Very often you can hear that in JavaScript "Functions are first-class citizens".

All it means is that it's possible to:

• Assign a function to a variable
• Pass a function to another function (callback)
• Return a function from another function

Store functions in variables

const sayHi = () => console.log('Hi!');

sayHi();
// Hi!

Pass a function to another function (callback)

function sayHi() {
  console.log('Hi!');
} 

// Whatever function, you pass to it, it's going to
// call it twice
const repeatTwice = (callback) => {
  callback();
  callback();
}

repeatTwice(sayHi);
// Hi!
// Hi!

Return a function from another function

function getSayHi(name) {
  return function() {
    return `Hi, ${name}!`
  }
}

const sayHiToJohn = getSayHi('John');
const sayHiToMary = getSayHi('Mary');

sayHiToJohn();
// Hi, John!

sayHiToMary();
// Hi, Mary!

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References

• MDN: First-class Function
• Syntax Podcast: Syntactic Sugar, Declarative and First Class Citizens? What does that even mean?