Refactor code-style

*   Add more docs to JSDoc
*   Add support for `null` in input of API types

Author: wooorm

lib/index.js

@@ -4,25 +4,32 @@
  */
 
 /**
- * Stringify one nlcst node or list of nodes.
+ * Get the text content of a node or list of nodes.
  *
- * @param {Root|Content|Array<Content>} node
- * @param {string} [separator='']
+ * Prefers the node’s plain-text fields, otherwise serializes its children, and
+ * if the given value is an array, serialize the nodes in it.
+ *
+ * @param {Root | Content | Array<Content>} value
+ *   Node or list of nodes to serialize.
+ * @param {string | null | undefined} [separator='']
+ *   Separator to use.
  * @returns {string}
+ *   Result.
  */
-export function toString(node, separator = '') {
+// To do next major: remove `separator`.
+export function toString(value, separator) {
   let index = -1
 
-  if (!node || (!Array.isArray(node) && !node.type)) {
-    throw new Error('Expected node, not `' + node + '`')
+  if (!value || (!Array.isArray(value) && !value.type)) {
+    throw new Error('Expected node, not `' + value + '`')
   }
 
   // @ts-expect-error Looks like a literal.
-  if (typeof node.value === 'string') return node.value
+  if (typeof value.value === 'string') return value.value
 
   /** @type {Array<Content|Root>} */
   // @ts-expect-error Looks like a list of nodes or parent.
-  const children = (Array.isArray(node) ? node : node.children) || []
+  const children = (Array.isArray(value) ? value : value.children) || []
 
   // Shortcut: This is pretty common, and a small performance win.
   if (children.length === 1 && 'value' in children[0]) {
@@ -36,5 +43,5 @@ export function toString(node, separator = '') {
     values[index] = toString(children[index], separator)
   }
 
-  return values.join(separator)
+  return values.join(separator || '')
 }