Page 172 - JavaScript

// Do something, then execute callback to log 'done'
doSomething(function () {
console.log('Done');
});

console.log('Doing something else');

// Outputs:
// "Doing something"
// "Done"
// "Doing something else"

The doSomething() method above executes synchronously with the callback - execution blocks until
doSomething() returns, ensuring that the callback is executed before the interpreter moves on.

Callbacks can also be used to execute code asynchronously:

doSomethingAsync(then) {
setTimeout(then, 1000);
console.log('Doing something asynchronously');
}

doSomethingAsync(function() {
console.log('Done');
});

console.log('Doing something else');

// Outputs:
// "Doing something asynchronously"
// "Doing something else"
// "Done"

The then callbacks are considered continuations of the doSomething() methods. Providing a
callback as the last instruction in a function is called a tail-call, which is optimized by ES2015
interpreters.

Error handling and control-flow branching

Callbacks are often used to provide error handling. This is a form of control flow branching, where
some instructions are executed only when an error occurs:

const expected = true;

function compare(actual, success, failure) {
if (actual === expected) {
success();
} else {
failure();
}
}

function onSuccess() {
console.log('Value was expected');
}

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