Can === hold when == doesn't?

Question

Are there any cases where

x == y //false
x === y //true

is that ever possible in JS? :)

I wouldnt think so. But then again, javascript is never short of surprises — AmmarCSE, May 22 '15 at 19:57
if the first line changed `x` or `y` (using a mutating getter or even an overloaded valueOf), then it could happen, but on it's face, no. — dandavis, May 22 '15 at 20:18
the most illustrative and educational way to see that it is not possible is to look at this table https://dorey.github.io/JavaScript-Equality-Table/ — jJ', May 25 '15 at 03:55

score 10 · Answer 1 · answered May 22 '15 at 19:57

10

No. That's never possible. === checks for type and equality. == just checks for equality. If something isn't == it can never be ===.

answered May 22 '15 at 19:57

Justin Niessner

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score 8 · Accepted Answer · answered May 22 '15 at 19:59

It'd be impossible. == compares value, while === compares value and type. Your case would require an impossible condition.

a === b -> (typeof(a) == typeof(b)) && (value(a) == value(b))
a == b ->  (value(a) == value(b))

You couldn't have the value comparisons in the == case be true while requiring the exact same comparison in === become false.

score 3 · Answer 3 · answered May 22 '15 at 19:59

3

== - Returns true if the operands are equal.

=== - Returns true if the operands are equal and of the same type.

So, I'll say not possible.

answered May 22 '15 at 19:59

Arup Rakshit

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score 2 · Answer 4 · answered May 22 '15 at 20:09

Briefly, if === is true, then == will return true. If === returns false, then == may or may not return false.

Examples:

5===5 is true, which means that 5==5 also must be true.

'5'===5 is false, and '5'==5 is true.

'6'===5 is false, and '6'==5 is also false.

This behavior is because a===b checks to make sure that the value and type of a and b are equal, while a==b only checks to make sure that their values are equal.

Can === hold when == doesn't?

4 Answers4