--description--

A problem posed by Paul Graham is that of creating a function that takes a single (numeric) argument and which returns another function that is an accumulator. The returned accumulator function in turn also takes a single numeric argument, and returns the sum of all the numeric values passed in so far to that accumulator (including the initial value passed when the accumulator was created).

--instructions--

Create a function that takes a number $n$ and generates accumulator functions that return the sum of every number ever passed to them.

Rules:

Do not use global variables.

Hint:

Closures save outer state.

--hints--

accumulator should be a function.

assert(typeof accumulator === 'function');

accumulator(0) should return a function.

assert(typeof accumulator(0) === 'function');

accumulator(0)(2) should return a number.

assert(typeof accumulator(0)(2) === 'number');

Passing in the values 3, -4, 1.5, and 5 should return 5.5.

assert(testFn && testFn(5) === 5.5);

--before-each--

const testFn = typeof accumulator === 'function' && accumulator(3);
if (testFn) {
  testFn(-4);
  testFn(1.5);
}

--seed--

--seed-contents--

function accumulator(sum) {

}

--solutions--

function accumulator(sum) {
  return function(n) {
    return sum += n;
  };
}