What is unit testing?

Unit testing is a best practice when developing software. To break it down you write a bit of code and perform a test again't to see if it works as expected leaving less holes for bugs etc. A simple example would be

Creating a function that only returns an int from a string passed in javascript for example. That function isn't suppose to return no other value. So you code out the function a simple test would be having the return variable be tested to see if it is actually a an integer. After the test you will be sure that your code works as expected and code that utilizes this function will not break in let's say a calculation if a string was to be returned when the code was expecting an int it may cause your application to behave unexpectedly.

At least that's how I think about it. There are many practices I.E Test Driven Development

It's code, your code here's an example

// return the number value of a string I.E "3" = 3;

function returnNumber(number){

    return number; // this should be parsed to an integer     

}

Say we use return number in a mathematical equation an sent in a letter instead of a number

var x = 3;

var y = returnNumber("a");

x * y = ?

This is where your code would break

A simple example to unit test the code would be to identify why the code break then solve it and make it unbreakable

example

function returnNumber(number){

 if(isNaN(number)){ // if the number sent in is not a number
           return 0; // just return 0
 }else{
          return number; // this should be parsed to an int
 }

}

Lets try our equation again

var x = 3;

var y = returnNumber("a");

// in this case why would be 0

x * y = ? // so 3 * 0 = 0

Notice our code didn't break even though we sent a letter instead of a number

var x = 3;

var y = returnNumber("3");

// y would be 3

x * y = ?// and 3 * 3 = 9

and out code works as intended too without breaking even if a number wasn't used