Replace all occurrences of a string in JavaScript

To replace all occurrences of a string in JavaScript, you can use the replace() method of a string along with a regular expression with the global flag /g. Here’s an example:

let str = "Hello, world! Hello, John! Hello, Jane!";
let newStr = str.replace(/Hello/g, "Hi");
console.log(newStr); // "Hi, world! Hi, John! Hi, Jane!"

In the example above, the regular expression /Hello/g matches all occurrences of the string “Hello” in the original string str. The replace() method replaces each match with the string “Hi” and returns a new string newStr.

Note that the global flag /g is necessary to replace all occurrences of the string. Without the global flag, the replace() method would only replace the first occurrence of the string.

In Summary , replacing a single time, use:

const res = str.replace('Hello', "Hi");

For replacing multiple times, use:

const res = str.replace(/Hello/g, "Hi")

Replace all commas with underscore in javascript:

let str = "Hello,How,are,you";
let newStr = str.replace(/,/g, "_"); 
or 
let newStr = str.replaceAll(",", "-");
console.log(newStr); // "Hi_How_are_you"