JavaScript has many operators for many different situations. We've already learned about a two types of operators:
+
, -
, *
, and /
. =
Let's now take the time to learn about more JavaScript operators used for assignment and comparison. For a list of all JavaScript operators, visit the MDN reference page on Expressions and Operators. We'll be linking to specific sections within the MDN "Expressions and Operators" page throughout this lesson.
We've already seen how we can use the =
assignment operator to set a variable equal to a value.
> const favoriteNumber = 42;
> favoriteNumber;
42
The =
is called an assignment operator because it assigns the value on the right of the operator to the variable on the left. In the above example, favoriteNumber
is assigned the value 42.
Another assignment operator is the +=
operator, because it also assigns a new value to the variable on the left based on the value to the right.
> let myNumber = 5;
> myNumber += 1;
> myNumber;
6
There is an assignment operator for each of the mathematical functions: addition, subtraction, multiplication and division.
+=
-=
*=
/=
When you use any of these assignment operators, the value of the variable on the left side is changed by the math operation and value on the right. Let's do one of each:
> let testNumber = 10;
> testNumber += 5;
> testNumber;
15
> testNumber -= 9;
> testNumber;
6
> testNumber *= 3;
> testNumber;
18
> testNumber /= 2
> testNumber;
9
Visit this link for the MDN reference page on assignment operators:
Assignment operators change the value of the variable on the left of the operator. Comparison operators do not change any values, but return the booleans true
or false
depending on whether the JavaScript expression evaluates as true or false.
> const myNumber = 5;
> myNumber < 10;
true
> myNumber > 10;
false
Comparison operators are also called relational operators, because they help find the relationship between two operands, asking questions like "is 10 bigger than 5?".
>
>
means "greater-than (and not equal to)":
3 > 4
evaluates to false
.3 > 3
also evaluates to false
, because 3 is equal to 3 and not greater.3 > 2
evaluates to true
.<
<
is the opposite of >
. It means "less-than (and not equal to)":
3 < 5
evaluates to true
. 3 < 3
evaluates to false
because they are equal.>=
>=
is the same as >
, except it evaluates to true
if the two sides are equal:
3 >= 3
evaluates to true
3 >= 2
also evaluates to true
.<=
<=
is the opposite of >=
. It means "less-than-or-equal-to":
3 <= 3
evaluates to true
because 3 is equal to 3. 3 <= 1
evaluates to false
3 <= 5
evaluates to true
.In the above examples, notice that the comparison operators return one of two values: true
or false
. Notice that there are no quotes around these values. true
and false
aren't strings — they're called booleans. They simply represent being true or false.
Visit this link for the MDN reference page on relational operators:
We can also compare the equality of two operands with equality operators. This means we can check to see whether or not two operands have the same value. Equality operators return a boolean; true
or false
.
> const myNumber = 5;
> myNumber === 10;
false
> myNumber === 5;
true
Notice the triple equal signs syntax: ===
. This is a type of equality operator called strict equality. We use 3 equal signs ===
when we are asking if two operands are equal to each other. When we're assigning a variable to a value, such as const myNumber = 5
we use a single equal sign =
. Mixing these up is one of the easiest syntax errors to make.
JavaScript also has an equal operator with 2 equal signs, ==
, but it is almost never used and you should generally avoid it. Try out this example in the DevTools Console:
> const myNumber = 5;
> myNumber === 5;
true
> myNumber === "5"
false
> myNumber == "5"
true
The double equals operator returns true
when comparing 5 == "5"
, indicating that a number and a string are the same. Whereas the strict equality operator with 3 equal signs returns false
when comparing 5 === "5"
, because a number and a string are not the same data type. With the double equals operator, JavaScript will make the assumption that you want the two different data types to be evaluated the same. As a developer you may not be expecting that assumption and this can lead to confusing bugs in your code.
Take some time to play around more with the double equals operator and the strict equality operator by visiting the MDN documentation:
We can also check the opposite of equality — not being equal — with the inequality operator !==
.
> const myNumber = 5;
> myNumber !== 10;
true
> myNumber !== 5;
false
Equality operators work for strings as well.
> const greeting = "hello world";
> greeting === "hello world";
true
> greeting === "goodbye";
false
> greeting;
"hello world"
Notice that if you type greeting
after using the equality operators, you will see that the variable greeting
still contains the string "hello world"
. Comparison and equality operators do not change the value of the variable. Let's look at another example to illustrate that important difference between assignment operators and comparison/equality operators.
> let myNumber = 5;
> myNumber === 5;
true
> myNumber === 10;
false
> myNumber = 10;
> myNumber === 10;
true
> myNumber === 5;
false
Notice that we use let
instead of const
here because we reassign the value of myNumber
to 10. We wouldn't be able to do that if myNumber
were a constant variable declared with const
.
Here are some more examples of equality operators.
===
===
means "equal-to".
5 === 5
or "cat" === "cat"
evaluate to true
3 === 5
or "cat" === "dog"
evaluate to false
.!==
!==
means "not-equal-to". It is the opposite of ===
.
"cat" !== "dog"
evaluates to true
5 !== 5
evaluates to false
, because saying that 5 is not equal to 5 is not true.Visit this link for the MDN reference page on equality operators:
Boolean: true
and false
are booleans. They are not strings — they simply represent being true or false.
Assignment operator: Changes the value of the variable on the left of the operator.
Comparison/relational operator: Does not change any values, but returns a boolean (true
or false
) depending on whether the statement evaluates as true or false.
Be aware of the difference between the =
assignment operator and the ===
comparison operator.
=
assign variable on left of operator value on right of operator+=
increase value of variable on left of operator by value on right of operator-=
decrease value of variable on left of operator by value on right of operator*=
multiply value of variable on left of operator by value on right of operator/=
divide value of variable on left of operator by value on right of operator===
is equal to!==
is not equal to>
is greater than>=
is greater than or equal to<
is less than<=
is less than or equal to