String concaternation in javascript

Question

I try to build and a JSON object which eventually will save to a file. But I found out if I use to approach one "element+= element", it will throw "Invalid string length" String. But for approach 2, I can concat a very long String and save a file. (The file is approximate 50mb). So i want to know what is wrong with my first approach?

const person = {
    id: 1,
    name: "john"
}
personJson = JSON.stringify(person);
personJson = personJson + ',';

let element = personJson;
for (let index = 0; index < 500; index++) {
     element += element; 
}

let element = personJson;
for (let index = 0; index < 100000; index++) {
     element = element + personJson; 
}

Answer 1

In both the approaches you're producing different results i.e. Suppose personJson contains string "ab"

1. element = element + element; is producing the results

   "ab" + "ab"
   
   "abab" + "abab"
   
   "abababab" + "abababab"
   

and so on... That's why string length is increasing exponentially.

2. element = element + personJson;

   "ab" + "ab"
   
   "abab" + "ab"
   
   "abababab" + "ab"
   

and so on... which makes your string length grow linear.

For better understanding, run below code and see the result:

const person = {
    id: 1,
    name: "john"
}
personJson = JSON.stringify(person);
personJson = personJson + ',';

let element = personJson;
for (let index = 0; index < 500; index++) {
     element += element;
     console.log(element.length); 
}