Fix to add warning when non-strings are given as children

src/react-markdown.js

@@ -98,9 +98,21 @@ function ReactMarkdown(options) {
     .use(options.rehypePlugins || [])
     .use(filter, options)
 
+  let children = options.children
+
+  if (typeof children !== 'string') {
+    if (children !== undefined && children !== null) {
+      console.warn(
+        `[react-markdown] Warning: please pass a string as \`children\` (not: \`${children}\`)`
+      )
+    }
+
+    children = ''
+  }
+
   /** @type {Root} */
   // @ts-ignore we’ll throw if it isn’t a root next.
-  const hastNode = processor.runSync(processor.parse(options.children || ''))
+  const hastNode = processor.runSync(processor.parse(children))
 
   if (hastNode.type !== 'root') {
     throw new TypeError('Expected a `root` node')

test/react-markdown.test.js

@@ -44,6 +44,39 @@ test('should warn when passed `source`', () => {
   console.warn = warn
 })
 
+test('should warn when passed non-string children (number)', () => {
+  const {error, warn} = console
+  console.error = jest.fn()
+  console.warn = jest.fn()
+  // @ts-ignore runtime
+  expect(renderHTML(<Markdown children={1} />)).toEqual('')
+  expect(console.warn).toHaveBeenCalledWith(
+    '[react-markdown] Warning: please pass a string as `children` (not: `1`)'
+  )
+  console.error = error
+  console.warn = warn
+})
+
+test('should warn when passed non-string children (boolean)', () => {
+  const {error, warn} = console
+  console.error = jest.fn()
+  console.warn = jest.fn()
+  // @ts-ignore runtime
+  expect(renderHTML(<Markdown children={false} />)).toEqual('')
+  expect(console.warn).toHaveBeenCalledWith(
+    '[react-markdown] Warning: please pass a string as `children` (not: `false`)'
+  )
+  console.error = error
+  console.warn = warn
+})
+
+test('should not warn when passed `null` as children', () => {
+  expect(renderHTML(<Markdown children={null} />)).toEqual('')
+})
+test('should not warn when passed `undefined` as children', () => {
+  expect(renderHTML(<Markdown children={undefined} />)).toEqual('')
+})
+
 test('should warn when passed `allowDangerousHtml`', () => {
   const warn = console.warn
   console.warn = jest.fn()