Variables

• A variable is a name for a value.
• You give a value a name using the assignment operator
  • It looks like an equal sign but is not a comparison.
  • often preceded by let as in "Let there be light" or "Let X equal 10".

let color = "blue"
let fruit = "berry"

• Anywhere you would use a literal value, you can use a variable instead.

color + fruit       // "blueberry"
fruit.toUpperCase() // "BERRY"

• ...so pick good names :-)

Let there be confusion

• let is just one way to declare a variable in JavaScript
• var is a lot like let but has wider scope which is sometimes bad
• If you don't use either let or var then the variable becomes global (which is dangerous)
• Moral: always use let unless you have a good reason not to

Don't let me down

Unfortunately, in JavaScript you can only use let once per variable name (in a given scope), otherwise you will get an error:

Identifier 'x' has already been declared

That means that when you're in the JavaScript console, if you see this error then try again without the let

> let x = 1
undefined
> let x = x + 2
SyntaxError: Identifier 'x' has already been declared
> x = x + 2
3

• also confusing: the value of a let is undefined, but the value of a normal assignment is the value being assigned

The Warehouse Metaphor

Think of memory as a giant warehouse.

Like this warehouse from the movie Raiders of the Lost Ark, computer memory is vast and filled with boxes of various sizes.

The Warehouse Metaphor Explained

If memory is a giant warehouse...

...and memory locations are boxes in that warehouse

...then a value is the contents of a box

...and a variable is a label you stick on the outside of the box

Variables are documentation

Which is clearer, this:

60 * 60 * 24

or this:

let secondsPerMinute = 60
let minutesPerHour = 60
let hoursPerDay = 24
let secondsPerDay = secondsPerMinute * minutesPerHour * hoursPerDay

?

Lab: Play In Console

Let's spend a few minutes just playing around with variables in the JavaScript console.

Some things to try:

• assign your birth year to a variable, then calculate your current age in years
• write a poem (with at least two lines) and assign it to a variable
• print the poem to the console with proper line formatting
• assign your best friend's name to a variable
• YELL YOUR BEST FRIEND'S NAME
• get a new best friend
• and YELL THEIR NAME TOO

The Pointer Metaphor

let snack = "Apple"

Think of a variable as an arrow pointing to a value.

Changing Variables

You can assign and reassign variables at will.

color = "blue"     // assign 'blue' to color
fruit = "berry"    // assign 'berry' to fruit
color + fruit      // 'blueberry'

color = "black"    // 'black'
color + fruit      // 'blackberry'

Reaassignment only changes the name of an object. It does not change the data inside the object.

This is analogous to removing a label from one box and placing it on a different box.

Tip: Did you get an Identifier 'color' has already been declared error? Try again without the let, or restart your JavaScript console (in a Browser, Reload the page; in a Terminal, quit and relaunch node).

Many pointers can point to the same thing

let fruit = "Apple"
let snack = fruit

After this both snack and fruit are pointing to the same value

This is analogous to placing two labels on the same box.

Return values are new

Most messages return new values:

let fruit = "banana"
let snack = fruit.toUpperCase()

"banana" and "BANANA" are two different values in memory. The original value is still sitting around and still pointed to by fruit.

Changing Values

Most messages do not change the data inside the object.

let color = "blue"
color.toUpperCase()     // "BLUE"
color                   // "blue"

This is true for all strings, since strings in JavaScript are immutable. Any message that transforms a string will return you an entirely new string.

But some messages do change the contents!

Changing Values Example

Let's say we have a friend named Joe and his birthday is Independence Day, 1990.

We will use the built-in JavaScript Date type to represent a year+month+day.

let independenceDay1990 = new Date(1990, 6, 4)
independenceDay1990.toDateString()    // 'Wed Jul 04 1990'
let joesBirthday = independenceDay1990

Then we learn that Joe's birthday is actually Bastille Day. No problem, we'll just tweak the variable.

joesBirthday.setDate(14)
joesBirthday.toDateString()           // 'Sat Jul 14 1990'

But what happened to the original date?

independenceDay1990.toDateString()    // 'Sat Jul 14 1990'

Oops! Our program now thinks Independence Day 1990 was on July 14. This is a problem. What's the solution?

Constants: Variables that Aren't Variable

• the keyword const is just like let, but also prevents reassignment

const pi = 3.14159;

• the value of a const is constant after it's been set once
  • if you try to change it, you get an error

pi = 7;
TypeError: Assignment to constant variable.

WARNING: const prevents reassignment but does not prevent changing the insides of objects (like the dates in the previous slide).

Summary: Variables

• variables are names for memory locations, which hold values
• declaring a variable says what its scope is
• assigning a variable changes which location it points to
• you can have many names for the same location
• sometimes values can change on the inside of a location
  • (which is useful but could cause bugs)