I Built a Free AI Grammar Checker That Runs Entirely in the Browser

#webdev #tutorial #javascript #ai

A few months ago I got tired of copy-pasting my writing between five different tools — Grammarly for grammar, Hemingway for readability, some word counter for stats. So I built AI Grammar Checker — a single tool that does all of that in one shot, for free, with no signup.

What it does

You paste English text, hit check, and get back:

Sentence-by-sentence grammar corrections with explanations
Style polish — passive voice, weak adverbs, wordy phrases flagged
Flesch readability score so you know if your text is actually readable
Hemingway-style highlights — hard sentences, very hard sentences, adverb count
CEFR level (A1–C2) so non-native speakers know where they stand
Tone detection — formal, casual, neutral

All client-side except the AI check call. No server stores your text.

The stack is stupidly simple

HTML + vanilla JS + CSS -> DeepSeek API -> Result rendered directly in the DOM

No React. No Next.js. , style.css, andindex.html.

Here's roughly how the grammar check works:

async function runGrammarCheck(text, intensity) {
const systemPrompt =nglish editor...`;

const response = awa{
method: 'POST',
headers: { 'Conten' },
body: JSON.stringify({
model: 'deepseek
messages: [
{ role: 'syste},
{ role: 'user', content: text }
],
temperature: 0.3
})
});

return response.json();
}

The intensity slider ao be more or lessaggressive — low only flags errors, high rewrites for style too.

Why DeepSeek over OpenAI

Cost — ~1/10th the price of GPT-4o, my API bill is under $2/month
Quality gap is tinyple test set
No content moderation false positives — academic/medical writing flagged by OpenAI work