null vs undefined

Similarities
Both are primitives
Both are falsy:
Boolean(undefined) //-> false
Boolean(null) //-> false
Differences
undefined means that a variable has not been declared, or it has been declared but has not yet been assigned a value.
null is an assignment value that means “no value”. Javascript itself never sets a value to null
Difference in typeof:
typeof null; //-> "object"
typeof undefined; //-> "undefined"
undefined is not a valid JSON while null is.
Tips
Check if a variable is null:
variable === null;
Check if a variable is undefined:
typeof variable === 'undefined';
​
// ⬇️ or this, but will throw ReferenceError if variable wasn't defined
variable === undefined;
Comparison (Equality returns true and identity returns false) :
undefined == null; //-> true
​
undefined === null; //-> false

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