When using double equals in JavaScript we are testing for loose equality. Double equals also performs type coercion.
Type coercion means that two values are compared only after attempting to convert them into a common type.
An example will illustrate this. Recall earlier when we tested the following with strict equality:
77 === '77'
// false (Number v. String)
77 does not strictly equal '77' because they have different types. However, if we were to test these values with loose equality…
77 == '77'
// true
You can see we get true. That because of type coercion. JavaScript will actually try to convert our values into a like type. In this case, it succeeds. The string value of '77' can easily be converted into the number value of 77. Since 77 equals 77, we get our answer of true.
Lets look at one more example.
Recall earlier when we tested with strict equality if false equals 0:
false === 0
// false (Different type and different value)
This is obviously false. However, if we run the same equation with loose equality…
false == 0
// true
We get true? Why is this? It has to do with falsy values in JavaScript. We’ll explore this concept in the next section.
Falsy Values
Okay, so why does false == 0 in JavaScript? It’s complex, but it’s because in JavaScript 0 is a falsy value.
Type coercion will actually convert our zero into a false boolean, then false is equal to false.
There are only six falsy values in JavaScript you should be aware of:
false — boolean false
0 — number zero
“” — empty string
null
undefined
NaN — Not A Number
Falsy Value Comparison
The following you can consider to be ‘rules’ of falsy values. These are things you should ultimately memorize if you will be working with JavaScript often.
false, 0, and ""
When comparing any of our first three falsy values with loose equality, they will always be equal! That’s because these values will all coerce into a false boolean.
false == 0
// true
0 == ""
// true
"" == false
// true
2. null and undefined
When comparing null and undefined, they are only equal to themselves and each other:
null == null
// true
undefined == undefined
// true
null == undefined
// true
If you try to compare null to any other value, it will return false.
3. NaN
Lastly, NaN is not equivalent to anything. Even cooler, it’s not even itself!
NaN == null
// false
NaN == undefined
// false
NaN == NaN
// false
Key Takeaways
As you’ve seen, type coercion can get a bit crazy in JS. Unless you’re very familiar with JavaScript, loose equality can lead to more headaches than it’s worth. Memorizing the six falsy values and the rules associated with them can go a long way towards understanding loose equality.
Triple Equals is superior to double equals. Whenever possible, you should use triple equals to test equality. By testing the type and value you can be sure that you are always executing a true equality test.
Type coercion means that two values are compared only after attempting to convert them into a common type.
An example will illustrate this. Recall earlier when we tested the following with strict equality:
77 === '77'
// false (Number v. String)
77 does not strictly equal '77' because they have different types. However, if we were to test these values with loose equality…
77 == '77'
// true
You can see we get true. That because of type coercion. JavaScript will actually try to convert our values into a like type. In this case, it succeeds. The string value of '77' can easily be converted into the number value of 77. Since 77 equals 77, we get our answer of true.
Lets look at one more example.
Recall earlier when we tested with strict equality if false equals 0:
false === 0
// false (Different type and different value)
This is obviously false. However, if we run the same equation with loose equality…
false == 0
// true
We get true? Why is this? It has to do with falsy values in JavaScript. We’ll explore this concept in the next section.
Falsy Values
Okay, so why does false == 0 in JavaScript? It’s complex, but it’s because in JavaScript 0 is a falsy value.
Type coercion will actually convert our zero into a false boolean, then false is equal to false.
There are only six falsy values in JavaScript you should be aware of:
false — boolean false
0 — number zero
“” — empty string
null
undefined
NaN — Not A Number
Falsy Value Comparison
The following you can consider to be ‘rules’ of falsy values. These are things you should ultimately memorize if you will be working with JavaScript often.
false, 0, and ""
When comparing any of our first three falsy values with loose equality, they will always be equal! That’s because these values will all coerce into a false boolean.
false == 0
// true
0 == ""
// true
"" == false
// true
2. null and undefined
When comparing null and undefined, they are only equal to themselves and each other:
null == null
// true
undefined == undefined
// true
null == undefined
// true
If you try to compare null to any other value, it will return false.
3. NaN
Lastly, NaN is not equivalent to anything. Even cooler, it’s not even itself!
NaN == null
// false
NaN == undefined
// false
NaN == NaN
// false
Key Takeaways
As you’ve seen, type coercion can get a bit crazy in JS. Unless you’re very familiar with JavaScript, loose equality can lead to more headaches than it’s worth. Memorizing the six falsy values and the rules associated with them can go a long way towards understanding loose equality.
Triple Equals is superior to double equals. Whenever possible, you should use triple equals to test equality. By testing the type and value you can be sure that you are always executing a true equality test.
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