Complete JavaScript Course: Build a Real World App from Scratch/

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Function Contents

Function Contents

This lesson explains the structure of a function in detail.

We'll cover the following...

Return value

Here is a variation of our example program.

Javascript (babel-node)
function sayHello() {
  return "Hello!";
}
console.log("Start of program");
const message = sayHello(); // Store the function return value in a variable
console.log(message);       // Show the return value
console.log("End of program");

Run this code, and you’ll see the same result as before. In this example, the body of the sayHello() function has changed: the statement console.log("Hello!") was replaced by return "Hello!". The keyword return indicates that the function will return a value, which is specified immediately after the keyword. This return value can be retrieved by the caller.

Javascript (babel-node)
// Declare myFunction
function myFunction() {
  let returnValue;
  // Calculate return value
  // returnValue = ...
  return returnValue;
}
// Get return value from myFunction
const result = myFunction();
// ...

This return value can be of any type (number, string, etc). However, a function can return only one value. Retrieving a function’s return value is not mandatory, but in that case the return value is “lost”. If you try to retrieve the return value of a function that does not actually have one, we get the JavaScript value undefined.

Javascript (babel-node)
function myFunction() {
  // ...
  // No return value
}
const result = myFunction();
console.log(result); // undefined

A function ...