Function Declarations and Expressions

Functions package behaviour so it can be called with different inputs. A function declaration defines a named function:

function add(a, b) {
  return a + b;
}
console.log(add(2, 3)); // 5

A function expression assigns a function value to a variable:

const multiply = function (a, b) {
  return a * b;
};

Function declarations are initialized during their scope's setup, so they can generally be called before their textual declaration. A const function expression cannot be accessed before its initialization.

Return values#

A function without an explicit return produces undefined. return exits the current function and sends a result to the caller. Logging a value is not the same as returning it.

function double(value) {
  return value * 2;
}
const result = double(4);

Design small functions#

A good function usually has a clear responsibility, meaningful parameters, and a predictable return value. Keep unrelated side effects separate when practical. Avoid depending on global variables if the needed value can be passed as an argument.

Practice#

Create a calculateTotal function that receives price and quantity and returns their product. Then call it with several different inputs.