Introduction To Programming
10-152-310

Learning Plan 3 : Math and Operators

Equality Operators

Operator Function
== Tests whether two expressions are equal
!= Tests whether two expressions are not equal
=== Tests whether two expressions are really equal
!== Tests whether two expressions are really not equal

Using the Equality Operators

These operators are used in "if" and "while" statements. We'll add those in the next unit but here's some samples showing what they look like.

Operators That Test for Equality

Equality Example

Boolean Expressions

Boolean expressions are just statements that are either true or false.

Does 5 equal 5?

        (5 === 5)    True!

Does "Fred" equal "Barney"?

        ("Fred" === "Barney")   False!

If a variable named 'count' has a value of 3. What is result of this expression?

        (count === 3)    True!

How about this?

        (100 === "100")   ???

We can see the results of an equality test with a simple document.write();

Testing for Equality with Type Conversion (==)

The == operator has a trick. It will convert types to match before it does the equality test.

        (100 == "100")   What will this be? It's true!

The "100" will first be converted to the number 100 and then compared to 100.

It does NOT convert the case of strings.

You can use variables in the test

Testing for Inequality With Type Conversion (!=)

The != operator will convert types and then test to see if the values are not equal. This can be confusing, if the values are not equal then the test returns true.

The 25 will be converted to "25" and then tested for inequality with the "25". Because they are equal the test will return false.

This test will return true because 10 is not equal to 11.

Just convert ("Bob" != "Bill") into a question: "Is 'Bob' different from 'Bill'?" Yes, they are different.

Testing for Equality Without Type Conversion (===)

The === operator will not do any conversions, it will just do a strict comparison. If the types don't match then you get false. This is the equality operator that we'll use most of the time.

If they are the same type then it's just the same as ==

This will now return false.

Note: This is called "Strict Equality" and is usually what you should use. Only use the non-strict == when you know that's what you want.

Important Note: It is a course standard that you use === whenever possible, unless directed to do otherwise.

Testing for Inequality Without Type Conversion (!==)

The !== tests for inequality with no type conversion.

This works like != with no type conversion so we often get different results.

For numbers it's just the same as !=.

The problem with == (And why you shouldn't use it).

JavaScript's auto-conversion can sometimes give us unexpected results. Here's an example. What do you think this should be?

        ('' == 0)  true or false?

Is that what we want to happen? What happens with ===?

The course standard is to always use === and !== whenever possible.