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