Understanding JS: Soft (==) vs Hard Equality (===)
January 02, 2020
Let’s see them in more detail.
The Soft Equality
It only checks for equality in value, not in type.
For example if we assign the number 3 to a variable like this:
const a = 3;
This works fine, but there are some slight problems with double equals operator. Let’s test again with the string “3”:
As you can see here, the returned output is
true when in reality it should be
a is not a
string but a
number. So what happened so that we get
" " == 0; << true " " == "0"; << false false == "0"; << true "1" == true; << true "2" == true; << false "true" == true; << false null == undefined; << true
The snippet above shows that values that are not actually equal have a tendency to be reported as being equal to each other.
The Hard Equality
Unlike the soft equality, this equality operator checks for both the value and the type. It is also called a stricter equality test and returns true only if the two elements are the same.
Let’s use the same example as above with hard equality and see how it will behave.
a === 3; << true a === "3"; << false null === undefined << false
As you can see,
a is equal to number 3, but not equal to the string 3. The hard equality operator also correctly reports that
undefined are two different values.
I have written a tutorial about
undefined that you can check here
Which one to use?
This is a good question, and the simple and quick answer is it depends upon your need. If your program does not need to check for the value type before doing operations (i.e you’re certain that both sides will still have the same type), you can go for soft equality. In the other case and by default, I would suggest to always go for hard equality. Some bugs in program are due to the use of soft equality and it is often difficult to discover them.