Javascript Non-Promise Thenables

Introduction

Both Promise.resolve() and Promise.reject() accept non-promise thenables as arguments.

When passed a non-promise thenable, these methods create a new promise that is called after the then() function.

A non-promise thenable is created when an object has a then() method that accepts a resolve and a reject argument, like this: 

let thenable = {
    then: function(resolve, reject) {
        resolve(42);
    }
};

You can call Promise.resolve() to convert thenable into a fulfilled promise: 


let thenable = {/*from www .  j a v  a2  s  . co m*/
    then: function(resolve, reject) {
        resolve(42);
    }
};

let p1 = Promise.resolve(thenable);
p1.then(function(value) {
    console.log(value);     // 42
});

We can use Promise.resolve() to create a rejected promise from a thenable: 

let thenable = {/*from w w  w.j a v  a2s .  c  o m*/
    then: function(resolve, reject) {
        reject(42);
    }
};

let p1 = Promise.resolve(thenable);
p1.catch(function(value) {
    console.log(value);     // 42
});



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