JavaScript Data Types: Primitive vs Reference
Jul 13, 2025 am 02:43 AM
JavaScript's data types are divided into primitive types and reference types. Primitive types include string, number, boolean, null, undefined, and symbol. The values are immutable and copy copies when assigned, so they do not affect each other; reference types such as objects, arrays and functions store memory addresses, and variables pointing to the same object will affect each other. Typeof and instanceof can be used to determine types, but pay attention to the historical problem of typeof null. Understanding these two types of differences can help write more stable and reliable code.
JavaScript data types are divided into two categories: Primitive and Reference. Understanding the difference between them is critical to writing more stable and predictable code.
Primitive types are immutable values
There are six primitive data types in JavaScript: string , number , boolean , null , undefined , and symbol (new in ES6). These types of values are immutable, which means that you cannot change an original value itself, you can only replace it with a new value.
for example:
let name = "Tom"; name.toUpperCase(); // Returns "TOM", but name itself is still "Tom"
This means that the operation of the original type will not modify the original value, but will return a new value. When you assign a primitive type to another variable, you actually copy the copy of this value:
let a = 10; let b = a; b = 20; console.log(a); // Still 10
Therefore, the assignment and operation of the original type are independent and do not affect each other.
The reference type points to the memory address
Reference types mainly include objects (Object), arrays (Array) and functions (Function). They store "references" to a location in memory, not the actual data itself.
See an example:
let obj1 = { name: "Alice" };
let obj2 = obj1;
obj2.name = "Bob";
console.log(obj1.name); // Output Bob Here obj2 does not copy obj1 's content, but points to the same memory address. Therefore, modifying the attributes of obj2 will also affect obj1 .
The same situation will also occur when the function passes arguments:
function changeName(user) {
user.name = "Changed";
}
let person = { name: "Original" };
changeName(person);
console.log(person.name); // Output ChangedBecause what is passed in is a reference to the object, modifications to the object inside the function will affect the outside.
Tips for judging types
To determine whether a variable is a primitive type or a reference type, you can use typeof and instanceof :
-
typeofrecognizes most primitive types (exceptnullwill return"object") -
instanceofis used to determine the specific constructor of reference type
For example:
typeof 42; // "number"
typeof "hello"; // "string"
typeof true; // "boolean"
typeof null; // "object" (this is a historical issue with JS)
typeof {}; // "object"
typeof function(){}; // "function" (special reference type) If you want to accurately judge null , you can use === null to handle it separately.
Basically that's it. Understanding the difference between primitive types and reference types can help you avoid many unexpected problems, especially when dealing with complex structures or function parameter passing.
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