String.prototype.replaceAll() not working

There is no replaceAll function in JavaScript.

You can use a regex with a global identifier as shown in pst's answer:

a.replace(/:/g,"hi");

An alternative which some people prefer as it eliminates the need for regular expressions is to use JavaScript's split and join functions like so:

a.split(":").join("hi");

It is worth noting the second approach is however slower.

Update: All recent versions of major browsers, as well as NodeJS 15+ now support replaceAll

Original:

There is no replaceAll in JavaScript: the error console was probably reporting an error.

Instead, use the /g ("match globally") modifier with a regular expression argument to replace:

const a = "::::::";
const replaced = a.replace(/:/g,"hi");
console.log(replaced);

The is covered in MDN: String.replace (and elsewhere).