Call function with the specified parameter number

In javascript, there are some situation we only want to call a function with one, two or three parameter(s), but input of it maybe there are a lot of parameters. For example, we want to repeat every element in array one time:

const data = [2, 11, 6, 10];
// expected [22, 1111, 66, 1010]

What will you do?

Simplest, we can do:

const data = [2, 11, 6, 10];

data.map(el => el.toString()).map(el => el.repeat(2)).map(el => Number(el));

// output [22, 1111, 66, 1010]

Yeah, that ok, but we can do something to make it more elegant :)

we have a repeat function like this:

function repeat(inp, times){
    if(undefined === times) {
        times = 2;
    }

    let result = "";
    for (let i = 0; i < times; ++i) {
        result += inp;
    }
    return result;
}

And …

const data = [2, 11, 6, 10];

data.map(Number.prototype.toString).map(repeat).map(Number);

// But ...
// Uncaught TypeError: Function.prototype.apply was called on undefined, which is a undefined and not a function

What happen?

Because toString function of class Number can receive more than 1 parameter, so when you call that, it will use second value of map function as radix number.

map will put to callback function 3 parameter: value, index and this array

Resolve it…

We can use a technique to help the function will receive only one parameter, Write unique function:

function unique (fn) {
    return (...params) => fn.hasOwnProperty("arguments") ? fn(params[0]) : fn.apply(params[0]);
}

unique function will help fn function invoke with only one parameter,

Now re-write it:

const data = [2, 11, 6, 10];

data.map(unique(Number.prototype.toString)).map(repeat).map(Number);

// (4) [0, 11, 66, 101010]

Soah, what happen?

Problem occurs with repeat function as well. So, rewrite it

const data = [2, 11, 6, 10];

data.map(unique(Number.prototype.toString)).map(unique(repeat)).map(Number);
// output (4) [22, 1111, 66, 1010]

Ok, maybe you not feel elegant, but you can consider it as alternative solution to resolve problem if possible :)