Asynchronous programming in JS

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what is synchronous and asynchronous programming?

Synchronous Programming

• As the name suggests, the tasks under sync. programming will be executed one by one.
• Execution of a task is dependent on the previous task.
• Any error in a task will break the chain of tasks
• In javascript tasks are executed synchronously by default.

Asynchronous Programming

• In async. programming tasks are independent of each other.
• Execution of a task is NOT dependent on other tasks.
• Javascript use callback queue to perform async. tasks.
• There are many ways to perform async programming in js such as:
  • Callbacks
  • Promises
  • async and await keywords

Promises in javascript

• Promises is one of the way to perform async. programming in js.
• We have to understand promise lifecycle to understand promises completely.

Promise Lifecycle

• Pending State: It is the initial state of a promise.
  • This means that promise is not started executing, it is neither resolved nor rejected.
• Resolve: This means promise is resolved successfully and is getting executed safely
• Reject: This state reflects an error during execution of promise. Error handler will be invoked when an error occurs.

Handlers in promises

• .then helps in chaining multiple promises and helps executing them one after another
• .catch this block is invoked whenever an error is encountered. It is also called error handler.
• .finally It will get executed irrespective of the execution of promise. Resolving or rejection of a promise don't effect execution of finally block.

Async and await

• async and await keywords are also used to use perform async tasks in javascript.
• They act as syntactical sugar over promises to simplify the code and to give it a procedural view.
• We wrap our promise using an async function.
• await keyword is used to wait for the result of the async function or promise.
• meanwhile other functionalities will work as usual.
• refer to this code to understand the use of async and await keyword.
• we can also chain tasks using async functions like promises.

// function declaration
async function fun(){}

// function invoking
fun()
.then()
.then()