refactor: make internal properties read-only and add instance check

---
type: pre_commit_static_analysis_report
description: Results of running static analysis checks when committing changes.
report:
  - task: lint_filenames
    status: passed
  - task: lint_editorconfig
    status: passed
  - task: lint_markdown
    status: passed
  - task: lint_package_json
    status: na
  - task: lint_repl_help
    status: passed
  - task: lint_javascript_src
    status: passed
  - task: lint_javascript_cli
    status: na
  - task: lint_javascript_examples
    status: na
  - task: lint_javascript_tests
    status: na
  - task: lint_javascript_benchmarks
    status: na
  - task: lint_python
    status: na
  - task: lint_r
    status: na
  - task: lint_c_src
    status: na
  - task: lint_c_examples
    status: na
  - task: lint_c_benchmarks
    status: na
  - task: lint_c_tests_fixtures
    status: na
  - task: lint_shell
    status: na
  - task: lint_typescript_declarations
    status: passed
  - task: lint_typescript_tests
    status: na
  - task: lint_license_headers
    status: passed
---

Author: kgryte

lib/node_modules/@stdlib/ndarray/dtype-ctor/README.md

@@ -148,7 +148,7 @@ var v = dt.enum;
 // returns <number>
 ```
 
-If a data type does not have a corresponding known enumeration constant, the returned value is the enumeration constant for a user-defined data type.
+If a data type does not have a corresponding known enumeration constant, the returned value is `-1`.
 
 **Note**: enumeration constants should be treated as **opaque** values. One should **not** assume that a data type has a specific enumeration constant value.
 

lib/node_modules/@stdlib/ndarray/dtype-ctor/docs/repl.txt

@@ -134,7 +134,7 @@
     Enumeration constant for the data type.
 
     If a data type does not have a corresponding known enumeration constant, the
-    returned value is the enumeration constant for a user-defined data type.
+    returned value is -1.
 
     Returns
     -------

lib/node_modules/@stdlib/ndarray/dtype-ctor/docs/types/index.d.ts

@@ -145,7 +145,7 @@ declare class DataType<T = unknown> {
 	*
 	* ## Notes
 	*
-	* -   If a data type does not have a corresponding known enumeration constant, the returned value is the enumeration constant for a user-defined data type.
+	* -   If a data type does not have a corresponding known enumeration constant, the returned value is `-1`.
 	*
 	* @example
 	* var dt = new DataType( 'float64' );

lib/node_modules/@stdlib/ndarray/dtype-ctor/lib/main.js

@@ -35,7 +35,6 @@ var dtype2desc = require( '@stdlib/ndarray/base/dtype-desc' );
 var dtype2char = require( '@stdlib/ndarray/base/dtype-char' );
 var dtype2alignment = require( '@stdlib/ndarray/base/dtype-alignment' );
 var bytesPerElement = require( '@stdlib/ndarray/base/bytes-per-element' );
-var dtypes = require( '@stdlib/ndarray/dtypes' );
 var format = require( '@stdlib/string/format' );
 
 
@@ -50,13 +49,16 @@ var format = require( '@stdlib/string/format' );
 */
 function isDataType( value ) {
 	return (
-		typeof value === 'object' &&
-		value !== null &&
-		value.constructor.name === 'DataType' &&
-		isString( value.char ) &&
-		isString( value.description ) &&
-		isString( value.byteOrder ) &&
-		hasProp( value, 'value' )
+		value instanceof DataType ||
+		(
+			typeof value === 'object' &&
+			value !== null &&
+			value.constructor.name === 'DataType' &&
+			isString( value.char ) &&
+			isString( value.description ) &&
+			isString( value.byteOrder ) &&
+			hasProp( value, 'value' )
+		)
 	);
 }
 
@@ -96,13 +98,13 @@ function DataType( value, options ) {
 	}
 	if ( isDataTypeString( value ) ) {
 		type = 'builtin';
-	} else if ( isStructConstructorLike( value ) ) {
-		type = 'struct';
 	} else if ( isDataType( value ) ) {
 		// Clone the input data type:
 		return new DataType( value.value, {
 			'description': value.description
 		});
+	} else if ( isStructConstructorLike( value ) ) {
+		type = 'struct';
 	} else {
 		throw new TypeError( format( 'invalid argument. First argument must be either a supported data type string, a struct constructor, or another data type instance. Value: `%s`.', value ) );
 	}
@@ -117,14 +119,14 @@ function DataType( value, options ) {
 	} else {
 		opts = {};
 	}
-	this._value = value;
-	this._description = opts.description || ( dtype2desc( value ) || '' );
-	this._char = dtype2char( value ) || '';
-	this._enum = resolveEnum( value ) || dtypes.userdefined_type;
-	this._alignment = dtype2alignment( value ) || -1;
-	this._byteLength = bytesPerElement( value ) || -1;
-	this._byteOrder = 'host'; // TODO: consider supporting little-endian and big-endian byte orders
-	this._type = type;
+	setReadOnly( this, '_value', value );
+	setReadOnly( this, '_description', opts.description || ( dtype2desc( value ) || '' ) );
+	setReadOnly( this, '_char', dtype2char( value ) || '' );
+	setReadOnly( this, '_enum', resolveEnum( value ) || -1 );
+	setReadOnly( this, '_alignment', dtype2alignment( value ) || -1 );
+	setReadOnly( this, '_byteLength', bytesPerElement( value ) || -1 );
+	setReadOnly( this, '_byteOrder', 'host' ); // TODO: consider supporting little-endian and big-endian byte orders
+	setReadOnly( this, '_type', type );
 	return this;
 }
 
@@ -266,12 +268,12 @@ setReadOnlyAccessor( DataType.prototype, 'description', function get() {
 *
 * ## Notes
 *
-* -   If a data type does not have a corresponding known enumeration constant, the returned value is the enumeration constant for a user-defined data type.
+* -   If a data type does not have a corresponding known enumeration constant, the returned value is `-1`.
 *
 * @name enum
 * @memberof DataType.prototype
 * @readonly
-* @type {NonNegativeInteger}
+* @type {integer}
 *
 * @example
 * var dt = new DataType( 'float64' );