Promises

https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Promise

Promise

var promiseInstance = new Promise(function(resolve, reject) {
    // call resolve(...) to resolve the request
    // call reject(...) to reject the request
});

const someFn = function() {
    var respPromise = new Promise(function(resolve, reject) {
        // call resolve(...) to resolve the request
        // call reject(...) to reject the request
    });

    return respPromise;
}

const promise1 = new Promise((resolve, reject) => {
  setTimeout(() => {
    resolve('foo');
  }, 300);
});

promise1.then((value) => {
  console.log(value);
  // Expected output: "foo"
});

console.log(promise1);
// Expected output: [object Promise]

Promise.resolve

The Promise.resolve() static method "resolves" a given value to a Promise. If the value is a promise, that promise is returned; if the value is a thenable, Promise.resolve() will call the then() method with two callbacks it prepared; otherwise the returned promise will be fulfilled with the value.

An example could be that you call a function and wrap that call in Promise.resolve(someFn()). The function you call could then return a value (e.g. a boolean) or it could return back a Promise instance itself. You can then handle the response of that function as if it returns a promise.

const promise1 = Promise.resolve(123);

promise1.then((value) => {
  console.log(value);
  // Expected output: 123
});

const someHandlerFn = function(someParam) {
    var respPromise = new Promise(function(resolve, reject) {
        if (typeof someParam === 'string' && someParam.length > 0) {
            resolve(true);
        } 
        else {
            reject();
        }
    });

    return respPromise;
};


try {
    Promise.resolve(someHandlerFn('some string')).then(
        function(resp) {
            // Handle resp
        },
        function() {
            // Handle rejection
        }
    );
}
catch(e) {
    console.warn(e);
}