add-front-matter.js

const { batch } = require('frontmatter-file-batcher');

// The name of the file to modify.
const file_name = process.argv[2];

// The name of the new front matter property to add.
let fm_property = process.argv[3];

// The value of the new front matter property
let fm_value = process.argv[4];

batch(`content/${file_name}`, 1, async ({ goods, actions }) => {
    const { update, save } = actions;

    var data = {};
    data[fm_property] = { $set: fm_value };

    const newData = update({ data });

    await save(newData);

    console.log('Just saved:', goods.path);
});