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
_ 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 will be the substring that matches the entire regex, you should take a look at String.replace to see how replace with a function works.
{0}will capture0, eg, so the second argument passed toformatwill be the replacement{0},_will be{0}(the entire regex) andiwill be0(the first capturing group), the arrow function returnsvalues[0]for the first match replacing{0}withk.