I have a parameter for you

We wrote a function, called it and counted the miles accrued for the flight to Irkutsk. And what about the flight to Kamchatka? How do you calculate the result of the second trip?

However many times we call the calculateMiles() function, the result will be the same, because inside the function, there are variables with fixed values ​​suitable only for the flight to Irkutsk, that is, only for one case.

Inside the function in the variable distance we store the distance to Irkutsk. To find miles for another flight, you need to change the value of this variable. But we will not rewrite the function with a different distance value. We will make the function universal so that it can be written once, and then used for different cases. To make the function really universal, you must use parameters.

Parameters are values ​​we pass to the function. With their help, we can calculate the result of its operation for specific cases.

Let’s see how it works.

var showTime = function (hours, minutes) {
  console.log('Current time: ' + hours + ':' + minutes);
};

showTime(3, 15);    // Displays: Current time: 3:15
showTime(16, 20);   // Displays: Current time: 16:20

For the function to work with parameters, they must be somehow expressed and given names. Let’s figure out how we can do it.

1. Set the parameters.

var showTime = function (hours, minutes) {
  …
};

When the function is declared, in the parentheses of the function itself, the parameter is given a name. This parameter starts behaving like a variable and can be used in the function code. If there are several of them, the parameters are separated with a comma.

2. Use the parameters.

var showTime = function (hours, minutes) {
  console.log('Current time: ' + hours + ':' + minutes);
};

Parameters work just like variables. We substitute them for fixed values in the operation inside the function. When the code is executed, each parameter is substituted with its value.

3. The function receives parameter values

showTime(3, 15);    // Displays: Current time: 3:15
showTime(16, 20);   // Displays: Current time: 16:20

In the previous steps, we examined how the function works with the received parameters. But we have not yet discussed how to make the function receive them. At the time of the function call, we indicate in parentheses those values that appear in the parameters. We write showTime (16, 20) and instead of hours in the body of the function 16 is substituted, and instead of minutes the number 20.

Let’s improve our code, finally make the calculateMiles function a universal one and calculate the miles accrued for the second flight.

By the way, we say “the function takes parameters”, but at the same time we “pass arguments to the function”. Why the arguments? It was about the parameters! The fact of the matter is that parameters are values ​​that we set at the time a function is declared. We also use them in the function body. And arguments are values ​​that we transfer to a function when it is called.