WHAT IS PROMISE Description. A Promise is a proxy for a value not necessarily known when the promise is created. It allows you to associate handlers with an asynchronous action's eventual success value or failure reason PROMISE() METHOD 1 STAGE PENDING 2 STAGE FULFILLED--->FIXED->ANY-PLACE 3 STAGE REJECTD----->ANOTHER DAY promise() resolve() reject() | | then() catch() notes after resolve then js callback fn called then() if reject then callback fn catch called JS PROMISE SYNTAX let prom =new promise(); let prom new promise(function(){ }); let prom=new promise(function(resolve,reject){ //condition }); Examle 1 case 1 //let complete=true;// then success let complete=false;// then falied let prom =new Promise(function(resolve,reject){ if(complete){ resolve("i am done.") }else{ reject("i am failed.") } }) console.log(prom) /*OUTPUT //let complete=true;// then success Promise[[Prototype]]: Promise[[PromiseState]]: "fulfilled" [[PromiseResult]]: "i am done" let complete=false;// then falied Promise[[Prototype]]: Promise [[PromiseState]]: "rejected" [[PromiseResult]]: "i am failed." pro.html:16 Uncaught (in promise) i am failed. */ Examle 2 // case 2 function promiss(complete){ return new Promise(function(resolve,reject){ if(complete){ resolve("i am done.") }else{ reject("i am failed.") } }) } //console.log(promiss(true)) console.log(promiss(false)) // case 4 if show pending stage function promiss2(complete){ return new Promise(function(resolve,reject){ console.log("fetching data please wait pending stage") setTimeout(()=>{ if(complete){ resolve("i am successful.") }else{ reject("i am failed.") } },1000 ) }) } let onfulfilment=(result)=>{ console.log(result) } let onRejection=(error)=>{ console.log(error) } // for calling use callback fn then catch promiss2(true).then(onfulfilment) promiss2(false).catch(onRejection) or promiss2(false).then(onfulfilment).catch(onRejection) /* output fetching data please wait pending stage i am successful. i am failed */ // EXAMPLE DIVISION function promdiv(a,b){ return new Promise(function(resolve,reject){ console.log("fetching Your data please wait") var c= a/b; setTimeout(()=>{ if(a,b){ resolve(`Your answer":${c}`) }else{ reject(`I AM FAILED TO Calculate`) } },3000) }) } promdiv(26,2).then((result)=>{// pass value div console.log(result); }).catch((error)=>{ console.log(error) }) /*output fetching Your data please wait Your answer":2.5 */ //promisse with ajax step 1 copy cdn <script src="https://cdnjs.cloudflare.com/ajax/libs/jquery/3.5.1/jquery.min.js"></script> step 2 json placeholder for fake data go recourse post copy url:https://jsonplaceholder.typicode.com/posts index.html <script> function promdiv(){ return new Promise(function(resolve,reject){ console.log("fetching Your data please wait") setTimeout(()=>{ $.get("https://jsonplaceholder.typicode.com/posts",function(data){ console.log(data); }).fal((err=>{ reject("failed to json load") })) },2000) }) } promdiv().then((result)=>{ console.log(result); }).catch((error)=>{ console.log(error) }) </script> topic What is Promise.all() if we have so many promise function then we have to use many catch and then function so javascrits E6 have new method promise.all promise() promise() promise() promise() | promise.all() resolve() reject() if all promise resolve if any promiser reject | | then() catch() Promise SYNTAX let p1=new Promise(function(resolve,reject){ console.log("first Promise"); resolve("prmise") }) let p2=new Promise(function(resolve,reject){ console.log("second Promise"); resolve("second ") }) let p3=new Promise(function(resolve,reject){ console.log("third Promise"); resolve("prmise") }) calling Promise.all([p1,p2,p3]).then().catch(); Example 2. let p1=new Promise(function(resolve,reject){ setTimeout(()=>{ console.log("first promise has resolved") resolve(10)// pass result value },1*1000); }) let p2=new Promise(function(resolve,reject){ setTimeout(()=>{ console.log("second promise has resolved") resolve(20) // pass result value },2*1000);//2second }) let p3=new Promise(function(resolve,reject){ setTimeout(()=>{ console.log("third promise has resolved") resolve(30)// pass result value },3*1000);//3second }) // Promise.all([p1,p2,p3]).then().catch(); Promise.all([p1,p2,p3]).then((result)=>{ for( var i in result){total+=result[i]} console.log(`Results:${result}`) console.log(`Total:${total}`) }).catch((error)=>{ console.log(`Errors:${error}`) }); /* output:first promise has resolved promiss-all.html:20 second promise has resolved promiss-all.html:26 third promise has resolved promiss-all.html:32 Results:10,20,30 Total:60 */