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

Javscript string are immutable Check this answer. https://stackoverflow.com/questions/51185/are-javascript-strings-immutable-do-i-need-a-string-builder-in-javascript — yue you, Feb 08 '18 at 19:01
Possible duplicate of [Are JavaScript strings immutable? Do I need a "string builder" in JavaScript?](https://stackoverflow.com/questions/51185/are-javascript-strings-immutable-do-i-need-a-string-builder-in-javascript) — Terry, Mar 24 '18 at 15:58

score 7 · Accepted Answer · edited Feb 05 '22 at 08:59

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'

@JonasW. Thanks for your suggestion – yue you Feb 08 '18 at 19:16 — yue you, Feb 08 '18 at 19:16

score 3 · Answer 2 · answered Feb 08 '18 at 19:03

3

Strings in JavaScript are immutable. This means that you cannot modify an existing string, you can only create a new string.

var test = "first string";
test = "new string"; // same variable now refers to a new string

answered Feb 08 '18 at 19:03

Elliot B.

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score 3 · Answer 3 · answered Feb 08 '18 at 19:12

3

You try to mutate a string which not possible, because strings are immutable. You need an assignment of the new value.

Below a fancy style to change a letter at a given position.

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

s.a = Object.assign(s.a.split(''), { 0: "9" }).join('');

console.log(s.a);

answered Feb 08 '18 at 19:12

Nina Scholz

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so can you think of mutable example? const egg = { name: "Humpty Dumpty" }; egg.isBroken = false; console.log(egg); – Yellowfun Feb 08 '18 at 21:06
pardon, i don't understand. – Nina Scholz Feb 08 '18 at 21:08
const egg mutated in my example? – Yellowfun Feb 09 '18 at 09:37
2

`const` means only that you can not assign something to egg, but not to the properties. this is always possible. – Nina Scholz Feb 09 '18 at 09:39
than it is the good example of mutation? Or it's not mutation? – Yellowfun Feb 09 '18 at 09:40
2

it is mutation, obvious. but not a good (opinion based) example, just an example. – Nina Scholz Feb 09 '18 at 09:43

vol7ron · Answer 4 · 2018-03-24T16:31:05.520

You are trying to mutate the string using element accessor, which is not possible. If you apply a 'use strict'; to your script, you'll see that it errors out:

'use strict';

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

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

If you want to replace the character of the string, you'll have to use another mechanism. If you want to see that Objects are mutable, simply do s.a = '9' instead and you'll see the value of a has been changed.

'use strict';

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

s.a = s.a.replace(/./,'9')
console.log(s.a);

How to make mutation of a string? JavaScript

4 Answers4

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