Point-Free

[Point-free] is a programming paradigm in which function definitions do not identify the arguments (or "points") on which they operate.

Gotchas in JavaScript

As much as point-free style is useful in other (functional) languages, in JavaScript it often brings problems that might not be worth the conciseness it brings. I still use it sometimes when the function called is under my control. After these experiences I will be more careful with it, though.

Gotcha #1: Function taking more parameters than you expected

const nums = ["25", "45", "11"];
// logs [ 25, 45, 11 ]
console.log(nums.map((num) => Number.parseInt(num)));
// logs [ 25, NaN, 3 ]
console.log(nums.map(Number.parseInt));

The reason for this is that while Number.parseFloat only takes one argument – the string to parse – Number.parseInt takes an additional optional argument – the radix of the number to be output (for example 16 for hexadecimal strings). Thus when used in a map like that this is what actually happens:

console.log(
  nums.map((item, index, array) =>
    Number.parseInt(/* string: */ item, /* radix: */ index, array)
  )
);

As we can see the radix argument of Number.parseInt is set using the index of the current item. That explains the 3 output for the 11 input as 3 is 11 in binary.

This is the first type of issue that can arise from point-free in JavaScript: functions taking more arguments than you expect.

There is no fool-proof way to protect yourself against this other than using point-free only with functions you know the signature of and know are not going to change, otherwise your code can break unexpectedly.

Solution: Ramda unary

Gotcha #2: Unexpected this

const obj = {
  message: "Hello",
  getMessage() {
    console.log(this.message);
  },
};

// Broken
setTimeout(obj.getMessage, 0);

There are two way to fix this:

// Fix A - closure
setTimeout(() => obj.getMessage(), 0);
// Fix B - binding
setTimeout(obj.getMessage.bind(obj), 0);

The first one uses a closure to implicitly set this of the getMessage call to the proper value.

The second (point-free) one makes use of the bind method to set the value of this explicitly.