Defining and calling functions

Learn to define and call functions with the function keyword.

What is a function?

In JavaScript, a function is a reusable block of code. You define it once and call it as many times as you need. It starts with the function keyword, followed by a name, parentheses for parameters and a body in curly braces.

Example: function greet() { console.log("Hello"); }. Defining the function does not run it; only calling it does. The statements inside the body run at that moment.

Calling a function

To run a function you defined, you add () after its name. This is called calling the function. For example, the line greet(); runs greet and executes the code in its body.

If you forget the parentheses, the function is not called; you only reference the function object. That is the difference between greet and greet(): the first is the definition, the second is the call.

Exercise Greeting function

Define a function called greet. Its body should print a welcome message with console.log. Then call the function.

Show example solution

function greet() {
  console.log("Welcome");
}

greet();

Test yourself

Which of these is a valid function declaration?
- greet() { console.log("Hi"); }
- function greet() { console.log("Hi"); }
- function greet() => { console.log("Hi"); }
- greet: function { console.log("Hi"); }
Show answer

function greet() { console.log("Hi"); } — Correct. The function keyword + name + () + {} form is the right declaration.

Call the function

function greet() {
  console.log("Hi");
}
____;

Show answer

function greet() {
  console.log("Hi");
}
greet();

Match the function parts

function

Keyword
greet

Function name
()

Parameter list
{ }

Function body

Next lesson: The return statement

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