Declaring and calling functions

What is a function?

A function is a group of statements designed to perform a particular task. When called, its statements execute in order. JavaScript programs use named functions, anonymous functions, nested functions, callbacks, load-event handlers, arrow functions, and immediately invoked function expressions.

The basic shape is:

function functionName(parameter1, parameter2) {
  // Statements to execute
}

Two ways to define a function

The two forms introduced here are a function declaration and a function expression.

A declaration has the familiar named-function form:

function functionName(parameter1, parameter2) {
  // Statements to execute
}

A function expression can assign an anonymous function to a variable:

const functionName = function(parameter1, parameter2) {
  // Statements to execute
}

Their differences become especially important when we discuss hoisting later. For now, recognize both forms. The placeholder names in these three syntax examples have been translated from Korean.

Calling a function

Once a function is defined, call it by writing its name followed by parentheses containing the arguments.

Open the original function-call example on CodePen.

Editorial correction: the original article said missing arguments always cause an error. In JavaScript, a missing argument ordinarily gives its parameter the value undefined unless a default applies. Whether that leads to an error depends on what the function does with it.

Next, we will learn about arrow functions.