JavaScript Callbacks Continuation (synchronous and asynchronous)
Example
Callbacks can be used to provide code to be executed after a method has completed:
/**
* @arg {Function} then continuation callback
*/
function doSomething(then) {
console.log('Doing something');
then();
}
// 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.
