JavaScript Code to Fix Undefined Is Not a Function Callback Errors

These ready-to-use JavaScript utilities help you fix undefined is not a function error in JavaScript callbacks by implementing defensive programming patterns. Copy these code snippets to prevent callback-related errors in your applications.

Essential Callback Utilities #

Safe Callback Executor #

The most important utility for preventing callback errors:

Callback Validator Class #

A comprehensive validation system for callbacks:

class CallbackUtils {
    // Check if value is a valid callback
    static isValidCallback(callback) {
        return callback && typeof callback === 'function';
    }
    
    // Execute callback with error handling
    static execute(callback, ...args) {
        if (!this.isValidCallback(callback)) {
            return { success: false, error: 'Invalid callback' };
        }
        
        try {
            const result = callback(...args);
            return { success: true, result };
        } catch (error) {
            return { success: false, error: error.message };
        }
    }
    
    // Create a safe wrapper for any callback
    static createSafeWrapper(callback, defaultCallback = null) {
        return (...args) => {
            if (this.isValidCallback(callback)) {
                return this.execute(callback, ...args);
            } else if (this.isValidCallback(defaultCallback)) {
                return this.execute(defaultCallback, ...args);
            }
            return { success: false, error: 'No valid callback available' };
        };
    }
    
    // Create callback with timeout protection
    static withTimeout(callback, timeoutMs = 5000) {
        return (...args) => {
            if (!this.isValidCallback(callback)) {
                throw new Error('Invalid callback provided');
            }
            
            return new Promise((resolve, reject) => {
                const timer = setTimeout(() => {
                    reject(new Error('Callback timeout'));
                }, timeoutMs);
                
                try {
                    const result = callback(...args);
                    clearTimeout(timer);
                    resolve(result);
                } catch (error) {
                    clearTimeout(timer);
                    reject(error);
                }
            });
        };
    }
}

Function Parameter Validators #

Utilities to validate function parameters including callbacks:

Async Callback Utilities #

Promise-based Callback Wrapper #

Convert callbacks to promises with error handling:

function promisifyCallback(callbackFunction) {
    return function(...args) {
        return new Promise((resolve, reject) => {
            if (typeof callbackFunction !== 'function') {
                reject(new Error('Provided callback is not a function'));
                return;
            }
            
            try {
                // Add callback to args that resolves/rejects promise
                const enhancedCallback = (error, result) => {
                    if (error) {
                        reject(error);
                    } else {
                        resolve(result);
                    }
                };
                
                callbackFunction(...args, enhancedCallback);
            } catch (error) {
                reject(error);
            }
        });
    };
}

// Usage example
const asyncOperation = promisifyCallback((data, callback) => {
    setTimeout(() => {
        callback(null, `Processed: ${data}`);
    }, 1000);
});

// Now can use with async/await
async function example() {
    try {
        const result = await asyncOperation('test data');
        console.log(result);
    } catch (error) {
        console.error('Operation failed:', error);
    }
}

Callback Chain Manager #

Manage multiple callbacks safely:

Event-Safe Callback Utilities #

Event Listener with Callback Validation #

class SafeEventEmitter {
    constructor() {
        this.events = {};
    }
    
    // Add event listener with callback validation
    on(event, callback) {
        if (typeof callback !== 'function') {
            console.error(`Cannot add non-function listener for event: ${event}`);
            return this;
        }
        
        if (!this.events[event]) {
            this.events[event] = [];
        }
        
        this.events[event].push(callback);
        return this;
    }
    
    // Emit event with safe callback execution
    emit(event, ...args) {
        if (!this.events[event]) {
            return this;
        }
        
        const results = [];
        
        this.events[event].forEach((callback, index) => {
            try {
                if (typeof callback === 'function') {
                    const result = callback(...args);
                    results.push({ index, success: true, result });
                } else {
                    results.push({ index, success: false, error: 'Invalid callback' });
                }
            } catch (error) {
                results.push({ index, success: false, error: error.message });
            }
        });
        
        return results;
    }
    
    // Remove specific callback
    off(event, callbackToRemove) {
        if (!this.events[event]) {
            return this;
        }
        
        this.events[event] = this.events[event].filter(callback => callback !== callbackToRemove);
        return this;
    }
}

Quick Implementation Guide #

1. Basic Safety Check #

// Simplest protection - add this before any callback call
if (typeof callback === 'function') {
    callback(data);
}

2. With Default Fallback #

// Provide fallback behavior
function withFallback(callback, fallback = () => {}) {
    return typeof callback === 'function' ? callback : fallback;
}

3. Error-Safe Execution #

// Wrap callback calls in try-catch
function safeExecute(callback, ...args) {
    try {
        return typeof callback === 'function' ? callback(...args) : null;
    } catch (error) {
        console.error('Callback error:', error);
        return null;
    }
}

Usage Tips #

1. Always validate: Check typeof callback === 'function' before calling
2. Use defaults: Provide default empty functions when appropriate
3. Handle errors: Wrap callback execution in try-catch blocks
4. Store references: Keep original callback references in async operations
5. Document expectations: Clearly specify callback requirements in function documentation

These utilities provide robust protection against undefined callback errors while maintaining clean, readable code. For comprehensive error handling strategies, see our JavaScript error troubleshooting guide.