index.js

const creator = () => {
	// side properties
	const properties = ['top', 'right', 'bottom', 'left'];

	return {
		postcssPlugin: 'postcss-inset',
		Declaration: {
			'inset': (decl, { list }) => {
				// space-separated side values (top, right, bottom, left)
				const values = list.space(decl.value);

				// conditionally add a right value
				if (values.length === 1) {
					values.push(values[0]);
				}

				// conditionally add a bottom value
				if (values.length === 2) {
					values.push(values[0]);
				}

				// conditionally add a left value
				if (values.length === 3) {
					values.push(values[1]);
				}

				// only transform up to 4 side values
				if (values.length === 4) {
					// for each side property
					properties.forEach((property, index) => {
						// create a new declaration for the side
						decl.cloneBefore({
							prop: property,
							value: values[index]
						});
					});
				}

				decl.remove();
			},
		}
	}
}

creator.postcss = true

export default creator