Many comparison operators we know from maths:
- Greater/less than:
a > b,
a < b.
- Greater/less than or equals:
a >= b,
a <= b.
- Equality check is written as
a == b(please note the double equation sign
=. A single symbol
a = bwould mean an assignment).
- Not equals. In maths the notation is
a != b.
Just as all other operators, a comparison returns a value. The value is of the boolean type.
true– means “yes”, “correct” or “the truth”.
false– means “no”, “wrong” or “a lie”.
A comparison result can be assigned to a variable, just like any value:
To see which string is greater than the other, the so-called “dictionary” or “lexicographical” order is used.
In other words, strings are compared letter-by-letter.
The algorithm to compare two strings is simple:
- Compare first characters of both strings.
- If the first one is greater(or less), then the first string is greater(or less) than the second. We’re done.
- Otherwise if first characters are equal, compare the second characters the same way.
- Repeat until the end of any string.
- If both strings ended simultaneously, then they are equal. Otherwise the longer string is greater.
In the example above, the comparison
'Z' > 'A' gets the result at the first step.
"Glee" are compared character-by-character:
Gis the same as
lis the same as
ois greater than
e. Stop here. The first string is greater.
The comparison algorithm given above is roughly equivalent to the one used in book dictionaries or phone books. But it’s not exactly the same.
For instance, case matters. A capital letter
"A" is not equal to the lowercase
"a". Which one is greater? Actually, the lowercase
"a" is. Why? Because the lowercase character has a greater index in the internal encoding table (Unicode). We’ll get back to specific details and consequences in the chapter Strings.
When compared values belong to different types, they are converted to numbers.
For boolean values,
0, that’s why:
It is possible that at the same time:
- Two values are equal.
- One of them is
trueas a boolean and the other one is
falseas a boolean.
Boolean conversion uses another set of rules.
A regular equality check
== has a problem. It cannot differ
The same thing with an empty string:
That’s because operands of different types are converted to a number by the equality operator
==. An empty string, just like
false, becomes a zero.
What to do if we’d like to differentiate
A strict equality operator
=== checks the equality without type conversion.
In other words, if
b are of different types, then
a === b immediately returns
false without an attempt to convert them.
Let’s try it:
There also exists a “strict non-equality” operator
!==, as an analogy for
The strict equality check operator is a bit longer to write, but makes it obvious what’s going on and leaves less space for errors.
Let’s see more edge cases.
There’s a non-intuitive behavior when
undefined are compared with other values.
- For a strict equality check
These values are different, because each of them belongs to a separate type of its own.
- For a non-strict check
There’s a special rule. These two are a “sweet couple”: they equal each other (in the sense of
==), but not any other value.
- For maths and other comparisons
< > <= >=
null/undefinedare converted to a number:
Now let’s see funny things that happen when we apply those rules. And, what’s more important, how to not fall into a trap with these features.
null with a zero:
Yeah, mathematically that’s strange. The last result states that "
null is greater than or equal to zero". Then one of the comparisons above must be correct, but they are both false.
The reason is that an equality check
== and comparisons
> < >= <= work differently. Comparisons convert
null to a number, hence treat it as
0. That’s why (3)
null >= 0 is true and (1)
null > 0 is false.
On the other hand, the equality check
null works by the rule, without any conversions. They equal each other and don’t equal anything else. That’s why (2)
null == 0 is false.
undefined shouldn’t participate in comparisons at all:
Why does it dislike a zero so much? Always false!
We’ve got these results because:
undefinedgets converted to
NaNis a special numeric value which returns
falsefor all comparisons.
- The equality check
nulland no other value.
Why did we observe these examples? Should we remember these peculiarities all the time? Well, not really. Actually, these tricky things will gradually become familiar over time, but there’s a solid way to evade any problems with them.
Just treat any comparison with
undefined/null except the strict equality
=== with exceptional care.
Don’t use comparisons
>= > < <= with a variable which may be
null/undefined, unless you are really sure what you’re doing. If a variable can have such values, then check for them separately.
- Comparison operators return a logical value.
- Strings are compared letter-by-letter in the “dictionary” order.
- When values of different types are compared, they get converted to numbers (with the exclusion of a strict equality check).
==each other and do not equal any other value.
- Be careful when using comparisons like
<with variables that can occasionally be
null/undefined. Making a separate check for
null/undefinedis a good idea.