JavaScript For Loop Skip Elements - Debugging Utilities

When investigating why does my JavaScript for loop skip array elements, having the right debugging utilities can save hours of troubleshooting. These ready-to-use code snippets help identify and diagnose common array iteration problems.

Array Loop Debugger Utility #

This comprehensive utility automatically detects common issues that cause for loops to skip array elements:

Loop Iteration Monitor #

This utility monitors array changes during loop execution to catch mutation issues:

Safe Array Iterator #

A utility that provides safe array iteration patterns to prevent element skipping:

// Safe Array Iterator - Prevents common skipping issues
class SafeArrayIterator {
    static forwardSafe(array, callback, options = {}) {
        const { skipEmpty = true, handleErrors = true } = options;
        const results = [];
        
        for (let i = 0; i < array.length; i++) {
            // Skip empty slots if requested
            if (skipEmpty && !(i in array)) {
                continue;
            }
            
            try {
                const result = callback(array[i], i, array);
                results.push({ index: i, value: array[i], result });
            } catch (error) {
                if (handleErrors) {
                    console.error(`Error at index ${i}:`, error);
                    results.push({ index: i, value: array[i], error });
                } else {
                    throw error;
                }
            }
        }
        
        return results;
    }
    
    static backwardSafe(array, callback, options = {}) {
        const { skipEmpty = true, handleErrors = true } = options;
        const results = [];
        
        // Iterate backwards - safe for array modifications
        for (let i = array.length - 1; i >= 0; i--) {
            if (skipEmpty && !(i in array)) {
                continue;
            }
            
            try {
                const result = callback(array[i], i, array);
                results.unshift({ index: i, value: array[i], result });
            } catch (error) {
                if (handleErrors) {
                    console.error(`Error at index ${i}:`, error);
                    results.unshift({ index: i, value: array[i], error });
                } else {
                    throw error;
                }
            }
        }
        
        return results;
    }
    
    static withSnapshot(array, callback) {
        // Work with a copy to prevent mutation issues
        const snapshot = [...array];
        const results = [];
        
        for (let i = 0; i < snapshot.length; i++) {
            if (i in snapshot) {
                const result = callback(snapshot[i], i, snapshot);
                results.push({ index: i, value: snapshot[i], result });
            }
        }
        
        return results;
    }
}

Quick Diagnostic Functions #

Essential one-liner utilities for common debugging scenarios:

Usage Instructions #

Basic Debugging #

// Quick check for common issues
const myArray = [1, , 3, 4];
const report = debugArrayLoop(myArray);
// Check report.issues for problems

Monitor Array Mutations #

// Track changes during iteration
monitorArrayLoop(myArray, (element, index, array) => {
    // Your loop logic here
    // Mutations will be automatically detected
});

Safe Iteration Patterns #

// Forward iteration (safe for reading)
SafeArrayIterator.forwardSafe(array, (element, index) => {
    console.log(element);
});

// Backward iteration (safe for modifications)
SafeArrayIterator.backwardSafe(array, (element, index, arr) => {
    if (condition) arr.splice(index, 1);
});

Summary #

These debugging utilities help identify why JavaScript for loops skip array elements by automatically detecting sparse arrays, array mutations, and iteration issues. Use them during development to catch common pitfalls before they become production bugs.

Keep these utilities in your development toolkit for quick diagnosis of array iteration problems and implement the safe iteration patterns to prevent element skipping issues.