How do I auto-complete words in insert mode without any plugins?

Explanation

Vim has a powerful built-in completion system that requires zero plugins. Pressing <C-n> in insert mode searches forward through all buffers, included files, and tags for words matching what you have typed so far. Pressing <C-p> searches backward. A popup menu appears with candidates, and you can keep pressing <C-n>/<C-p> to cycle through them.

How it works

• Start typing a word in insert mode
• Press <C-n> to trigger keyword completion and open the popup menu
• <C-n> selects the next match, <C-p> selects the previous match
• Press <CR> or simply keep typing to accept the current selection
• Press <C-e> to dismiss the popup and return to what you originally typed
• Press <C-y> to accept the current selection and close the popup without inserting a trailing character

Vim scans multiple sources for completions, controlled by the complete option. By default it searches the current buffer, other open buffers, and tag files.

Example

You are editing a file that contains the variable calculateTotalRevenue. On a new line, type calc and press <C-n>:

calculateTotalRevenue

Vim finds the match and inserts it. If there are multiple matches (e.g., calculateTax, calculateDiscount), a popup menu appears and you cycle through them with <C-n> and <C-p>.

Specialized completion modes

Vim offers targeted completion with <C-x> followed by a second key:

<C-x><C-l>  Line completion (match entire lines)
<C-x><C-f>  Filename completion
<C-x><C-]>  Tag completion
<C-x><C-k>  Dictionary completion
<C-x><C-o>  Omni completion (language-aware)
<C-x><C-n>  Current buffer keywords only

Tips

• <C-n> pulls from all sources in complete; <C-x><C-n> restricts to the current buffer only — use the latter when you have many buffers open and want faster, more relevant results
• <C-x><C-f> is invaluable for completing file paths while typing import statements or shell commands
• <C-x><C-l> completes entire lines — perfect for duplicating similar lines like struct fields or test assertions
• Set completeopt=menu,menuone,noselect for a smoother popup experience: show the menu even with one match and don't auto-select the first entry
• The popup menu supports fuzzy-ish matching — if you type cTR, <C-n> may still find calculateTotalRevenue depending on your complete settings
• Omni completion (<C-x><C-o>) provides language-aware completions when a filetype plugin defines an omnifunc — this works out of the box for HTML, CSS, JavaScript, Python, and many other languages
• You never leave insert mode during completion — this is faster than switching to normal mode to yank and paste