Apply Custom Function to List Items

Want to transform your list items programmatically? This Apply Custom Function to List Items tool lets you write JavaScript functions that process each item individually. Perfect for data transformation, text manipulation, and batch processing tasks. Whether you need simple case changes or complex custom logic, this browser-based tool executes your code safely and shows results instantly.

How to Use:

1. Enter your list data into the Input List area
   • One item per line gives the cleanest processing results
   • Skip empty lines excludes blank entries from transformation
   • Trim whitespace removes extra spaces before processing

2. Write your custom function in the JavaScript code area
   • Use item as the parameter name for each list element
   • Pass index adds the item position as a second parameter
   • Filter null results removes empty or null function outputs

3. Choose output formatting from the radio options
   • Line per item keeps simple one-item-per-line format
   • Comma separated creates a single line with comma delimiters
   • JSON array generates valid JSON for import into other tools
   • Numbered list adds sequential numbering to results

4. Copy your transformed results with the Copy button
   • Output updates live as you modify the function code
   • Export saves your processed data as a downloadable file
   • Import loads existing files for batch transformation

What Apply Custom Function to List Items Can Do:

This tool brings the power of JavaScript programming to list processing without requiring any development setup. Write functions that transform, filter, or analyze your data using the full JavaScript language features.

The preset functions handle common transformations like uppercase, lowercase, reverse, and slugify operations. Use these as starting points for more complex custom functions.

Advanced Function Development:

The pass index feature gives your function access to each item’s position in the list. Create functions that behave differently based on whether an item is first, last, or at a specific position.

Write conditional logic that processes different items differently. For example, capitalize the first word of each item while leaving the rest unchanged, or add prefixes only to odd-numbered items.

Error handling shows you exactly what went wrong when functions fail. Debug your code iteratively and see results update in real-time as you fix issues.

Data Processing Applications:

Content managers clean up imported data by removing special characters, normalizing formatting, or extracting specific parts of each item. The slugify preset converts titles into URL-friendly formats automatically.

Web developers transform user input into database-ready formats. Convert form responses into consistent formats or extract email domains from address lists.

Data analysts preprocess survey responses or social media data. Write functions that extract hashtags, count words, or categorize responses based on keywords.

Technical and Educational Uses:

Students learning JavaScript practice array processing and string manipulation with real data. The tool provides immediate feedback and error messages for learning purposes.

System administrators process log entries or configuration files. Write functions that parse timestamps, extract IP addresses, or format system data for reports.

API developers test data transformation logic before implementing in production code. Prototype complex data processing pipelines with sample data.

Example:

Here’s how custom functions transform a sample list:

Original list:

apple pie
banana split
cherry tart

Uppercase function:

javascript

return item.toUpperCase();

Result:

APPLE PIE
BANANA SPLIT
CHERRY TART

Custom extraction function:

javascript

return item.split(' ')[0];

Result:

apple
banana  
cherry

Index-based function:

javascript

return `${index + 1}: ${item.charAt(0).toUpperCase() + item.slice(1)}`;

Result:

1: Apple pie
2: Banana split
3: Cherry tart

Apply Custom Function to List Items Table:

This table shows how different function types transform list data:

Common Use Cases:

Web developers transform user input data into consistent formats for database storage and API processing. Content creators clean up imported lists by removing special characters and normalizing text formatting. Data analysts preprocess survey responses and social media data before analysis and reporting. Students practice JavaScript programming concepts with real-world data transformation examples. System administrators process configuration files and log entries with custom parsing functions.