index.js

const elements = ['svg', 'Svg']

const plugin = ({ types: t }) => ({
  visitor: {
    JSXOpeningElement: {
      enter(path) {
        if (
          !elements.some((element) =>
            path.get('name').isJSXIdentifier({ name: element }),
          )
        )
          return

        const requiredAttributes = ['width', 'height']
        const attributeValue = '1em'

        path.get('attributes').forEach((attributePath) => {
          if (!attributePath.isJSXAttribute()) return
          const index = requiredAttributes.indexOf(attributePath.node.name.name)

          if (index === -1) return

          const value = attributePath.get('value')
          value.replaceWith(t.stringLiteral(attributeValue))
          requiredAttributes.splice(index, 1)
        })

        requiredAttributes.forEach((attribute) => {
          path.pushContainer(
            'attributes',
            t.jsxAttribute(
              t.jsxIdentifier(attribute),
              t.stringLiteral(attributeValue),
            ),
          )
        })
      },
    },
  },
})

export default plugin