Defining functions

Learn to define functions with function and arrow function.

Defining functions

A function is a reusable block of code. JavaScript has two common forms: the function declaration and the function expression.

Function declaration: function greet(name) { return 'Hello ' + name; }. The name comes first; the function can be called before its definition (hoisting).

Function expression: const greet = function (name) { return 'Hello ' + name; };. Here the function is assigned to a variable; it cannot be called before the definition. Both are valid; this lesson uses the declaration form.

Parameter and argument

The names in a function definition are parameters; the values passed at call time are arguments. The two are often confused but are different concepts.

Example: in function greet(name) { ... }, name is a parameter. In the call greet('Ayşe'), 'Ayşe' is an argument. A parameter is a placeholder; an argument is the real value put in that place.

In JavaScript, if an argument is not given for a parameter, its value is undefined. This lets incomplete calls continue silently, so it is important to pass all the arguments you need.

Exercise Greet function

Define a function greet(name). Its body should return the string 'Hello ' + name. Print the result of greet('Ayşe') with console.log.

Show example solution

function greet(name) {
  return 'Hello ' + name;
}
console.log(greet('Ayşe'));

Test yourself

Which is a function declaration?
- const add = (a, b) => a + b;
- const add = function (a, b) { return a + b; };
- function add(a, b) { return a + b; }
- add(a, b) => { return a + b; }
Show answer

function add(a, b) { return a + b; } — the form starting with the function keyword is a declaration.

Two-parameter function

function add(a, ____) { return a + b; }

Show answer

function add(a, b) { return a + b; }

Match the function parts

function

Keyword
greet

Function name
name

Parameter
'Ayşe'

Argument

Next lesson: Return values

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