A callback is a function that is passed into another function.
// calculate sales tax
function calcTax(bill,callback){
var tax = bill * .06;
callback(tax);
}
// calculate deduction
function calcDeduction(tax){
var total = tax * .20;
// log deduction
console.log(total);
}
// Make Message
function makeMessage(tax){
var message = "You paid " + tax + " in sales tax.";
console.log(message);
}
➼ calculate tax -
The bill and a callback are passed into this function. When the function's code is
done executing, the callback will be executed with tax as the parameter.
➼ calcDeduction -
This is a function that takes in tax as a parameter and calculates the amount of a deduction.
It then displays the amount.
➼ makeMessage -
This is a function that takes in tax as a parameter and displays a friendly message with the amount.
var myBill = 100;
// display message
calcTax(myBill,makeMessage);
// display deduction
calcTax(myBill,calcDeduction);
➼ calcTax: senario 1 -
myBill and makeMessage are passed into calcTax.
➼ calTax: senario 2 -
myBill and calcDeduction are passed into calcTax
➼ callback functions -
➼ makeMessage is passed into ➼ calcTax as a callback.
➼ calcDeduction is passed into ➼ calcTax as a callback.
The code above is not a realistic example of callbacks. It is meant to be a simple example of the concept. In the next lesson, we will look at a realistic example.
When a callback is passed into a function, the parent function becomes asynchronous. In the example above, makeTax would be an asynchronous function. makeTax takes in two parameters: the amount of the bill and a callback function. Once all the code in makeTax is done executing, you can pick a callback function to pass in. It can be any function that takes the tax calculation as a parameter.
✶ Database Queries
✶ Reading Files
✶ Creating Files
✶ Parse Data
Tasks that take longer require callbacks. Say you are displaying a blog page. The blog articles are stored in a PostgreSQL database. Making the blog takes a long time, because it has to interact with the database. The other page elements are made with html and client side JavaScript. While makeBlog is waiting for a response, the rest of the page can begin displaying,
db.query(statement,values, function(err,res) {
if (err) {
throw err;
}
else {
var blog = JSON.stringify(res.rows);
makeBlog(blog);
}
});
NodeJS Queries covers queries in more detail.
