Promise in Javascript

Promise basically used for async, because other function will execute if before function is fine.

–JS

var fPromise = new Promise(function (resolve, reject){
    var isTrue = true;
    if(isTrue){
        resolve('True');
    } else {
        reject();
    }
});

fPromise.then(function(fromRes){
    console.log('this is ' + fromRes);
});

If Promise not completed.

–JS

var fPromise = new Promise(function (resolve, reject){
    var isTrue = false;
    if(isTrue){
        resolve('True');
    } else {
        reject('Not True');
    }
});

fPromise.then(function(fromRes){
    console.log('this is ' + fromRes);
}).catch(function(fromRej){
    console.log('this is ' + fromRej);
})

Now Promises one by one.

–JS

var fPromise = function(){
    return new Promise(function(resolve, reject){
        resolve('done fPromise completed.');
    })
};

var sPromise = function(msg){
    return new Promise(function(resolve, reject){
        resolve(msg + ' done sPromise');
    })
};

var tPromise = function(msg){
    return new Promise(function(resolve, reject){
        resolve(msg + ' done tPromise');
    })
};

fPromise().then(function(res){
    return sPromise(res);
}).then(function(res){
    return tPromise(res);
}).then(function(res){
    console.log('done by ' + res);
})