How to make mutation of a string? JavaScript

Question

if objects are mutable by default why in this case it dosen't work? How to make mutation value of the key "a" in the object "s"?

var s = {
  a: "my string"
};

s.a[0] = "9"; // mutation
console.log(s.a); // doesn't work

Answer 1

You are trying to change a primitive String, which is immutable in Javascript.

For exmaple, something like below:

var myObject = new String('my value');
var myPrimitive = 'my value';

function myFunc(x) {
  x.mutation = 'my other value';
}

myFunc(myObject);
myFunc(myPrimitive);

console.log('myObject.mutation:', myObject.mutation);
console.log('myPrimitive.mutation:', myPrimitive.mutation);

Should output:

myObject.mutation: my other value
myPrimitive.mutation: undefined

---

But you can define a function in primitive String's prototype, like:

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

var hello="Hello World"
hello = hello.replaceAt(2, "!!")) //should display He!!o World

Or you can just assign another value to s.a, as s.a = 'Hello World'