What is Promise.all

5 minute frontend interview prep

Promise.all() will accept an array of promises and returns a promise that resolves when all the promises in the array are fullfilled or when the iterable contains no promises . it rejects when any one of those promise rejects ,

it’s like the proverb building reputation takes years but breaking can take one bad move , now Let's implement promise.all()

const promiseAll = function(tasks){
// for storing results 
let results = [];

// to track how many promises are completed
let totalPromise  = 0
// returns a new promise 

return new Promise((res,rej)=>{
    tasks.forEach((promise,index)) =>{
         // if promises pass
      promise.then((val)=>{
        // store it in results in order
          results[index] = val;
               totalPromise+=1

        // if all promises aree completed then
        if(totalPromises === tasks.length){
         res(results)
              }

          }
     // if promise fails
         .catch(err=> rej(err))
       }
    }
)
}

This is basic implementation of promise.all() , always remember it will return a promise which will resolve if all promises in the array are fullfilled and it will reject or gives an error even one promise is rejected