JSFiddle - React, Tailwind, and code Playground

JavaScript

String.prototype.replaceFromIndex=function(index, length, replace) {
    return this.substr(0, index) + replace + this.substr(index+length);
}

var search = '[BACK]';
var str="itss[BACK][BACK][BACK][BACK][BACK][BACK] it's a test stringgg[BACK][BACK]";

while((index = str.indexOf(search)) >= 0){
    str = str.replaceFromIndex(index-1, search.length+1, '');    
}

document.write(str);
alert(str);