Move internal modules to “internal” folder.

.internal/baseToString.js

@@ -0,0 +1,35 @@
+import arrayMap from './.internal/arrayMap.js';
+import isSymbol from './isSymbol.js';
+
+/** Used as references for various `Number` constants. */
+const INFINITY = 1 / 0;
+
+/** Used to convert symbols to primitives and strings. */
+const symbolProto = Symbol ? Symbol.prototype : undefined;
+const symbolToString = symbolProto ? symbolProto.toString : undefined;
+
+/**
+ * The base implementation of `toString` which doesn't convert nullish
+ * values to empty strings.
+ *
+ * @private
+ * @param {*} value The value to process.
+ * @returns {string} Returns the string.
+ */
+function baseToString(value) {
+  // Exit early for strings to avoid a performance hit in some environments.
+  if (typeof value == 'string') {
+    return value;
+  }
+  if (Array.isArray(value)) {
+    // Recursively convert values (susceptible to call stack limits).
+    return `${ arrayMap(value, baseToString) }`;
+  }
+  if (isSymbol(value)) {
+    return symbolToString ? symbolToString.call(value) : '';
+  }
+  const result = `${ value }`;
+  return (result == '0' && (1 / value) == -INFINITY) ? '-0' : result;
+}
+
+export default baseToString;