Closure.md

Closure

A programming pattern that allows variables to be accessible from an outer lexical scope inside nested blocks of code.

Adding example

An example of closures can be seen in this function that returns another function. The trick to understanding this is understanding how to implement/test the function.

function translate2d(dx, dy) {
  return function(x, y) {
    return [x + dx, y + dy];
  };
};

Since translate2d returns a function we should store the results in a var that is callable:

// sets dx, dy with 1, 1
const translator = translate2d(1, 1);

// sets x, y and executes the inner function
translator(4, 6) // [5, 7]

After the variable calls, we can see that the addition on line 12 takes place.

For the first call 'dx' and 'dy' are being set to (1, 1), respectively, becasue translate2d returns the inner function.

So when we call translator, we're making the inner call with new x and y values. Executing the addition of x+dx and y+dy.

The closure is that the dx, and dy vars were saved when we set the translator variable.