let
currentTempC
=
22
;
// degrees Celsius
The let keyword is new in ES6; prior to ES6, the only option was
the var keyword, which we will discuss in
This statement does two things: it declares (creates) the variable
currentTempC
and
assigns it an initial value. We can change the value of
currentTempC
at any time:
currentTempC
=
22.5
;
Note that we don’t use
let
again;
let
specifically declares a variable, and you can only
do it once.
With numbers, there’s no way to associate units with the value.
That is, there’s no language feature that allows us to say that
currentTempC
is in degrees Celsius, thereby producing an error if
we assign a value in degrees Fahrenheit. For this reason, I chose to
add “C” to the variable name to make it clear that the units are
degrees Celsius. The language can’t enforce this, but it’s a form of
documentation that prevents casual mistakes.
When you declare a variable, you don’t have to provide it with an initial value. If you
don’t, it implicitly gets a special value,
undefined
:
let
targetTempC
;
// equivalent to "let targetTempC = undefined";
You can also declare multiple variables with the same
let
statement:
let
targetTempC
,
room1
=
"conference_room_a"
,
room2
=
"lobby"
;
In this example, we’ve declared three variables:
targetTempC
isn’t initialized with a
variable, so it implicitly has the value
undefined
;
room1
is declared with an initial
value of
"conference_room_a"
; and
room2
is declared with an initial value of
"lobby"
.
room1
and
room2
are examples of string (text) variables.
A constant (new in ES6) also holds a value, but unlike a variable, can’t be changed
after initialization. Let’s use a constant to express a comfortable room temperature
and a maximum temp (
const
can also declare multiple constants):
const
ROOM_TEMP_C
=
21.5
,
MAX_TEMP_C
=
30
;
It is conventional (but not required) for constants that refer to a specific number or
string to be named with all uppercase letters and underscores. This makes them easy
to spot in your code, and is a visual cue that you shouldn’t try to change their value.
34 | Chapter 3: Literals, Variables, Constants, and Data Types
