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NodeJS: Callbacks

A callback is a function that is passed into another function.



Hover over the ➼ words in bold.

          // 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 realilistic 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.

Callbacks & & Asynchronous Code

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 taxs the tax calculation in as a parameters.

Common Asynchronous NodeJS functions

✶ Database Queries
✶ Reading Files
✶ Creating Files
✶ Parse Data


Coursera

Realistic Example

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.

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