Add no-useless-promise-resolve-reject rule (#1623)

Co-authored-by: Sindre Sorhus <[email protected]>
Co-authored-by: fisker Cheung <[email protected]>

Author: 3 people

configs/recommended.js

@@ -47,6 +47,7 @@ module.exports = {
 		'unicorn/no-unused-properties': 'off',
 		'unicorn/no-useless-fallback-in-spread': 'error',
 		'unicorn/no-useless-length-check': 'error',
+		'unicorn/no-useless-promise-resolve-reject': 'error',
 		'unicorn/no-useless-spread': 'error',
 		'unicorn/no-useless-undefined': 'error',
 		'unicorn/no-zero-fractions': 'error',

docs/rules/no-useless-promise-resolve-reject.md

@@ -0,0 +1,41 @@
+# Disallow returning/yielding `Promise.resolve/reject()` in async functions
+
+✅ *This rule is part of the [recommended](https://github.com/sindresorhus/eslint-plugin-unicorn#recommended-config) config.*
+
+🔧 *This rule is [auto-fixable](https://eslint.org/docs/user-guide/command-line-interface#fixing-problems).*
+
+Wrapping a return value in `Promise.resolve` in an async function is unnecessary as all return values of an async function are already wrapped in a `Promise`. Similarly, returning an error wrapped in `Promise.reject` is equivalent to simply `throw`ing the error. This is the same for `yield`ing in async generators as well.
+
+## Fail
+
+```js
+const main = async foo => {
+	if (foo > 4) {
+		return Promise.reject(new Error('🤪'));
+	}
+
+	return Promise.resolve(result);
+};
+
+async function * generator() {
+	yield Promise.resolve(result);
+	yield Promise.reject(error);
+}
+```
+
+## Pass
+
+```js
+const main = async foo => {
+	if (foo > 4) {
+		throw new Error('🤪');
+	}
+
+	return result;
+};
+
+async function * generator() {
+	yield result;
+	throw error;
+}
+```

readme.md

@@ -81,6 +81,7 @@ Configure it in `package.json`.
 			"unicorn/no-unused-properties": "off",
 			"unicorn/no-useless-fallback-in-spread": "error",
 			"unicorn/no-useless-length-check": "error",
+			"unicorn/no-useless-promise-resolve-reject": "error",
 			"unicorn/no-useless-spread": "error",
 			"unicorn/no-useless-undefined": "error",
 			"unicorn/no-zero-fractions": "error",
@@ -195,6 +196,7 @@ Each rule has emojis denoting:
 | [no-unused-properties](docs/rules/no-unused-properties.md) | Disallow unused object properties. |  |  |  |
 | [no-useless-fallback-in-spread](docs/rules/no-useless-fallback-in-spread.md) | Forbid useless fallback when spreading in object literals. | ✅ | 🔧 |  |
 | [no-useless-length-check](docs/rules/no-useless-length-check.md) | Disallow useless array length check. | ✅ | 🔧 |  |
+| [no-useless-promise-resolve-reject](docs/rules/no-useless-promise-resolve-reject.md) | Disallow returning/yielding `Promise.resolve/reject()` in async functions | ✅ | 🔧 |  |
 | [no-useless-spread](docs/rules/no-useless-spread.md) | Disallow unnecessary spread. | ✅ | 🔧 |  |
 | [no-useless-undefined](docs/rules/no-useless-undefined.md) | Disallow useless `undefined`. | ✅ | 🔧 |  |
 | [no-zero-fractions](docs/rules/no-zero-fractions.md) | Disallow number literals with zero fractions or dangling dots. | ✅ | 🔧 |  |

rules/no-useless-promise-resolve-reject.js

@@ -0,0 +1,171 @@
+'use strict';
+const {matches, methodCallSelector} = require('./selectors/index.js');
+const {getParenthesizedRange} = require('./utils/parentheses.js');
+
+const MESSAGE_ID_RESOLVE = 'resolve';
+const MESSAGE_ID_REJECT = 'reject';
+const messages = {
+	[MESSAGE_ID_RESOLVE]: 'Prefer `{{type}} value` over `{{type}} Promise.resolve(error)`.',
+	[MESSAGE_ID_REJECT]: 'Prefer `throw error` over `{{type}} Promise.reject(error)`.',
+};
+
+const selector = [
+	methodCallSelector({
+		object: 'Promise',
+		methods: ['resolve', 'reject'],
+	}),
+	matches([
+		'ArrowFunctionExpression[async=true] > .body',
+		'ReturnStatement > .argument',
+		'YieldExpression[delegate=false] > .argument',
+	]),
+].join('');
+
+const functionTypes = new Set([
+	'ArrowFunctionExpression',
+	'FunctionDeclaration',
+	'FunctionExpression',
+]);
+function getFunctionNode(node) {
+	let isInTryStatement = false;
+	let functionNode;
+	for (; node; node = node.parent) {
+		if (functionTypes.has(node.type)) {
+			functionNode = node;
+			break;
+		}
+
+		if (node.type === 'TryStatement') {
+			isInTryStatement = true;
+		}
+	}
+
+	return {
+		functionNode,
+		isInTryStatement,
+	};
+}
+
+function createProblem(callExpression, fix) {
+	const {callee, parent} = callExpression;
+	const method = callee.property.name;
+	const type = parent.type === 'YieldExpression' ? 'yield' : 'return';
+
+	return {
+		node: callee,
+		messageId: method,
+		data: {type},
+		fix,
+	};
+}
+
+function fix(callExpression, isInTryStatement, sourceCode) {
+	if (callExpression.arguments.length > 1) {
+		return;
+	}
+
+	const {callee, parent, arguments: [errorOrValue]} = callExpression;
+	if (errorOrValue && errorOrValue.type === 'SpreadElement') {
+		return;
+	}
+
+	const isReject = callee.property.name === 'reject';
+	const isYieldExpression = parent.type === 'YieldExpression';
+	if (
+		isReject
+		&& (
+			isInTryStatement
+			|| (isYieldExpression && parent.parent.type !== 'ExpressionStatement')
+		)
+	) {
+		return;
+	}
+
+	return function (fixer) {
+		const isArrowFunctionBody = parent.type === 'ArrowFunctionExpression';
+
+		let text = errorOrValue ? sourceCode.getText(errorOrValue) : '';
+
+		if (errorOrValue && errorOrValue.type === 'SequenceExpression') {
+			text = `(${text})`;
+		}
+
+		if (isReject) {
+			// `return Promise.reject()` -> `throw undefined`
+			text = text || 'undefined';
+			text = `throw ${text}`;
+
+			if (isYieldExpression) {
+				return fixer.replaceTextRange(
+					getParenthesizedRange(parent, sourceCode),
+					text,
+				);
+			}
+
+			text += ';';
+
+			// `=> Promise.reject(error)` -> `=> { throw error; }`
+			if (isArrowFunctionBody) {
+				text = `{ ${text} }`;
+				return fixer.replaceTextRange(
+					getParenthesizedRange(callExpression, sourceCode),
+					text,
+				);
+			}
+		} else {
+			// eslint-disable-next-line no-lonely-if
+			if (isYieldExpression) {
+				text = `yield${text ? ' ' : ''}${text}`;
+			} else if (parent.type === 'ReturnStatement') {
+				text = `return${text ? ' ' : ''}${text};`;
+			} else {
+				if (errorOrValue && errorOrValue.type === 'ObjectExpression') {
+					text = `(${text})`;
+				}
+
+				// `=> Promise.resolve()` -> `=> {}`
+				text = text || '{}';
+			}
+		}
+
+		return fixer.replaceText(
+			isArrowFunctionBody ? callExpression : parent,
+			text,
+		);
+	};
+}
+
+/** @param {import('eslint').Rule.RuleContext} context */
+const create = context => {
+	const sourceCode = context.getSourceCode();
+
+	return {
+		[selector](callExpression) {
+			const {functionNode, isInTryStatement} = getFunctionNode(callExpression);
+			if (!functionNode || !functionNode.async) {
+				return;
+			}
+
+			return createProblem(
+				callExpression,
+				fix(callExpression, isInTryStatement, sourceCode),
+			);
+		},
+	};
+};
+
+const schema = [];
+
+/** @type {import('eslint').Rule.RuleModule} */
+module.exports = {
+	create,
+	meta: {
+		type: 'suggestion',
+		docs: {
+			description: 'Disallow returning/yielding `Promise.resolve/reject()` in async functions',
+		},
+		fixable: 'code',
+		schema,
+		messages,
+	},
+};