How do you implement private methods in a JavaScript class?
Aug 01, 2025 am 04:36 AM
In JavaScript, private methods are implemented by preceding the method name with # symbols, which is part of the ES2022 standard. 1. Use # prefix to define private methods, which can only be accessed inside the class, and external calls will report syntax errors; 2. Private methods cannot be enumerated and will not appear in Object.keys() or for...in loops; 3. Support static private methods, which can only be called through this inside the class; 4. Underscore naming (such as _method) is only a convention and is not truly private; 5. Closure or WeakMap was the way to simulate private methods in the old era, and is not recommended now; 6. The best practice is to use # syntax to ensure true privateness, such as #logTransaction safely hides in the BankAccount class to prevent external interference. Therefore, the #methodName() syntax should be used to define private methods, which is a clear, safe and standard practice in modern JavaScript.
In JavaScript, private methods in classes are implemented using the # symbol as a prefix for the method name. This is part of the official class fields and methods privacy proposal , which is now supported in modern JavaScript (ES2022 ).
? Using the # syntax (Standard way)
To define a private method, prepend # to the method name. These methods can only be accessed within the class itself — attempting to call them from outside will result in a syntax error.
class MyClass {
publicMethod() {
this.#privateMethod(); // OK: called inside the class
}
#privateMethod() {
console.log("This is a private method!");
}
}
const instance = new MyClass();
instance.publicMethod(); // "This is a private method!"
instance.#privateMethod(); // SyntaxError: Private field '#privateMethod' must be declared in an enclosing class? Key Points About Private Methods
- Name must start with
#– both when defining and calling. - Only accessible within the class – not even in subclasses.
- Not enumerable – they won't show up in
Object.keys()orfor...inloops. - Static private methods are also supported:
class MyClass {
static #privateStaticMethod() {
console.log("Private static method");
}
static publicStaticMethod() {
this.#privateStaticMethod(); // OK
}
}
MyClass.publicStaticMethod(); // "Private static method"
MyClass.#privateStaticMethod(); // SyntaxError? What Doesn't Work (Common Misconceptions)
Underscore prefix (
_method) : This is just a naming convention to indicate privacy, but the method is still publicly accessible.
class MyClass { _privateByConvention() { ... } // Not actually private }Closure-based patterns or WeakMaps : These were used before ES2022 to simulate privacy, but they're more complex and not idiomatic anymore for simple cases.
? Best Practice Today
Use the # syntax for true private methods:
class BankAccount {
#balance = 0;
deposit(amount) {
if (amount > 0) {
this.#balance = amount;
this.#logTransaction("deposit", amount);
}
}
#logTransaction(type, amount) {
console.log(`${type}: $${amount} at ${new Date().toISOString()}`);
}
} Now #logTransaction is safely hidden from external interference.
Basically, just use #methodName() — it's clean, safe, and standard.
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