string placeholder replacement program in javascript

Question

function format(template, ...values) {
    return template.replace(/{(\d+)}/g, (_,i) => values[i]);
}

The above solution returns a formatted string. for example format("{0}{1}{2}{1}{0}", "k", "a", "y") returns "kayak".

I know the string replace function in JS , Here I didn't get the arrow function

but how the i value is incrementing? i mean i am not able to understand how the placeholders are replaced by the values with the arrow function — Bala Krishna Kesani, Feb 02 '20 at 05:51
The second argument is the capture group. `{0}` will capture `0`, eg, so the second argument passed to `format` will be the replacement — CertainPerformance, Feb 02 '20 at 05:53
The regex matches constructs like `{0}`, `_` will be `{0}` (the entire regex) and `i` will be `0` (the first capturing group), the arrow function returns `values[0]` for the first match replacing `{0}` with `k`. — Titus, Feb 02 '20 at 05:57
I am only confused about the i variable incrementing part , there is no loop here then how i value is going from 0 to 2 — Bala Krishna Kesani, Feb 02 '20 at 06:03
It is not incrementing, `i` is the first capturing group (`(\d+)`) which is one or more consecutive numbers (`\d` number, `+` one or more), for the string `{0}{1}{2}{1}{0}`, the arrow function will be called 5 times, on time for each of the numbers in the string. — Titus, Feb 02 '20 at 06:06
@CertainPerformance please reopen this question, i am still looking for a clear explanation, i got the regex part I am editing my question — Bala Krishna Kesani, Feb 02 '20 at 06:07
The `_` is just an argument that goes unused. It's just a variable name, it doesn't mean anything — CertainPerformance, Feb 02 '20 at 06:08

score 0 · Accepted Answer · answered Feb 02 '20 at 06:11

_ in fat arrow function is called as throwaway variable.

The underscore symbol _ is a valid identifier in JavaScript, and in your example, it is being used as a function parameter.

A single underscore is a convention used by some javascript programmers to indicate to other programmers that they should "ignore this binding/parameter". Since JavaScript doesn't do parameter-count checking the parameter could have been omitted entirely.

This symbol is often used (by convention again) in conjunction with fat-arrow functions to make them even terser and readable, like this:

const fun = _ => console.log('Hello, World!')
fun()

In this case, the function needs no params to run, so the developer has used the underscore as a convention to indicate this. The same thing could be written like this:

const fun = () => console.log('Hello, World!')
fun()

The difference is that the second version is a function with no parameters, but the first version has a parameter called _ that is ignored. These are different though and the second version is safer, if slightly more verbose (1 extra character).

Also, consider a case like

arr.forEach(function (_, i) {..})

Where _ indicates the first parameter is not to be used.

i tried to remove the underscore and ran the solution i got undefined in the output,if possible please breakdown the entire solution — Bala Krishna Kesani, Feb 02 '20 at 06:15
@BalaKrishnaKesani That is because in that case, `i` will be the substring that matches the entire regex, you should take a look at [`String.replace`](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/String/replace#Specifying_a_function_as_a_parameter) to see how replace with a function works. — Titus, Feb 02 '20 at 06:19

string placeholder replacement program in javascript

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