Undefined

undefined is a value in JavaScript that is used to represent nothingness. When a variable is declared, it is assigned to undefined by default.

var bb;
bb === undefined; // true

Remember that variables can be reassigned to other values after they are initially declared.

var abc;
abc === undefined; // true
abc = "happy";
abc === undefined // false

Functions that do not use the return keyword to explicitly return a value will return undefined by default.

function boring() {}
var result = boring();
result === undefined; // true

Functions can perform operations, but will still return undefined if the return keyword is forgotten.

function add(a, b) {
  a + b;
}
var result = add(44, 55);
result === undefined; // true

undefined is a separate type:

typeof(undefined); // "undefined"

Strings ("bob", "i like cheese"), numbers (4.55, 77), and booleans (true, false) are other types we've already covered in this book.

Question Click to View Answer

What does the following code print to the console?

var x = 11;
console.log(x);

11

The variable x is assigned to the value 11 and is then printed to the console.

What does the following code print to the console?

var lol;
var zyz;
console.log(lol);

undefined

The variable lol is declared but is not assigned to any value. When variables are declared but not assigned to a value, they are automatically assigned to undefined.

What does the following code print to the console?

function fullName(x, y) {
  x + " " + y;
}
console.log(fullName("Mr", "Powers"));

undefined

The fullName() function does not use the return keyword, so nothing is returned from the function. JavaScript returns undefined from a function when the return keyword does not specify a return value.

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