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https://www.robinwieruch.de/javascript-variable-question-mark

JavaScript Variable with Question Mark

If you are new to JavaScript, the question mark after a variable may be confusing to you. Let's shed some light into it. The question mark in JavaScript is commonly used as conditional operator -- called ternary operator when used with a colon (:) and a question mark (?) -- to assign a variable name conditionally.

const isBlack = false;
 
const text = isBlack ? 'Yes, black!' : 'No, something else.';
 
console.log(text);
// "No, something else."

Either the expression is true and returns the value after the question mark (?) or the expression is false and returns the value after the colon (:).

This kind of JavaScript variable declaration is used as a shorthand though. You can achieve the same with the "if-else"-statement in JavaScript as conditional operator in contrast to the ternary operator, but it turns out more verbose:

const isBlack = false;
 
let text;
if (isBlack) {
  text = 'Yes, black!';
} else {
  text = 'No, something else.';
}
 
console.log(text);
// "No, something else."

If this is not what you are looking for, then maybe you are searching for JavaScript's optional chaining feature. It is used to assign a variable conditionally:

const person = {
  name: 'Robin Wieruch',
  pet: {
    name: 'Trixi',
  },
};
 
const petName = person.pet?.name;
console.log(petName);
// "Trixi"

If the person has no pet, the output would be undefined without throwing a JavaScript exception.

const person = {
  name: 'Robin Wieruch',
};
 
const petName = person.pet?.name;
console.log(petName);
// undefined

When this feature was not available in JavaScript, it was common to use the AND (&&) operator or the ternary operator (?:) from before to avoid any JavaScript exceptions:

const person = {
  name: 'Robin Wieruch',
};
 
let petName = person.pet && person.pet.name;
console.log(petName);
// undefined
 
petName = person.pet ? person.pet.name : undefined;
console.log(petName);
// undefined

Most commonly you will find the question mark in JavaScript for these two use cases. Either it is used as shorthand conditional operator instead of the commonly used "if-else"-statement or as optional chaining operator to assign variables conditionally without hitting an exception.