How do I prepend // to every line using one Vim command?

Explanation

When you need to comment a whole block quickly, :global combined with :normal is faster than recording a macro or entering Visual Block mode. This pattern executes a Normal-mode edit on every matching line, so it scales well when your file has scattered sections that still follow a clear match rule. It is especially useful for temporary instrumentation and fast code reviews where you want to disable code without deleting it.

How it works

:g/^/normal I// <CR>

• :g/^/ matches every line (^ is start-of-line, present on all non-empty lines and also empty lines)
• normal runs a Normal-mode command for each matched line
• I// inserts // at the first non-blank character of each line
• <CR> executes the Ex command when typed interactively

Example

Before:

call_api()
render_view()

After:

// call_api()
// render_view()

Tips

• Use a narrower match to target only certain lines, for example :g/TODO/normal I//
• Replace I// with any repetitive prefix operation, such as I# for shell scripts