Empowering Today’s Learners to Become Tomorrow’s Leaders
Functions are a way of packaging functionality that you wish to reuse. It's possible to define a body of code as a function that executes when you call the function name in your code. This is a good alternative to repeatedly writing the same code. You have already seen some uses of functions previously. For example:
let myVariable = document.querySelector('h1');
alert('hello!');
These functions, document.querySelector and alert, are built into the browser.
If you see something which looks like a variable name, but it's followed by parentheses— () —it is likely a function. Functions often take arguments: bits of data they need to do their job. Arguments go inside the parentheses, separated by commas if there is more than one argument.
For example, the alert() function makes a pop-up box appear inside the browser window, but we need to give it a string as an argument to tell the function what message to display.
You can also define your own functions. In the next example, we create a simple function which takes two numbers as arguments and multiplies them:
function multiply(num1,num2) {
let result = num1 * num2;
return result;
}
Try running this in the console; then test with several arguments. For example:
multiply(4, 7);
multiply(20, 20);
multiply(0.5, 3);