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JavaScript Immutability

5 minute read

In programming, when talking about immutability, there are some closely related words and their meanings that we should take in to account. There's immutable (of course), writable, and constant. So, let's have a look at these before diving into JavaScript specifics (Object.freeze() and Object.seal() methods).


The definition of immutable is: not mutable; unchangeable; changeless. In object-oriented programming, an immutable object is defined as an object whose state cannot be modified after it is created. Immutability works on the value not the variable. Therefore, a variable can point to an immutable object then to another or even a mutable object throughout its life. However, the immutable object the variable originally pointed to still will remain unchanged. In most programming languages, including Java and JavaScript, a String object is immutable. For example:

    var name = "Chris"; //defining a string object
    name.toLowerCase(); //doesn't alter the original object, this call does nothing
    var lowerCaseName = name.toLowerCase(); //the method calls returns a new immutable object but does not change the original

    name = "Ryan"; //the original object does not change, only the variable points to a different object


The definition of constant is: not changing or varying; uniform; regular; invariable. In programming, a constant is defined as a variable that cannot be changed. Therefore, seemingly making it not a "variable". A constant can be seen as somewhat of the opposite of an immutable. An immutable cannot change, yet, a variable pointing to it can. A constant variable cannot change its value, yet, it is possible for parts of the value to change. In Java, you denote a variable or field as constant using the final keyword. In JavaScript (ECMAScript 6), you denote a variable or field as constant using the const keyword. For example:

    const num = 23; //variable num is a constant and its value will always be 23.
    const obj = {"key", "value"}; //obj value will always be this object
    obj = {"otherKey", "otherValue"}; //will not work, cannot change the value of obj
    obj.key = "otherValue"; //will work. obj value stays the same yet its objects values may change


The definition of writable is: capable of recording data. In object-oriented programming, writable refers to whether a property's value can change or not with an assignment operator. Usually, writable is a property itself which takes a boolean value. If true, the properties value can be changed (or written to), if false, the properties value cannot be changed. It's applied to a property in an object (which is basically a variable belonging to that object) and states whether its value can be changed. If set to false, that property becomes like a constant for the specific object and its value cannot be changed, making it immutable. Note, as stated before with immutable objects, being immutable doesn't mean part of the immutable objects values can't change. So, a property which is not writable can be seen like a combination of constant and immutable. This is useful when needed to create read-only properties. For example, in JavaScript you could do:

   Object.defineProperty(obj, "key", {writable: false});

Where, in the above code, obj refers to the object where you're adding the property, key defines the property name, and writable is set to false making it a read-only property.


The Object.freeze() method became available in ECMAScript 5.1 with some added functionality coming in ECMAScript 6 (freezing primitive values). This method prevents an object from being modified in any way including adding, deleting, or editing properties, making the object effectively immutable. How to use it:


The object entered as a parameter becomes immutable. Also, the method returns the immutable object, so if you needed to assign it to a variable you could. Note, that if a frozen objects property is an object, that objects values can still be changed. In this case, you would need to use a loop to go through all of the properties to make sure each of their properties are frozen too.


The Object.seal() method, also added in ECMAScript 5.1, is very similar to the Object.freeze() method. The major difference is that Object.seal() allows its already existing properties to be edited. If a property is an object, its properties can still be edited after being frozen or sealed. However, if a property is a primitive, it can be edited or changed if it is sealed, but if it is frozen it may not be edited. The method is simply used like:


The object entered as a parameter becomes sealed, meaning nothing can be added or removed from it. Also, it returns that same object if you would like to assign it to a variable.


Immutability is an important ability for object-oriented programming. It allows for the development of better applications and API's. Code becomes thread safe and less error prone. JavaScript provides, starting with ECMAScript 5.1, a couple of approaches for immutability. These approaches are the freeze() and seal() methods of Object. Also, we've seen approaches to providing similar concepts of immutability, such as, the const keyword and the writable property within the defineProperty() method of Object. This understanding should help us build better, safer, and more robust JavaScript applications and API's. If you found any errors in my code or writings, or have any input, please feel free to leave a comment! And, of course, for more detail on the subjects I recommend using Mozilla Developer Network (MDN). 


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