JS: undefined | JavaScript | Tutorials by PiTribe

JavaScript undefined

A uninitialized variable is undefined.


var v;
console.log(typeof v);

undefined

Value of a variable is undefined until set. This can also be said that undefined is the default value of a variable when declared only.


var v;
console.log('old:', typeof v);
v = 'PiTribe';
console.log('new:', typeof v);

old: undefined new: PiTribe

undefined refers an uninitialized variable.

undefined can be assigned to a variable, explicitly.


var v;
console.log('old:', typeof v);
v = 'PiTribe';
console.log('new:', typeof v);
v = undefined;
console.log('last:', typeof v);

old: undefined new: PiTribe last: undefined

undefined is a falsy in logical comparisions.


var v;	//v is undefined

if(v)
alert('Truly');
else
alert('Falsy');

Falsy

undefined implies that the variable is virgin. A variable can be set to undefined again later in the program, there is no prohibtion.

undefined is truly (true) with double equal == operator.


var v;	//v is undefined

if(v == null)
alert('Truly');
else
alert('Falsy');

Truly

This is because JavaScript cast/convert true to null. But this is not the case with tripple equal === operator.


var v;	//v is undefined

if(v === null)
alert('Truly');
else
alert('Falsy');

Falsy

Arithmetic operations with undefined result NaN i.e. +, -, /, * & % operators etc.


var v;	//v is undefined

var u = v + 1;
alert (u);

NaN

This is because JavaScript cast/convert true to null. But this is not the ase with tripple equal === operator.