Javascript undefined Type

Introduction

Javascript undefined type has only one value, which is the special value undefined.

When a variable is declared using var or let but not initialized, it is assigned the value of undefined: 

let message; 
console.log(message == undefined);  // true

In this example, the variable message is declared without initializing it.

When compared with the literal value of undefined, the two are equal.

This example is identical to the following: 

let message = undefined; 
console.log(message == undefined);  // true

Here the variable message is initialized to be undefined.

By default, any uninitialized variable gets the value of undefined.

A variable containing the value of undefined is different from a variable that hasn't been defined.

Consider the following: 

let message;   // this variable is declared but has a value of undefined 
console.log(message);  // "undefined" 
console.log(age);      // causes an error

In this example, the first console.log displays the variable message, which is "undefined".

In the second console.log, an undeclared variable called age is passed into the console.log() function.

It causes an error because the variable hasn't been declared.

We can call typeof on an undeclared variable.

Calling delete on an undeclared variable won't cause an error. 

typeof aVariable; //undefined
delete aVariable; //throw error in strict mode

It throws an error in strict mode.

The typeof operator returns "undefined" on an uninitialized variable.

It also returns "undefined" when called on an undeclared variable. 

let message;   // this variable is declared but has a value of undefined 
console.log(typeof message);  // "undefined" 
console.log(typeof age);      // "undefined"

Note

It is advisable to always initialize variables.

That way, when typeof returns "undefined", we know that it's because a given variable hasn't been declared rather than was not initialized.




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