undefined and null

let name = 'John';

console.log(name); // John

But we can also define a variable but not assign it like this

let noname;

console.log(noname); // undefined

• note undefined is used in JavaScript to represent the absence of a value

  • undefined is set implicitly behind the scenes

Write a conditional to check if variable has a value (and is not undefined)

let anonymous;
if (anonymous === undefined) {
  console.log('Please provide a value');
} else {
  console.log(anonymous);
}
// will output a terminal log 'Please provide a value'

undefined for function arguments

Where else will we see undefined set for us?

• You will also see this with your functional arguments

// Undefined for function arguments
let square = function(num) {
  console.log(num);
};

square();

• If you do not provide an argument to the function JavaScript will set num equal to undefined

  • It won’t crash the program
  • It won’t alert you that the argument is required (there’s no way to do that)
  • remember When an argument is not provided but it is named in the function definition, undefined is assigned as its value

undefined and function return values

• remember When we return from a function we can store its value in a variable
• We will get undefined twice
• When you do not explicitly return a value from a function JavaScript will explicitly return undefined for that function

// Undefined for function arguments
let square = function(num) {
  console.log(num);
};

let result = square();

console.log(result);

• note undefined as function return default value (if none provided)

Can we manually assign undefined? Yes we can

• If you defined a variable and set its value you can remove that value and replace it with null like in the following example:

let age = 27;
age = undefined;
console.log(age); // undefined

• note We can explicitly assign null to a variable

Houston we have a problem

• When I manually assign undefined to a variable I cause a problem

  • I’m not sure if age is undefined because it was never given a value
  • Or if it is undefined because the variable was explicitly cleared
  • We lose the context between the explicit JavaScript language assigning undefined and a coder manually assigned undefined

  • note This context is IMPORTANT

    • To preserve that context JavaScript gave us a different type that also represents a sort of emptiness and this is called null

Use null instead of undefined when you want to explicitly clear a variable value

// using null
let age = 27;

age = null;
console.log(age); // null