Closures

map.js

'use strict';

function mul(x) {
  return x * x;
}

function mulWVariance(variance, x) {
  return x * x + variance;
}

function mulWVarianceCallBack(variance) {
  return function(x) {
    return x * x + variance;
  };
}

console.log([1, 2, 3].map(mul)); // [1, 4, 9]

console.log([1, 2, 3].map(mulWVariance.bind(null, 2))); // [3, 6, 11]

console.log([1, 2, 3].map(mulWVarianceCallBack(2))); // [3, 6, 11]

sum.js

function sum (a, b) {
  return b ? a + b : b => a + b
}

console.log( sum(5, 3) ); // 8
console.log( sum(5)(3) ); // 8