How to add a character to the beginning of every word in a string in javascript?

You could map the splitted strings and add the prefix. Then join the array.

var string = "Hello there! How are you?",
    result = string.split(' ').map(s => '#0' + s).join(' ');

console.log(result);

You could use the regex (\b\w+\b), along with .replace() to append your string to each new word

\b matches a word boundry

\w+ matches one or more word characters in your string

$1 in the .replace() is a backrefence to capture group 1

let string = "Hello there! How are you?";
let regex = /(\b\w+\b)/g;

console.log(string.replace(regex, '#0$1'));

You should use map(), it will directly return a new array :

let result = some_string.split(" ").map((item) => {
    return "#0" + item;
}).join(" ");

console.log(result);

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