Undefined Error V/s Reference Error

JavaScript Error Handling- at a glance

Undefined Error

An undeclared variable is assigned the value undefined at execution It is also of type Undefined. Any property that has not been assigned a value, assumes the undefined value.

var undefinedVar;
console.log(undefinedVar); // undefined
console.log(typeof undefinedVar); // "undefined"

undefined(); // undefined
console.log(typeof undefined); // "undefined"

In JavaScript there is Undefined (type), undefined (value) and undefined (variable).

Undefined (type) is a built-in JavaScript type.

undefined (value) Any property that has not been assigned a value, assumes the undefined value by default.

undefined (variable) is a global property whose initial value is undefined (value), Since its a global property we can also access it as a variable.

Something is undefined when it hasn’t been defined. If you call a variable or function without actually creating it yet the parser will give you an not defined error.

Simple fix to this,

var definedVar = 'Aastha';
console.log(undefinedVar); // Aastha
console.log(typeof undefinedVar); // String

function definedFunction(){
  return "Defined now"
}
console.log(undefined()); // Defined now
console.log(typeof undefined); // function

Reference Error

ReferenceError is thrown when trying to access a previously undeclared variable. A ReferenceError is thrown when trying to dereference a variable that has not been declared.

function undefineddemo() {
  console.log(variablenotdefined);
}
undefineddemo(); // here variablenotdefined is not referenced 

Fix, Now this will give undefined TypeError

function undefineddemo() { 
   var variablenotdefined
   console.log(variablenotdefined);
}
undefineddemo(); // Here variablenotdefined is not defined

Quick Fix,

function undefineddemo() { 
   var variabledefined = 100;
   console.log(variabledefined);
}
undefineddemo(); // Here variablenotdefined is not defined

That’s all folks!