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Convert SRT to VTT with Node.js

Convert subtitle formats in Node.js with the SubtoVTT API. Native fetch and FormData example, credit billing explained. v1 · stable

Convert SRT to VTT from Node.js using the native fetch and FormData APIs — no dependencies, no SDK. Node 18+ ships both, so this is the lowest-friction path for serverless functions, API routes, or any JavaScript backend.

Pick Node.js when your app is already JavaScript end to end (Express, Fastify, Vercel functions) and you want the converted file to stay in the same language as the rest of your stack.

Convert SRT to VTT

const fs = require("node:fs");

const API_URL = "https://api.example.com/api/v1/convert";
const API_KEY = process.env.SUBTOVTT_API_KEY;

async function convertSrtToVtt() {
    const form = new FormData();
    form.append("file", new Blob([fs.readFileSync("movie.srt")]), "movie.srt");
    form.append("from", "srt");
    form.append("to", "vtt");

    const response = await fetch(API_URL, {
        method: "POST",
        headers: { Authorization: `Bearer ${API_KEY}` },
        body: form,
    });

    if (!response.ok) {
        throw new Error(
            `Conversion failed: ${response.status} ${await response.text()}`,
        );
    }

    console.log("Cues:", response.headers.get("x-cue-count"));
    fs.writeFileSync("movie.vtt", Buffer.from(await response.arrayBuffer()));
}

convertSrtToVtt().catch(console.error);

Set SUBTOVTT_API_KEY in your environment before running (100 free credits on key creation). The converted file is returned as the response body; here it's written straight to movie.vtt.

The x-cue-count response header reports how many cues the converter wrote. To apply formatting or timing fixes during the conversion, append fields like shift_ms or casing to the FormData.

Note: a failed request surfaces as !response.ok — log await response.text() to see the API's error message. Confirm Node 18+ if you see FormData is not defined.

Convert between any formats

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