Closure is when a function "remembers" the variables around it even when that function is executed elsewhere"

Shows an example of closure when you return a new function from a surrounding function. This is not the only way to create closure. Other examples he mentions: click handler, timeouts, ajax calls, callbacks

  function foo() {
  var count = 0;
  return function () {
    return count++;
  };
}

var x = foo();

x(); // 0
x(); // 1
x(); // 2

Make it more "functional" by currying or partial application

  function sumX(x) {
  return function(y) {
    return x + y;
  };
}

var add10 = sumX(10);

add10(3);   // 13
add10(14);  // 24

In this pattern, we still have closure, but ​x​ never changes. There's no side effect, where we get a different result on each run.