• A VARIABLE is a NAME for a VALUE.
  • You give a value a name using the assignment operator =
  • Anywhere you use a value, you can use a variable instead.
let color = "blue"
let fruit = "berry"
color + fruit       // "blueberry"
fruit.toUpperCase() // "BERRY"

Let vs Var vs Cont

  • let declares a variable in JavaScript
  • const declares a variable that cannot be reassigned
  • var is like let but is no longer recommended

Rebinding Let

You can only use let once per variable name, per scope

let name = 'Grace';
let name = 'Hopper';
// Identifier 'name' has already been declared

The Warehouse Metaphor

Think of memory as a giant warehouse. Computer memory is vast and filled with boxes of various sizes.

Warehouse from Raiders of the Lost Ark

The Warehouse Metaphor Explained

If memory is a giant warehouse...

  • Memory locations are boxes in that warehouse
  • Then a value is the contents of a box
  • And a variable is a label you apply to the outside of the box

Variables as Documentation

Which is clearer?

60 * 60 * 24
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
  • Get a new best friend

Variables Point at Values

let snack = "Apple"


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

Changing Variables

You can assign and reassign variables at will

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

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

Many Variables One Value

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

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

Many messages do not change the data inside the object.

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

This is true for all primitive values.

Constants: Variables that Aren't Variable

  • const is just like let, but also prevents reassignment
  • The value of a const is constant after it's been set once
const pi = 3.14159;
pi = 7;
TypeError: Assignment to constant variable.

Whileconst prevents reassignment, it does not prevent changing the properties within objects

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