How do I trigger language-aware autocompletion in Vim?

Explanation

The <C-x><C-o> keystroke triggers omni completion, Vim's built-in language-aware completion system. It uses filetype-specific logic to suggest completions based on the structure of the programming language you're editing.

How it works

1. Ensure filetype detection is enabled: :filetype plugin on
2. In insert mode, press <C-x><C-o>
3. Vim invokes the omnifunc set for the current filetype
4. A popup menu shows language-aware suggestions

What it knows about

Vim ships with omni completion for several languages:

• HTML/CSS: Tag names, attributes, CSS properties and values
• JavaScript: Object properties, DOM methods
• Python: Module members, function signatures (with pythoncomplete)
• SQL: Table names, column names, SQL keywords
• XML: Tag completion based on DTD
• PHP: Functions, class methods

Example

In an HTML file:

<div cl

Pressing <C-x><C-o> after cl suggests class, clear, etc.

Setting up for other languages

" Set omnifunc manually for a filetype
autocmd FileType python setlocal omnifunc=pythoncomplete#Complete
autocmd FileType javascript setlocal omnifunc=javascriptcomplete#CompleteJS
autocmd FileType css setlocal omnifunc=csscomplete#CompleteCSS

Tips

• Check the current omnifunc with :set omnifunc?
• For more advanced completion, pair with LSP clients (vim-lsp, coc.nvim, nvim-lspconfig)
• <C-n> / <C-p> cycle through results in the popup
• Omni completion is the foundation that many plugins build upon