When does JavaScript variable return undefined?

if we didnt assign the value for the given input

In JavaScript, undefined means a variable has been declared but has not yet been assigned a value, such as: var TestVar; alert(TestVar); //shows undefined alert(typeof TestVar); //shows undefined. null is an assignment value.

A JavaScript variable returns undefined when it has not been assigned a value. For example: var a = "Test String"; // defined var b; // undefined if (a === undefined) { var1 = "a is undefined"; } else { var1 = "a is defined"; } if (b === undefined) { var2 = "b is undefined"; } else { var2 = "b is defined"; } res = var1 + ", " + var2; The result of res will be: a is defined, b is undefined