Learn how to define functions using function declarations in my blog post here.

In JavaScript, one of the ways to define a function is to use a function expression.

To define a function inside an expression, we can use the function keyword. In a function expression, the function name is usually omitted. A function with no name is called an anonymous function. A function expression is often stored in a variable in order to refer to it.

Consider the following function expression:

To declare a function expression:

• Declare a variable to make the variable’s name be the name, or identifier, of your function. Since the release of ES6, it is common practice to use const as the keyword to declare the variable.

• Assign as that variable’s value an anonymous function created by using the function keyword followed by a set of parentheses with possible parameters. Then a set of curly braces that contain the function body.

To invoke a function, write the name of the variable in which the function is stored followed by parentheses enclosing any arguments being passed into the function.

Example:

const rectangleArea = function (width, height) {
  const area = width * height
  return area
}

console.log(rectangleArea(6, 2))
// 12

Note: Unlike function declarations, function expressions are not hoisted; so they can’t be called before they are defined.