Javascript · Booleans
Truthy Falsy values
The full text of this chapter, to read or revisit. To practise, do the exercises.
▶ Do the exercisesExercise 1 — What are Truthy and Falsy Values?
In JavaScript, every value has an inherent boolean nature. When used in a boolean context (like an
if
statement), JavaScript automatically converts values to either
true
or
false
.
- A truthy value is a value that converts to
true
in a boolean context
- A falsy value is a value that converts to
false
in a boolean context
This means you don't always need to use comparison operators in conditionals! Let's see this in action:
let codemonName = "Aquachu";
// Instead of: if (codemonName !== "")
if (codemonName) {
console.log("You have a Codémon!");
}
let codemonLevel = 0;
// This won't print because 0 is falsy
if (codemonLevel) {
console.log("Level up!");
}
console.log("Level is:", codemonLevel);
Why is this useful?
Understanding truthy and falsy values makes your code cleaner and more concise. Instead of writing
if (value !== null && value !== undefined && value !== "")
, you can often just write
if (value)
!
Exercise 2 — The 6 Falsy Values
In JavaScript, there are exactly 6 falsy values. Everything else is truthy! Here they are:
1.
false
- the boolean false
2.
0
- the number zero
3.
""
- empty string
4.
null
- intentional absence of value
5.
undefined
- variable declared but not assigned
6.
NaN
- Not a Number (result of invalid math operations)
console.log("false:", Boolean(false));
console.log("0:", Boolean(0));
console.log("empty string:", Boolean(""));
console.log("null:", Boolean(null));
console.log("undefined:", Boolean(undefined));
console.log("NaN:", Boolean(NaN));
Memorize these 6!
These 6 falsy values are crucial to remember. A common mistake is thinking that other values like empty arrays
[]
or empty objects
{}
are falsy - but they're NOT! Only these 6 are falsy.
Exercise 3 — Identifying Falsy Values
Which of the following is NOT a falsy value in JavaScript?
Options
• ```"0"```
• ```0```
• ```null```
• ```undefined```
Hint : Remember: Only 6 values are falsy. The string ```"0"``` contains a character, so it's not an empty string.
Exercise 4 — Everything Else is Truthy!
Now for the good news: everything that isn't one of those 6 falsy values is truthy!
This includes some values that might surprise you:
-
"0"
- string with zero (not empty!)
-
"false"
- string with the word false
-
-1
or any non-zero number
-
" "
- string with a space
-
[]
- empty array
-
{}
- empty object
console.log("String \"0\":", Boolean("0"));
console.log("String \"false\":", Boolean("false"));
console.log("-1:", Boolean(-1));
console.log("Space string:", Boolean(" "));
console.log("42:", Boolean(42));
Watch out for strings!
A very common mistake:
"0"
and
"false"
are truthy because they are non-empty strings. Only the empty string
""
is falsy. Any string with content, even if it says "false", is truthy!
Exercise 5 — Which one is Falsy?
Which of these values will return
false
when converted to Boolean?
Options
• ```"0"```
• ```" "```
• ```0```
• ```[]```
Hint : Remember the 6 falsy values! Non-empty strings are truthy, even if they contain ```"0"``` or a space.
Exercise 6 — Which is Truthy?
Which of these values is truthy?
Options
• ```"false"```
• ```0```
• ```null```
• ```""```
Hint : Remember: ```"false"``` is a non-empty string, so it's truthy! Only the 6 specific falsy values are falsy.
Exercise 7 — Match Values to Their Nature
Match each value with whether it's truthy or falsy:
Items to match
• ""
• "false"
• 0
• -1
• null
• " "
Definitions
• Falsy
• Truthy
• Falsy
• Truthy
• Falsy
• Truthy
Exercise 8 — Negating Truthy and Falsy
What will this code output?
let codemonHP = 0;
console.log(!codemonHP);
Options
• ```true```
• ```false```
• ```0```
• ```undefined```
Hint : ```0``` is falsy. The ```!``` operator inverts it, so ```!0``` becomes ```true```!
Exercise 9 — Combining Truthy and Falsy
What will this code output?
let name = "Aquachu";
let hp = 0;
console.log(name && hp);
Options
• ```true```
• ```false```
• ```0```
• ```"Aquachu"```
Hint : ```&&``` checks if both sides are truthy. ```name``` is truthy, but ```hp``` is ```0``` (falsy). What does ```true && false``` give?
Exercise 10 — Truthy/Falsy Practice
Practice identifying truthy and falsy values!
Create these variables:
-
emptyName
with an empty string
-
zeroDamage
with
0
-
type
with
"fire"
Then check if each one is truthy or falsy by printing them with
Boolean()
.
Starter code
// Create your variables and tests here