Use replace with a regular expression with the global modifier for better browser support. (Check the browser compatibility table on MDN to see which version of each browser started supporting the replaceAll method.)

let string = ":insertx: :insertx: :inserty: :inserty: :insertz: :insertz:";
let newstring = string.replace(/:insertx:/g, 'hello!');
console.log(newstring);

For a more generic solution, we can escape regular expression metacharacters and use the RegExp constructor. You could also add the function to String.prototype as a polyfill.

(It is necessary to escape the string to replace so that characters that have special meanings in regular expressions will be interpreted literally, e.g. . will refer only to actual dots rather than any character.)

//Taken from https://developer.mozilla.org/en-US/docs/Web/JavaScript/Guide/Regular_Expressions
function escapeRegExp(string) {
  return string.replace(/[.*+?^${}()|[\]\\]/g, '\\$&'); // $& means the whole matched string
}
function replaceAll(str, match, replacement){
   return str.replace(new RegExp(escapeRegExp(match), 'g'), ()=>replacement);
}

console.log(replaceAll('a.b.c.d.e', '.', '__'));
console.log(replaceAll('a.b.c.d.e', '.', '$&'));

A specification-compliant shim can be found here.

.replaceAll will be available starting on Chrome 85. The current version is 83.

If you download Google Chrome Canary (which is on version 86), you'll be able to see that your code runs fine. Firefox is on version 78, and since .replaceAll has been available starting version 77, it works there too. It will work on current Safari as well. Microsoft Edge has it as unsupported.

You'll find supported browser versions at the bottom of the article in your question.

If you don't want to upgrade your Chrome nor use reg expressions (since they're less performant), you can also do this:

let string = ":insertx: :insertx: :inserty: :inserty: :insertz: :insertz:";
let newstring = string.split(":insertx:").join('hello!');

And you can, of course, attach to the String prototype if you'd like it everywhere. But since the real replaceAll is more feature filled (supports regex), you'd be instead safer doing:

String.prototype.replaceAllTxt = function replaceAll(search, replace) { return this.split(search).join(replace); }