Add no-this-assignment rule (#1018)

Co-authored-by: Sindre Sorhus <sindresorhus@gmail.com>

docs/rules/no-this-assignment.md

@@ -0,0 +1,52 @@
+# Disallow assigning `this` to a variable
+
+`this` should be used directly. If you want a reference to `this` from a higher scope, consider using [arrow function expression](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Functions/Arrow_functions) or [`Function#bind()`](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_objects/Function/bind).
+
+## Fail
+
+```js
+const foo = this;
+
+setTimeout(function () {
+	foo.bar();
+}, 1000);
+```
+
+```js
+const foo = this;
+
+class Bar {
+	method() {
+		foo.baz();
+	}
+}
+
+new Bar().method();
+```
+
+## Pass
+
+```js
+setTimeout(() => {
+	this.bar();
+}, 1000);
+```
+
+```js
+setTimeout(function () {
+	this.bar();
+}.bind(this), 1000);
+```
+
+```js
+class Bar {
+	constructor(fooInstance) {
+		this.fooInstance = fooInstance;
+	}
+	method() {
+		this.fooInstance.baz();
+	}
+}
+
+new Bar(this).method();
+```

index.js

@@ -70,6 +70,7 @@ module.exports = {
 				'unicorn/no-null': 'error',
 				'unicorn/no-object-as-default-parameter': 'error',
 				'unicorn/no-process-exit': 'error',
+				'unicorn/no-this-assignment': 'error',
 				'unicorn/no-unreadable-array-destructuring': 'error',
 				'unicorn/no-unsafe-regex': 'off',
 				'unicorn/no-unused-properties': 'off',

readme.md

@@ -64,6 +64,7 @@ Configure it in `package.json`.
 			"unicorn/no-null": "error",
 			"unicorn/no-object-as-default-parameter": "error",
 			"unicorn/no-process-exit": "error",
+			"unicorn/no-this-assignment": "error",
 			"unicorn/no-unreadable-array-destructuring": "error",
 			"unicorn/no-unsafe-regex": "off",
 			"unicorn/no-unused-properties": "off",
@@ -140,6 +141,7 @@ Configure it in `package.json`.
 - [no-null](docs/rules/no-null.md) - Disallow the use of the `null` literal.
 - [no-object-as-default-parameter](docs/rules/no-object-as-default-parameter.md) - Disallow the use of objects as default parameters.
 - [no-process-exit](docs/rules/no-process-exit.md) - Disallow `process.exit()`.
+- [no-this-assignment](docs/rules/no-this-assignment.md) - Disallow assigning `this` to a variable.
 - [no-unreadable-array-destructuring](docs/rules/no-unreadable-array-destructuring.md) - Disallow unreadable array destructuring. *(partly fixable)*
 - [no-unsafe-regex](docs/rules/no-unsafe-regex.md) - Disallow unsafe regular expressions.
 - [no-unused-properties](docs/rules/no-unused-properties.md) - Disallow unused object properties.

rules/no-this-assignment.js

@@ -0,0 +1,43 @@
+'use strict';
+const getDocumentationUrl = require('./utils/get-documentation-url');
+
+const MESSAGE_ID = 'no-this-assignment';
+const messages = {
+	[MESSAGE_ID]: 'Do not assign `this` to `{{name}}`.'
+};
+
+const variableDeclaratorSelector = [
+	'VariableDeclarator',
+	'[init.type="ThisExpression"]',
+	'[id.type="Identifier"]'
+].join('');
+
+const assignmentExpressionSelector = [
+	'AssignmentExpression',
+	'[right.type="ThisExpression"]',
+	'[left.type="Identifier"]'
+].join('');
+
+const selector = `:matches(${variableDeclaratorSelector}, ${assignmentExpressionSelector})`;
+
+const create = context => ({
+	[selector](node) {
+		const variable = node.type === 'AssignmentExpression' ? node.left : node.id;
+		context.report({
+			node,
+			data: {name: variable.name},
+			messageId: MESSAGE_ID
+		});
+	}
+});
+
+module.exports = {
+	create,
+	meta: {
+		type: 'suggestion',
+		docs: {
+			url: getDocumentationUrl(__filename)
+		},
+		messages
+	}
+};

test/no-this-assignment.js

@@ -0,0 +1,46 @@
+import {outdent} from 'outdent';
+import {test} from './utils/test.js';
+
+test.visualize({
+	valid: [
+		'const {property} = this;',
+		'const property = this.property;',
+		'const [element] = this;',
+		'const element = this[0];',
+		'([element] = this);',
+		'element = this[0];',
+		'property = this.property;',
+		'const [element] = [this];',
+		'([element] = [this]);',
+		'const {property} = {property: this};',
+		'({property} = {property: this});',
+		'const self = true && this;',
+		'const self = false || this;',
+		'const self = false ?? this;',
+		'foo.bar = this;',
+		'function foo(a = this) {}',
+		'function foo({a = this}) {}',
+		'function foo([a = this]) {}'
+	],
+	invalid: [
+		'const foo = this;',
+		'let foo;foo = this;',
+		'var foo = bar, baz = this;'
+	]
+});
+
+test.babel({
+	valid: [
+		outdent`
+			class A {
+				foo = this;
+			}
+		`,
+		outdent`
+			class A {
+				static foo = this;
+			}
+		`
+	],
+	invalid: []
+});

test/snapshots/no-this-assignment.js.md

@@ -0,0 +1,35 @@
+# Snapshot report for `test/no-this-assignment.js`
+
+The actual snapshot is saved in `no-this-assignment.js.snap`.
+
+Generated by [AVA](https://avajs.dev).
+
+## Invalid #1
+      1 | const foo = this;
+
+> Error 1/1
+
+    `␊
+    > 1 | const foo = this;␊
+        |       ^^^^^^^^^^ Do not assign `this` to `foo`.␊
+    `
+
+## Invalid #2
+      1 | let foo;foo = this;
+
+> Error 1/1
+
+    `␊
+    > 1 | let foo;foo = this;␊
+        |         ^^^^^^^^^^ Do not assign `this` to `foo`.␊
+    `
+
+## Invalid #3
+      1 | var foo = bar, baz = this;
+
+> Error 1/1
+
+    `␊
+    > 1 | var foo = bar, baz = this;␊
+        |                ^^^^^^^^^^ Do not assign `this` to `baz`.␊
+    `