Variables

let color = "blue"
let fruit = "berry"
color + fruit       // "blueberry"
fruit.toUpperCase() // "BERRY"

Let there be confusion

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 a syntax error:

Identifier 'x' has already been declared

That means that when you're in the node 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

The Warehouse Metaphor

Warehouse from Raiders of the Lost Ark

Think of memory as a giant warehouse.

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 seconds_per_minute = 60
let minutes_per_hour = 60
let hours_per_day = 24
let seconds_per_day = seconds_per_minute * minutes_per_hour * hours_per_day

?

Lab: Play In node

Let's spend a few minutes just playing around in node. Some things to try:

The Pointer Metaphor

let snack = "Apple"

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"
fruit = "berry"
color + fruit           // 'blueberry'

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

Changing a variable (using ASSIGNMENT) just 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 exit and relaunch node.

Many pointers can point to the same thing

let fruit = "Apple"
let snack = fruit

snack-fruit

After this both snack and fruit...

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()

fruit-banana-snack-banana

"banana" and "BANANA" are two different objects in memory

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 to some objects do change the contents!

Changing Values Example

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

let independence_day_1990 = new Date(1990, 6, 4)
independence_day_1990.toDateString()    // 'Wed Jul 04 1990'
let joes_birthday = independence_day_1990

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

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

But what happened to the original date?

independence_day_1990.toDateString()    // 'Sat Jul 14 1990'

Oops! Our program now thinks Independence Day 1990 was on July 14.

Summary: Variables

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Outline

[contents]

Variables Slides - Code Like This

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