What is the way to replace JavaScript buttons for Lightning experience?

Hi,

The Lightning Experience Configuration Converter recreates your org’s JavaScript buttons as Lightning components, quick actions, or other solutions — all without touching your original buttons. Before committing to any changes, you can preview the new component code or declarative steps and verify that the alternatives work as expected.

This tool currently converts some (but not all) JavaScript buttons that implement “url-hacks.” (If you’re not familiar with the term, a “url-hack” is how the community refers to a URL that sets predefined values.) This is the most common use case for JavaScript buttons. The Configuration Converter recreates these buttons as either quick actions, Lightning component actions, or custom buttons or links, based on what the JavaScript code is doing. JavaScript buttons can have varying degrees of complexity. Currently, the tool supports simpler “url-hack” implementations.

Hope this helps.