Snippet to simply generate Firestore Document ID outside of Firebase SDK. (Javascript, Node.js)

firestore-document-id.js

const { randomBytes } = require('crypto');

// Sources: https://github.com/firebase/firebase-js-sdk/blob/4090271bb71023c3e6587d8bd8315ebf99b3ccd7/packages/firestore/src/util/misc.ts#L27-L55
const newId = () => {
    // Alphanumeric characters
    const chars = 'ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789';
    // The largest byte value that is a multiple of `char.length`.
    const maxMultiple = Math.floor(256 / chars.length) * chars.length;

    let autoId = '';
    const targetLength = 20;
    while (autoId.length < targetLength) {
      const bytes = randomBytes(40);
      for (let i = 0; i < bytes.length; ++i) {
        // Only accept values that are [0, maxMultiple), this ensures they can
        // be evenly mapped to indices of `chars` via a modulo operation.
        if (autoId.length < targetLength && bytes[i] < maxMultiple) {
          autoId += chars.charAt(bytes[i] % chars.length);
        }
      }
    }

    return autoId;
}

// Usage:
//     newId() => 7MShqEngbz1ZgCk6U0zA