Callbacks
A callback function is a function passed into another function as an argument.
This can then be invoked inside the outer function to complete some kind of routine or action.
Synchronous callback
function greeting(name) {
alert('Hello ' + name);
}
function processUserInput(callback) {
var name = prompt('Please enter your name.');
callback(name);
}
processUserInput(greeting);
function ask(question, yes, no) {
if (confirm(question)) yes()
else no();
}
ask(
"Do you agree?",
function() { alert("You agreed."); },
function() { alert("You canceled the execution."); }
);
Asynchronous Callback
function asyncMethod(message, callback_function) {
setTimeout( function() {
console.log(message);
callback_function();
}, 400)
}
asyncMethod('Connect to Database', function() {
}
Nested Callbacks
asyncMethod('Connect to Database', function() {
asyncMethod('Verify User Details', function() {
asyncMethod('Check Permissions', function() {
asyncMethod('Update User', function() {} )
})
})
})
Best Practices
Arguments should be (err, results)
Always 'return' when you want to exit
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