Why does null equal undefined in one comparison but not in another?
When comparing using ==, undefined is equal to null.
true
However, when using !==, undefined is not equal to null.
true
This means that undefined and null are not strictly equal.
false
And in terms of inequality, undefined is also not not equal to null.
false
When it comes to variables like undefined and null, they may seem empty at first glance. However, the key to understanding their behavior lies in comparison operators. The == operator can perform type conversion while comparing values, whereas the === operator strictly tests for equality without type conversion. So in essence, null is essentially equivalent to undefined. It may sound confusing initially, but with a few examples, it becomes clearer:
'' == '0' // false
0 == '' // true
0 == '0' // true
false == 'false' // false
false == '0' // true
false == undefined // false
false == null // false
null == undefined // true
Kindly refer to my response below.
"==" essentially changes the type of the variable before conducting a comparison. i. Therefore, when undefined == null is evaluated, it returns true because both variables coerce to false (representing empty values) and are then deemed equal after comparison.
On the other hand, !== performs a strict comparison without altering the type. In the case of undefined, its type remains as "undefined," while for null, its type is "object," which can be confirmed using typeof.
ii. Hence, since the types differ,!== results in true: undefined !== null.
iii. Similarly, === conducts a strict comparison where undefined === null yields false due to mismatched types.
iv. Finally, undefined != null results in false, as != like == coerces both variables to false before comparing them, hence giving the appearance of equality and returning false.
== is used to compare variable values after converting them to the same type through type coercion.
=== compares both values and types of variables without performing any type coercion, returning true only if they are identical.
Here are some examples to illustrate this:
console.log(5 == "5"); // Returns true because no type coercion is done.
console.log(5 === "5"); // Returns false because although values are equal, data types (number vs string) are not.
console.log(undefined == null); // Returns true as no type coercion is performed and both values are falsy.
console.log(undefined !== null); // Returns true because there is a mismatch in data types due to type coercion.
console.log(undefined === null); // Returns false because strict comparison shows their different data types.
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