Closure

  • combination of a function enclosed with references to its surrounding state (lexical environment)
  • gives you access to an outer function's scope from an inner function

Application

  • primary mechanism used to enable data privacy
    • enclosed variables are only in scope within the containing (outer) function
    • can't get the data from an outside scope except through the object's privileged methods (any exposed method defined within the closure scope)
function counter() {
  var count = 0; //only accessible through methods returned

  return {
    inc: function () { count++; },
    dec: function () { count--; },
    getCount: function () { return count; }
  };
}

var count1 = new counter();

count1.inc();
count1.inc();
console.log(count1.getCount());

count1.dec();
console.log(count1.getCount());
  • partial application and currying
    • a function that takes a function with multiple parameters and returns a function with fewer parameters.
    • takes advantage of closure scope in order to fix parameters
const partialApply = (fn, ...fixedArg) => {
  return function (...remainingArg) {
    return fn.apply(this, fixedArg.concat(remainingArg));
  };
};

const add = (a, b) => a + b;

const add10 = partialApply(add, 10);

console.log(add10(5));

Resources