How to replace a part of specific string FROM string TO string?

One option could be to use a regex to match in a capturing group FROM and capture in a second group any character non greedy (.*?) followed by a lookahead (?= that asserts what what follows is TO

(FROM)(.*?)(?= TO)

Replace with group 1 followed by a whitespace and XXX

$1 XXX

var str = "This is my string that I need to replace a part FROM to a part TO.";
console.log(str.replace(/(FROM)(.*?)(?= TO)/, '$1 XXX'));

There are a few ways you can do this.

A - Using a Regular Expression

var str = "This is my string that I need to replace a part FROM to a part TO.";
var res = str.replace(/FROM(.*)TO/, "FROM XXX TO");

B - Finding the Index and Rebuilding the String

var posFrom = str.indexOf('FROM');
var posTo = str.indexOf('TO', posFrom);
if (posFrom !== -1 && posTo !== -1) {
    var res = str.substring(0, posFrom) + 'FROM XXX TO' + str.substring(posTo + 2)
}

In both approaches you have to make sure the string doesn't have other words that could be confused with the FROM and TO keywords.

Use String#replace with RegExp to mach:

• the beginning (FROM )
• the replaceable range as non-greedy .*?
• and the end ( TO)

and use special replacement patterns $1 and $2:

const string = "This is my string that I need to replace a part FROM to a part TO.";
const pattern = /(FROM ).*?( TO)/;
const replaced = string.replace(pattern, '$1XXX$2');

console.log(replaced);

Or a replacement function in .replace():

const string = "This is my string that I need to replace a part FROM to a part TO.";
const pattern = /(FROM ).*?( TO)/;
const replaced = string.replace(pattern, (fullMatch, fromPart, toPart) => {
  return fromPart + 'XXX' + toPart;
});

console.log(replaced);

To be able to change the beginning and end patterns to match (a.k.a. parameterize), construct your RegExp with the parameters in a function:

const string = "This is my string that I need to replace a part FROM to a part TO.";

function parameterizedReplace(string, from, to, replacement) {
  return string.replace(new RegExp('(' + from + ' ).*?( ' + to + ')'), '$1' + replacement + '$2');
}

const replaced = parameterizedReplace(string, 'FROM', 'TO', 'XXX');

console.log(replaced);

Or using template literals:

const string = 'This is my string that I need to replace a part FROM to a part TO.';

function parameterizedReplace(string, from, to, replacement) {
  return string.replace(new RegExp(`(${from} ).*?( ${to})`), `\$1${replacement}\$2`);
}

const replaced = parameterizedReplace(string, 'FROM', 'TO', 'XXX');

console.log(replaced);

The pattern on regex101

is the replace not enough simple? Be careful in case of multiple instances of the string you are searching all of them will be replaced.

var str = "This is my string that I need to replace a part FROM to a part TO.";
var res = str.replace("to a part", "XXX");

Further information at https://www.w3schools.com/jsref/jsref_replace.asp