Pass an ASYNC JavaScript function as parameter

July 10, 2023

I am trying to pass an async JS function as a function parameter, but I keep getting an undefined response. I found code in another SO answer which works when not using async/await, but when I tried to modify it to allow async/await I dont get the nested function result "5" returned. Why is this happening, and how can I fix it?

Code example (expected result print "5")

async function foo(x) {
    //sleep 5 sec
    new Promise((resolve) => setTimeout(resolve, 5000));
    console.log(x);
    return 5
 }

async function bar(func) {
    console.log(1)
    return await func();
    console.log(2)
 }

(async()=>{
    //alerts "Hello World!" (from within bar AFTER being passed)
    let keys=await bar(async function(){ await foo("Hello World!") });

    console.log(`result:`,keys)

})()

>Solution :

You are missing an await on the sleep if you want it to wait 5 seconds. You have no return in the function you are passing in so the function returns undefined.

async function foo(x) {
    //sleep 5 sec
    await new Promise((resolve) => setTimeout(resolve, 5000));
    console.log(x);
    return 5
 }

async function bar(func) {
    console.log(1)
    return await func();
    console.log(2)
 }

(async()=>{
    let keys=await bar(async function(){ return await foo("Hello World!") });
    console.log(`result:`,keys)
})()