ES6 what is this triple fat arrow syntax

Question

Ran into some code that looks like the following:

return store => next => action => {
    switch(action.type) {
    ...
    default:
      return next(action)
};

The entire sample is here: https://exec64.co.uk/blog/websockets_with_redux/

What is the triple arrow syntax here? I'm familiar with arrow functions, but I've never seen more than one used before to define a function.

Answer 1

It is an arrow function with argument store which returns another arrow function with argument next which returns another one with argument action . Analogy with regular functions would be:

return function (store) {
  return function(next) {
    return function(action) {
      switch(action.type) {
      ...
      default:
        return next(action)
    }
  }
}

Just to notice that this syntax:

const myFunction = someParam => someValue

is shorthand for:

const myFunction = someParam => {
  return someValue
}