal with this problems latest ECMAScript specification define async/await, feature supported by typeScript, but you can only 'await' functions that return a Promise.
This module 'promisify' your async function so they can be called as if they where synchronous ( as if the result was directly returned ).
#Usage:
import * as promisify from "ts-promisify";
/*
Here is a asyncFunction that take a number in input
and return a string after 100ms
*/
let myAsync = function (input: number,
callback: (error: Error, output1: string, output2: string) => void): void {
setTimeout(() => {
if( input === 2 ){
callback( new Error("expected error"), null, null);
}else{
callback(null, "=>" + input.toString() + "<=", "useless");
}
}, 100);
};
/*
Your have to be in an async function to use await
Just wrap the part of your code where you want to program
sequentially in an async closure
*/
(async () => {
for (let i of [1, 2, 3]) {
let [error, output] = await promisify.typed(myAsync)(i);
if (error) {
console.log(error.message);
continue;
}
console.log(output);
}
})();The output will be:
=>1<=
expected error
=>3<=The equivalent in a the traditional asynchronous paradigm would be:
myAsync(1, function (error, output) {
if (error) {
console.log(error.message);
} else {
console.log(output);
}
myAsync(2, function (error, output) {
if (error) {
console.log(error.message);
} else {
console.log(output);
}
myAsync(3, function (error, output) {
if (error) {
console.log(error.message);
} else {
console.log(output);
}
});
});
});You can bound your async function to a specifics object. If your function is overloaded or not typed precisely enough use promisify.generic instead of promisify.typed.
See ./src/test/example.ts for detailed exemples.