How Can a Curried JavaScript Function Incrementally Sum Values?

Incrementally Summing Values with a Variadic Curried Function

Can we create a JavaScript function sum that behaves as follows?

sum(1)(2) = 3
sum(1)(2)(3) = 6
sum(1)(2)(3)(4) = 10

In a nutshell, the function should allow us to accumulate a sum by repeatedly passing in additional values.

Initially, we were told that such a function is impossible in JavaScript. However, an alternative approach emerged: adding a sign in front of the sum function.

+sum(1)(2)(3)(4)

The sign, when used in conjunction with a function, coerces the function's returned value to a number. So, in this case, the function sum is effectively returning numbers that are then incremented upon each subsequent call.

To achieve the desired behavior, we can employ a currying technique in our sum function. Here's how the implementation looks like:

function sum(n) {
  var v = function(x) {
    return sum(n + x);
  };

  v.valueOf = v.toString = function() {
    return n;
  };

  return v;
}

console.log(+sum(1)(2)(3)(4)); // Output: 10

This implementation wraps the sum function in the v variable, which is then returned. The v function allows us to incrementally add values to the n variable. By coercing v to a number using the operator, we can obtain the accumulated sum.

In the example, the console.log statement outputs 10, which is the sum of 1, 2, 3, and 4.

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