Basics

Closures

A closure is the combination of a function and lexical environment which that function was declared. It executes with the scope that was defined in place, not with the state that's in place when it is executed. A function with lexical scope is able to access variables that were defined in the parent scope.

const prepareBark = dog => {
  const say = `${dog} barked!`
  return () => {
    console.log(say)
  }
}

const rogerBark = prepareBark(`Roger`)
const sydBark = prepareBark(`Syd`)

rogerBark() // Roger barked!
sydBark() // Syd barked

When we redefine a new say variable, it doesn't change the state of the first scope of prepareBark()

Why would you use one?

• Data privacy / emulating private methods with closures. Commonly used in the module pattern.

• Partial applications or currying.

References

• https://developer.mozilla.org/en-US/docs/Web/JavaScript/Closures