I'm currently doing the JustJavascript course, which I highly recommend, and I've learned how equality of values works in JavaScript.
There are 3 kinds of equality in JavaScript.
-
Same value equality:
Object.is(a, b)
. -
Strict equality:
a === b
(triple equals). -
Loose equality:
a == b
(double equals).
Same value equality
Object.is(a, b)
tells us if a
and b
are the same value:
Object.is(2, 2); // 🟢 true
Object.is(undefined, undefined); // 🟢 true
Object.is(null, null); // 🟢 true
Object.is(true, true); // 🟢 true
Object.is(1, 1); // 🟢 true
Object.is(-1, -1); // 🟢 true
Object.is("Hello", "Hello"); // 🟢 true
Object.is({}, {}); // 🔴 false
Object.is([], []); // 🔴 false
Strict equality
Strict equality works like Object.is
but there are two exceptions.
1. NaN === NaN
is false
, although they are the same value in JavaScript.
There are some ways to safely check if two values are NaN
:
Number.isNaN(variable)
Object.is(variable, NaN)
-
variable !== variable
NaN === NaN; // 🔴 false
Object.is(NaN, NaN); // 🟢 true
Number.isNaN(NaN); // 🟢 true
NaN !== NaN; // 🟢 true
2. -0 === 0
and 0 === -0
are true
, although they are different values in JavaScript.
In the common math that we all learn at school negative zero does not exist, but it exists in floating-point math for practical reasons.
0 === -0; // 🟢 true
Object.is(0, -0); // 🔴 false
Loose equality
Loose equality is very confusing and that's why it's recommended not to use it. As an example, see these cases:
[[]] == ""; // true
true == [1]; // true
false == [0]; // true
If you still want to learn how it works, you can read more about it here.
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