Web tricks: 2022

Saturday, October 15, 2022

How Promise.all works

Promise.all waits till all the promises passed to it are resolved. Here is an example code snippet:

async function fun1() {
    console.log("1 before wait");
    return new Promise((resolve, reject) => {
        setTimeout(() => { 
            console.log("1 after wait");
            resolve();
        }, 3000);
    });
}

async function fun2() {
    console.log("2 before wait");
    return new Promise((resolve, reject) => {
        return setTimeout(() => {
            console.log("2 after wait");
            resolve();
        }, 2000);
    });
}

async function main() {
    console.log("start");
    await Promise.all([fun1(), fun2()]);
    console.log("end");
}
main();

The above snippet outputs

start
1 before wait
2 before wait
2 after wait
1 after wait
end