We've talked about performing basic mathematical functions, and assigning values
to variables, but how would we check to see if a value is what we're expecting?
In addition to performing arithmetic and assigning value to variables,
JavaScript has additional operators for comparing values. The value returned by
a comparison is always true
or false
.
- Identify equality operators
- Compare numbers with the relational operators
There are four equality operators built into JavaScript:
- loose equality operator (
==
) - strict equality operator (
===
) - loose inequality operator (
!=
) - strict inequality operator (
!==
)
When writing JavaScript, you strongly prefer the strict operators, as the loose operators will return true even if the data types aren't the same. A string '42' is not the same as an integer 42. As developers we want to ensure that not only are the values the same, but also the data types.
The loose equality operator returns true
if two values are equal:
42 == 42
// => true
However, it will also return true
if it can perform a type conversion (e.g.,
changing the string '42'
into the number 42
) that makes the two values
equal:
42 == '42'
// => true
true == 1
// => true
'0' == false
// => true
null == undefined
// => true
' ' == 0
// => true
This is confusing and inaccurate! It makes no sense that the string '0'
is
equal to the boolean false
or that null
and undefined
— two completely
different data types — are equivalent.
You should prefer ===
for comparisons.
The strict equality operator returns true
if two values are equal without
performing type conversions. Even if the values on both sides of the operator
look similar (e.g., '42' === 42
), the ===
operator will only return true
if the data types also match:
42 === 42
// => true
42 === '42'
// => false
true === 1
// => false
'0' === false
// => false
null === undefined
// => false
' ' === 0
// => false
This is logical and accurate! Prefer ===
for comparisons.
The loose inequality operator is the opposite of ==
. It returns true
if
two values are not equal, performing type conversions as necessary:
9000 != 9001
// => true
9001 != '9001'
// => false
[] != ''
// => false
You should prefer !==
for comparisons.
The strict inequality operator returns true
if two values are not equal
and does not perform type conversions:
9000 !== 9001
// => true
9001 !== '9001'
// => true
[] !== ''
// => true
You should prefer !==
for comparisons.
There are four relational operators built in to JavaScript:
- greater than (
>
), - greater than or equals (
>=
) - less than (
<
) - less than or equals (
<=
)
These operators work in a very similar way to the equality operators:
88 > 9
// => true
However, beware of type conversion when comparing non-numbers against numbers. For instance, when a string is compared with a number, the JavaScript engine tries to convert the string to a number:
88 > '9'
// => true
If the engine can't convert the string into a number, the comparison will always
return false
:
88 >= 'hello'
// => false
88 <= 'hello'
// => false
Strings are compared with other strings lexicographically, meaning
character-by-character from left-to-right. The following returns false
because
the Unicode value of 8
, the first character in 88
, is less than the Unicode
value of 9
.
'88' > '9'
// => false
If you aren't sure what data type you are going to be receiving, but you still need to compare them, make sure that you tell JavaScript to convert the string to a number first, and then compare.
Top Tip: Stick to comparing numerical values with the relational operators and you'll be golden.
JavaScript contains both equality and comparison operators that assist us in writing functional code. Make sure you're preferring the strict equality operators, and only comparing numerical values with the relational operators, and you'll avoid those annoying troubleshooting errors that can drive you crazy!