promisify
Takes a function following the common error-first callback style, i.e. taking an (err, value) => ... callback as the last argument, and returns a version that returns promises.
import { promisify } from 'node:util';
import { stat } from 'node:fs';
const promisifiedStat = promisify(stat);
promisifiedStat('.').then((stats) => {
// Do something with `stats`
}).catch((error) => {
// Handle the error.
});Content copied to clipboard
Or, equivalently using async functions:
import { promisify } from 'node:util';
import { stat } from 'node:fs';
const promisifiedStat = promisify(stat);
async function callStat() {
const stats = await promisifiedStat('.');
console.log(`This directory is owned by ${stats.uid}`);
}
callStat();Content copied to clipboard
If there is an original[util.promisify.custom] property present, promisify will return its value, see Custom promisified functions.
promisify() assumes that original is a function taking a callback as its final argument in all cases. If original is not a function, promisify() will throw an error. If original is a function but its last argument is not an error-first callback, it will still be passed an error-first callback as its last argument.
Using promisify() on class methods or other methods that use this may not work as expected unless handled specially:
import { promisify } from 'node:util';
class Foo {
constructor() {
this.a = 42;
}
bar(callback) {
callback(null, this.a);
}
}
const foo = new Foo();
const naiveBar = promisify(foo.bar);
// TypeError: Cannot read properties of undefined (reading 'a')
// naiveBar().then(a => console.log(a));
naiveBar.call(foo).then((a) => console.log(a)); // '42'
const bindBar = naiveBar.bind(foo);
bindBar().then((a) => console.log(a)); // '42'Content copied to clipboard
Since
v8.0.0
external fun <TResult> promisify(fn: (callback: (err: Any?, result: TResult) -> Unit) -> Unit): () -> ERROR CLASS: Symbol not found for Promise<TResult>(source)
external fun promisify(fn: (callback: (err: Any?) -> Unit) -> Unit): () -> ERROR CLASS: Symbol not found for Promise<ERROR CLASS: Symbol not found for js.core.Void>(source)
external fun <T1, TResult> promisify(fn: (arg1: T1, callback: (err: Any?, result: TResult) -> Unit) -> Unit): (arg1: T1) -> ERROR CLASS: Symbol not found for Promise<TResult>(source)
external fun <T1> promisify(fn: (arg1: T1, callback: (err: Any?) -> Unit) -> Unit): (arg1: T1) -> ERROR CLASS: Symbol not found for Promise<ERROR CLASS: Symbol not found for js.core.Void>(source)
external fun <T1, T2, TResult> promisify(fn: (arg1: T1, arg2: T2, callback: (err: Any?, result: TResult) -> Unit) -> Unit): (arg1: T1, arg2: T2) -> ERROR CLASS: Symbol not found for Promise<TResult>(source)
external fun <T1, T2> promisify(fn: (arg1: T1, arg2: T2, callback: (err: Any?) -> Unit) -> Unit): (arg1: T1, arg2: T2) -> ERROR CLASS: Symbol not found for Promise<ERROR CLASS: Symbol not found for js.core.Void>(source)
external fun <T1, T2, T3, TResult> promisify(fn: (arg1: T1, arg2: T2, arg3: T3, callback: (err: Any?, result: TResult) -> Unit) -> Unit): (arg1: T1, arg2: T2, arg3: T3) -> ERROR CLASS: Symbol not found for Promise<TResult>(source)
external fun <T1, T2, T3> promisify(fn: (arg1: T1, arg2: T2, arg3: T3, callback: (err: Any?) -> Unit) -> Unit): (arg1: T1, arg2: T2, arg3: T3) -> ERROR CLASS: Symbol not found for Promise<ERROR CLASS: Symbol not found for js.core.Void>(source)
external fun <T1, T2, T3, T4, TResult> promisify(fn: (arg1: T1, arg2: T2, arg3: T3, arg4: T4, callback: (err: Any?, result: TResult) -> Unit) -> Unit): (arg1: T1, arg2: T2, arg3: T3, arg4: T4) -> ERROR CLASS: Symbol not found for Promise<TResult>(source)
external fun <T1, T2, T3, T4> promisify(fn: (arg1: T1, arg2: T2, arg3: T3, arg4: T4, callback: (err: Any?) -> Unit) -> Unit): (arg1: T1, arg2: T2, arg3: T3, arg4: T4) -> ERROR CLASS: Symbol not found for Promise<ERROR CLASS: Symbol not found for js.core.Void>(source)
external fun <T1, T2, T3, T4, T5, TResult> promisify(fn: (arg1: T1, arg2: T2, arg3: T3, arg4: T4, arg5: T5, callback: (err: Any?, result: TResult) -> Unit) -> Unit): (arg1: T1, arg2: T2, arg3: T3, arg4: T4, arg5: T5) -> ERROR CLASS: Symbol not found for Promise<TResult>(source)