Javascript Ajax Interview Questions

#What is a Promise?

A promise is an object that may produce a single value some time in the future: either a resolved value, or a reason that it’s not resolved (e.g., a network error occurred). A promise may be in one of 3 possible states: fulfilled, rejected, or pending. Promise users can attach callbacks to handle the fulfilled value or the reason for rejection.

The syntax of promise would be as below:

const promise = new Promise(function(resolve, reject) {
  // promise description
})
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#What are the states of promise?

Promises have three states: 1. Pending: This is an initial state of the Promise before an operation begins 2. Fulfilled: This state indicates that specified operation was completed.

3. Rejected: This state indicates that the operation did not complete. In this case an error value will be thrown.

#What is a callback hell?

Callback Hell is an anti-pattern with multiple nested callbacks which makes code hard to read and debug when dealing with asynchronous logic. The callback hell looks like below,

async1(function(){
    async2(function(){
        async3(function(){
            async4(function(){
                ....
            });
        });
    });
});
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