📝 Quick Notes #

🔢 Number #

Number is the set of all numbers: integers, floats, positives, negatives, Infinity, NaN, and so on. Numbers can do, well, numbery things, like addition (+), subtraction (-), modulo (%), and comparison (<).

🧩 Primitives in JS/TS #

In JavaScript, a primitive (primitive value, primitive data type) is data that is not an object and has no methods or properties. There are 7 primitive data types:

string • number • bigint • boolean • undefined • symbol • null

Key Characteristics: #

• All primitives are immutable — they cannot be altered
• It’s important not to confuse a primitive itself with a variable assigned a primitive value
• The variable may be reassigned to a new value, but the existing value cannot be changed in the ways that objects, arrays, and functions can be altered
• The language does not offer utilities to mutate primitive values

Auto-boxing Behavior: #

Primitives have no methods but still behave as if they do. When properties are accessed on primitives, JavaScript auto-boxes the value into a wrapper object and accesses the property on that object instead.

Example: "foo".includes("f") implicitly creates a String wrapper object and calls String.prototype.includes() on that object. This auto-boxing behavior is not observable in JavaScript code but is a good mental model of various behaviors — for example, why “mutating” primitives does not work (because str.foo = 1 is not assigning to the property foo of str itself, but to an ephemeral wrapper object).

🔒 Object.seal(…) and Object.freeze(…) #

Object.seal() #

In JavaScript, Object.seal() is used to restrict modifications to an object’s structure:

• ✅ Prevents adding or deleting properties
• ✅ Allows changing the values of existing ones
• ⚠️ Not recursive — sealing an object does not seal its nested objects

You must apply Object.seal() manually or via a recursive function for deeper levels.

Note: If you seal an object with nested objects inside, you can still modify those nested properties unless they are sealed individually.

Object.freeze() #

Object.freeze() makes an object completely immutable at the top level:

• ❌ Prevents addition, deletion, and value updates of its properties
• ⚠️ Like seal(), freeze() is also shallow by default
• Nested objects remain mutable unless recursively frozen using a utility function

Deep Sealing/Freezing #

To make an object fully sealed or frozen, including its nested levels, you need to write or use a deep sealing/freezing function that traverses and applies the method to each nested object.

Important: Both seal and freeze work only on objects — if you try to seal or freeze a primitive value like a string or number, it has no effect.

✅ Is value a valid Object? #

Reason: Some of them are primitive data types.

Misconception #

This is actually one of JavaScript’s most famous quirks. When you use the typeof operator on null, it returns “object”: javascripttypeof null // returns “object” However, this is widely considered a bug in JavaScript that has been kept for backward compatibility. According to the JavaScript specification, null is actually a primitive value, not an object.