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.

Documentation / Reference

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