no-unused-private-class-members

Disallow unused private class members

no-unused-private-class-members

Private class members that are declared but never used are likely an error due to incomplete refactoring. They take up space and can confuse readers.

Rule Type: Problem
Fixable: No

Rule Details

This rule reports unused private class members:

• Private fields/methods: Unused if their value is never read
• Private accessors: Unused if never accessed (read or written)

Examples

Incorrect Code

// Unused private field
class A {
    #unusedMember = 5; // Never accessed
}

// Only written, never read
class B {
    #usedOnlyInWrite = 5;
    method() {
        this.#usedOnlyInWrite = 42; // Write only - still unused
    }
}

// Only updates itself
class C {
    #usedOnlyToUpdateItself = 5;
    method() {
        this.#usedOnlyToUpdateItself++; // Read and write same value
    }
}

// Unused private method
class D {
    #unusedMethod() {} // Never called
}

// Unused accessor
class E {
    get #unusedAccessor() {}
    set #unusedAccessor(value) {}
}

Correct Code

// Private field that's read
class A {
    #usedMember = 42;
    method() {
        return this.#usedMember; // Value is read
    }
}

// Private method that's called
class B {
    #usedMethod() {
        return 42;
    }
    anotherMethod() {
        return this.#usedMethod(); // Method is called
    }
}

// Private accessor that's used
class C {
    #value;
    
    get #usedAccessor() {
        return this.#value;
    }
    set #usedAccessor(value) {
        this.#value = value;
    }
    
    method() {
        this.#usedAccessor = 42; // Accessor is used
    }
}

Understanding “Used”

Fields and Methods Must Be Read

class Example {
    #counter = 0;
    
    increment() {
        this.#counter++; // Used: reads old value, writes new value
    }
    
    reset() {
        this.#counter = 0; // Unused: only writes, never reads result
    }
}

Accessors Must Be Accessed

class Example {
    #value;
    
    // Both getter and setter must be accessed
    get #accessor() { return this.#value; }
    set #accessor(v) { this.#value = v; }
    
    method() {
        // Either reading OR writing counts as "used"
        this.#accessor = 10; // Setter is used
        console.log(this.#accessor); // Getter is used
    }
}

Common Patterns

Leftover from Refactoring

// After refactoring, this private method is no longer called
class UserManager {
    #validateEmail(email) { // Used to be called, now it's not
        return email.includes('@');
    }
    
    addUser(email) {
        // Validation was moved to separate validator class
        // Forgot to remove #validateEmail
    }
}

Write-Only Fields

Write-only private fields are considered unused since their values are never read.

class Logger {
    #logLevel = 'info';
    
    setLogLevel(level) {
        this.#logLevel = level; // Only writes, never reads
    }
    
    // Missing method that uses #logLevel!
}

// Fix: Add a method that reads it
class Logger {
    #logLevel = 'info';
    
    setLogLevel(level) {
        this.#logLevel = level;
    }
    
    shouldLog(messageLevel) {
        return this.#logLevel === messageLevel; // Now it's used!
    }
}

Self-Updating Fields

// This is flagged as unused
class Counter {
    #count = 0;
    
    increment() {
        this.#count++; // Reads and writes, but result isn't used elsewhere
    }
}

// Fix: Add getter to expose the value
class Counter {
    #count = 0;
    
    increment() {
        this.#count++;
    }
    
    getCount() {
        return this.#count; // Now the value is actually used
    }
}

Why Private Members?

Private members are often unused because:

1. Incomplete implementation - You planned to use it but didn’t
2. Refactoring artifact - Used to be needed, no longer is
3. Dead code - Experimental code that never got used

Public unused members might be part of your API. Private unused members are definitely dead code.

Refactoring Tips

Remove or Make Public

// If it's truly unused:
class Example {
    // #unusedPrivate = 5; // Delete it
}

// If it might be used by consumers:
class Example {
    unusedPublic = 5; // Make it public (won't be flagged)
}

Add Missing Usage

// Before: Method exists but is never called
class DataStore {
    #validateData(data) {
        return data != null;
    }
}

// After: Actually use it
class DataStore {
    #validateData(data) {
        return data != null;
    }
    
    save(data) {
        if (!this.#validateData(data)) { // Now it's used!
            throw new Error('Invalid data');
        }
        // save logic
    }
}

When Not to Use It

You might disable this rule if:

1. You’re in the middle of development and have placeholder private members
2. You want to keep private members for documentation purposes

However, it’s usually better to remove truly unused code.

// Temporarily disable while developing
/* eslint-disable no-unused-private-class-members */
class WorkInProgress {
    #futureFeature = null; // Will be used later
}
/* eslint-enable no-unused-private-class-members */

Comparison with no-unused-vars

• no-unused-vars - checks variables, parameters, imports
• no-unused-private-class-members - checks private class members specifically

Both rules work together to catch unused code.

Related Rules

• no-unused-vars
• no-unused-expressions