Can we modify the methods of immutable objects in Javascript?

Question

I'm going crazy with the concept of immutability in Javascript. The concept is explained like "after it has been created, it can never change." But what does it exactly mean? I understood the example of the content of a string

 var statement = "I am an immutable value";
 var otherStr = statement.slice(8, 17);

The second line in no way changes the string in statement. But what about the methods? Can you give an example of immutability in methods? I hope you can help me, thank you.

This is not in javascript but the same concept applies. stackoverflow.com/questions/279507/what-is-meant-by-immutable — toskv
– toskv, Commented Oct 4, 2015 at 18:11
Related: Why can't I add properties to a string object in javascript? — James Thorpe
– James Thorpe, Commented Oct 4, 2015 at 18:16

toskv · Accepted Answer · 2015-10-04 18:18:19Z

1

An example of where immutability in a string helps would be when you pass a string to a function so it can be used later (eg. in a setTimeout).

var s = "I am immutable";

function capture(a) {
  setTimeout(function() { // set a timeout so this happens after the s is changed
    console.log("old value: " + a); // should have the old value: 'I am immutable'
  }, 2000);
}

capture(s); // send s to a function that sets a timeout.
s += "Am I really?"; // change it's value
console.log("new value: " + s); // s has the new value here

This way you can be certain that no matter what changes you make to s in the global scope will not affect the value of a (old s) in the scope of the capture function.

You can check this working in this plunker

answered Oct 4, 2015 at 18:18

toskv

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2 Comments

GniruT Over a year ago

Hi, can we say the same if <s> is declared as <s = new String("I am immutable")>? Thanks

toskv Over a year ago

@GniruT yes, check this plnkr. plnkr.co/edit/UbObCDrYeFeixHAOWUqC?p=preview

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