What does immutable mean?

javascript variables immutability
alex picture alex · Jul 8, 2010 · Viewed 37.6k times · Source

If a string is immutable, does that mean that.... (let's assume JavaScript)

var str = 'foo';

alert(str.substr(1)); // oo

alert(str); // foo

Does it mean, when calling methods on a string, it will return the modified string, but it won't change the initial string?

If the string was mutable, does that mean the 2nd alert() would return oo as well?

Answer

kemiller2002 picture kemiller2002 · Jul 8, 2010

It means that once you instantiate the object, you can't change its properties. In your first alert you aren't changing foo. You're creating a new string. This is why in your second alert it will show "foo" instead of oo.

Does it mean, when calling methods on a string, it will return the modified string, but it won't change the initial string?

Yes. Nothing can change the string once it is created. Now this doesn't mean that you can't assign a new string object to the str variable. You just can't change the current object that str references.

If the string was mutable, does that mean the 2nd alert() would return oo as well?

Technically, no, because the substring method returns a new string. Making an object mutable, wouldn't change the method. Making it mutable means that technically, you could make it so that substring would change the original string instead of creating a new one.

Related questions

How can I determine if a variable is 'undefined' or 'null'?

How do I determine if variable is undefined or null? My code is as follows: var EmpName = $("div#esd-names div#name").attr('class'); if(EmpName == 'undefined'){ // DO SOMETHING }; <div id="esd-names"> <div id="name"></div&…

Read More
JavaScript check if variable exists (is defined/initialized)

Which method of checking if a variable has been initialized is better/correct? (Assuming the variable could hold anything (string, int, object, function, etc.)) if (elem) { // or !elem or if (typeof(elem) !== 'undefined') { or if (elem != null) {

Read More
How do JavaScript closures work?

How would you explain JavaScript closures to someone with a knowledge of the concepts they consist of (for example functions, variables and the like), but does not understand closures themselves? I have seen the Scheme example given on Wikipedia, but …

Read More