Callbacks are JavaScript's way of implementing asynchronous programming. A callback is a function that is passed as an argument to another function and is executed after its parent function has been completed.

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function callback() {

console.log("Callback function");

}

function parentFunction(callback) {

console.log("Parent function");

callback();

}

If the callback is an anonymous function, then we can also directly call the function at the time of parent call.

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function parentFunction(callback) {

console.log("Parent function");

callback();

}

parentFunction(function() {

console.log("Callback function");

});

These are a type of functions that are used to write the functions in a short way. They are anonymous functions, with a special syntax.

It is named an arrow function, because of the arrow symbol `=>` in the syntax.

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var functionName = (param1, param2, ...) => {

// function body

return value;

}

If the function has only one statement, then we can remove the curly braces and the return keyword.

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var functionName = (param1, param2, ...) => statement;

If the function has only one parameter, then we can remove the parenthesis.

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var functionName = param1 => {

// function body

return value;

}

They are generally used with JS's native functions like `map`, `filter`, `reduce`, etc. When we need to apply the same operation to a set of values.

These are the basics of asynchronous programming in JS. 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.

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