let str = 'Learning JS is easy, I like JS programming!';
let newStr = str.replace('JS','JavaScript');

// Replacing the first occurrence of the substring JS in given string

console.log(newStr);

/*
Output:
Learning JavaScript is easy, I like JS programming!
*/

// Using regular expression all occurences of a substring in a string can be replaced.
// global flag 'g' is used to replace all occurences of JS in a given string

let str = 'Learning JS is easy, I like JS programming!';
let newStr = str.replace(/JS/g,'JavaScript');

console.log(newStr);

/*
Output:
Learning JavaScript is easy, I like JavaScript programming!
*/

// To ignore cases for search and replacement, ignore flag 'i' can be used.

let str = 'Learning JS is easy, I like js programming!';
let newStr = str.replace(/JS/gi,'JavaScript');

console.log(newStr);

/*
Output:
Learning JavaScript is easy, I like JavaScript programming!
*/

// Instead of passing newSubString as a second argument to the replace(), we can pass replacement function

// Syntax is as follows :

let newStr = str.replace(substr | regexp, replacer);

// Using this syntax, the replace() method invokes the replacer function after the match has been performed. 

let str = "I want to learn nodejs and npm";
let regx = /nodejs|npm/gi;

// here replace() function change the substrings nodejs and npm to uppercase. 
// a replacer function is passed into the replace() function:

let newStr = str.replace(regx, (match) => { 
    console.log({match}); 
    return match.toUpperCase();
});

// match: is the matched substring.

console.log(newStr);

/*
Output:
{match: "nodejs"}
{match: "npm"}
I want to learn NODEJS and NPM 
*/

/*
Note: 
Use a regular expression with the global flag (g) to replace all occurrences of a substring with a new one.
*/