Javascript - Undefined

When the value of variable or property is not defined, it gets the type undefined.

It's handy to see if a variable is passed as an argument of a function.

Are undefined:

• a not declared variable
• a declared variable without value
• functions that return no values
• the void operator.

if (typeof sec === 'undefined') console.log("The variable sec is not defined");

The question mark permits to manage an undefined object.

For instance, if location is undefined, you can write:

country = location?.getCountry()

country will then be undefined.

• Reference/Global_Objects/undefined