A VALUE is a location in computer memory that stores DATA.

There are many kinds of values, including String, Number, Array, Date, ...

(The different kinds of values are called TYPES. Soon you will create your own types but for now, we will use the built-in ones.)


A number is what it sounds like -- any integer or decimal.


Lab: Playing with Numbers

Open up a node environment in your terminal (node Enter). Once you're in the node environment you should be able to write javascript, and run it inside your terminal. Try out the following equations:

  • Add two numbers together with the + operator
  • Multiply two numbers with the * operator
  • Divide two numbers using the / operator
  • Square a number by raising it to the power of 2 **2
  • Create a more complex mathematical equation. Useing amixture of different operators
  • Bonus: Weird math! Try the following equations:
    • 0.1 + 0.2, 2**53 and 2**53 + 1, and Infinity - Infinity

What have you noticed about math in JavaScript


A string is an object that's a collection of characters, like a word or a sentence.

"Cherry Pie"

Slicing and Dicing

Every string is made of lots of other strings.

You can pull out parts of a string with the slice method.

// this means "slice from character 0 to character 4"
"blueberry".slice(0, 4) 

// this means "slice from character 4 to the end

These start and end numbers are called indexes (or indices if you're feeling fancy).

MDN: slice

String Indexing

Humans like to start counting at 1, but computers like to start counting at 0.

Think of the indexes as pointing at the spaces between characters, as in this diagram:

| B | L | U | E | B | E | R | R | Y |
0   1   2   3   4   5   6   7   8   9

So with this picture in your mind, slice...

  • includes the character to the right of the start index
  • includes the character to the left of the end index...
  • ...but excludes the character to the right of the end index

Try various start and end values in the console and see what happens!

Lab: Playing with Strings

Open up a Node instance in your terminal, and let's play around with strings!

  • Add two strings together that use the same type of quotes (e.g.'Java' + 'Script')
  • Add two strings together using different types of quotes (e.g. `"I'm double quoted!" + 'I am single quoted.')
  • Subtract two strings (e.g. 'cats' - 'dogs')
  • Add a string to a number

What happens when you try to put quotes inside your string's quotes?


A boolean is a value that is either true or false, and is represented in JavaScript with the keywords true, and false.

Booleans in JavaScript are often the result of a comparison, and used to run code conditionally (more on that later). Pretty much any data in JavaScript can be evaluated as a boolean so often we use implicit truthy, or falsy values instead of the actual keywords true and false

(It's named after George Boole, a 19th-century mathematician who invented Boolean algebra.)

Booleans and Conditionals

Let's look ahead a little bit and use a simple conditional to evaluate various values. A conditional starts with the keyword if then takes a check in parentheses if('some condition') and finally a block of code defined by curly braces which only runs if the check evaluates to true

Don't worry too much about how conditionals work right now, we'll go into a lot more depth in them later on. For now just use the conditional given on the next slide to complete the lab, swapping out the check for different values to answer the questions

Lab: Is it True?

Given this conditional answer the following questions. If the check (currently the boolean true) is true it will print It is truthy to the console, otherwise nothing will print.

if(true) {
    console.log('It is truthy')
  • Is the boolean true truthy?
    • how about false?
  • Try replacing true with a string
    • Is it truthy?
  • Try replacing true with a number
    • Is it true?
    • What about the number 0?
  • Try replacing true with the expression 1 < 2
    • What happens if you switch the operator like so 1 > 2?


Values can be combined or manipulated using operators, like...

  • PLUS (+)
  • TIMES (*)
  • POWER (**)
  • DOT (.)
  • COMPARISON (===)

An operator sends a message to the value

  • e.g. 1 + 2 sends the number 1 the message please add 2 to yourself.

Dot is a special operator that sends arbitrary messages; we will learn more about her later.


When reading JavaScript code, if you ever see two slashes in a row, that means "everything after these slashes is a comment".

2 + 2    // makes four

A comment is a message for humans. JavaScript ignores everything to the right of the slashes, so you can explain what the nearby code does, or why it does it.

In these lessons, we often use comments to explain the result of executing the nearby code. In this case, we sometimes add an arrow to the comment:

2 + 2  //=> 4
3 + 5 // -> 8

JavaScript also has multi-line comments via /* ... */ but those are less common. They can also be used to comment out a section within a line:

/* This is
 * a multiline
 * comment! */

 console.log(x /*some variable from earlier*/);

Expression Evaluation

A snippet of JavaScript code is called an expression.

Whenever JavaScript encounters an expression, it tries to evaluate it, which means to convert it into a value.

A simple expression (like a plain number or a string) evaluates to just that value.

A more complicated expression with operators keeps applying those operators until it gets down to a single value.

You can think of evaluation as asking and answering a question.

2 + 2    // Question: What is 2 + 2?
4        // Answer: 4

// Q: What is the all-caps version of the string "apple"?
// A: the string "APPLE"

We say that a statement evaluates to a value, as in "2 plus 2 evaluates to 4". You can also say "the value of 2 + 2 is 4" or "the return value of 2 + 2 is 4".

Return Values

Sometimes the return value is the same as the original value.

4 * 1    // return value: 4

Sometimes the return value is a different value.

2 + 3    // return value: 5

Sometimes the return value is a different value and a different type.

"banana".length  // return value: 6

Sometimes the return value is a special value!

(5).length     // return value: undefined
5 / 0          // return value: Infinity
"cookie" * 10  // return value: NaN

Expression vs Statement Breakdown

Expression Statement
has a value does something
can be assigned to a var has a semicolon (optionally)
contains expressions

Read more here: (

Readings and Exercises: