27 Async Await

The purpose of async / await is to simplify the promise handling.

• async functions always return a promise.
• await waits until the promise is resolved.
• The await keyword can only be used inside an async function.
• await is like yield
• The behavior of async / await is similar to combining generators and promises.

1. Async Function

//This function returns promise object
async function logName(name) {
     return name;
}

//We are extracting the promise object using then
logName("Biswajit").then((result) => {
     console.log(result);
})

2. Async Await

async function logName(name) {
    
    const transform = new Promise((resolve, reject) =>{
        setTimeout(()=>resolve(name.toUpperCase()),5000);
    });

    //1. yield promises using await
    const result = await transform;
    
    //2. Return a promise
    return result;
}

logName("Biswajit").then((result) => {
    console.log(result);
})

This will wait until the promise for transform object is fulfilled. The promise will be assigned to the result and then we are unpacking the promise object using then.

3. Using Arrow Function

The same example mentioned before but using arrow function.

const logName = async (name) => {
    const transform = new Promise((resolve, reject) =>{
        setTimeout(()=>resolve(name.toUpperCase()),5000);
    });

    //1. yield promises using await
    const result = await transform;
    
    //2. Return a promise
    return result;
}

logName("Biswajit").then((result) => {
    console.log(result);
})

4. Error Handling

The error handling in async await is really simple. We need to use try catch blocks

const logName = async (name) => {

    try{
        const transform = new Promise((resolve, reject) =>{
            setTimeout(()=>resolve(name.toUpperCase()),2000);
            setTimeout(()=>reject("There is an error"),1000);
        });
    
        //1. yield promises using await
        const result = await transform;
        
        //2. Return a promise
        return result;
    }
    catch(error){
        console.log(error);
    }
    
}

logName("Biswajit").then((result) => {
    console.log(result);
})

The promise will send reject in 1 second, so it will throw error and will be caught in the catch block.

5. Simple Example

const fun1 = ( async ()=>{
  
    const num = new Promise((resolve, reject)=>{
        setTimeout(()=>{
            console.log("Hello");
             resolve(5);
          },2000);
    })

    const result = await num;
    return result;

});

//We can unpack the value from promise object using then
fun1().then((data)=>{
    console.log(data);
})


//We can assign the value returned from await function
const fun2 = async ()=>{
    const num1 = await fun1();
    console.log(num1);
}


//Invoke the function
fun2();