SpacingJS is a free Chrome add-on developed by Steven Lei that allows users to accurately measure the spacing between elements on webpages. This tool is particularly useful for web designers and developers who need to ensure precise spacing between different elements on their websites.

To use SpacingJS, simply hold down the Alt key (Windows) or Option key (Mac) and move the cursor over one element. Then, move the cursor to another element to measure the spacing between them. This add-on provides a quick and convenient way to measure the exact distance between various elements on a webpage.

With the latest update, version 1.0.7, users can now hold down the Shift key to delay the dismissal of measurement results. This feature makes it easier to take screenshots for reference or documentation purposes.

Overall, SpacingJS is a valuable tool for web designers and developers who need to ensure accurate spacing between elements on their webpages. Its simple and intuitive interface, combined with its precise measurement capabilities, make it a must-have add-on for anyone working in web design or development.