New: Adds prefer-object-spread rule (refs: eslint#7230)

conf/eslint-recommended.js

@@ -229,6 +229,7 @@ module.exports = {
         "prefer-const": "off",
         "prefer-destructuring": "off",
         "prefer-numeric-literals": "off",
+        "prefer-object-spread": "off",
         "prefer-promise-reject-errors": "off",
         "prefer-reflect": "off",
         "prefer-rest-params": "off",

docs/rules/prefer-object-spread.md

@@ -0,0 +1,50 @@
+# Prefer use of an object spread over `Object.assign` (prefer-object-spread)
+
+When Object.assign is called using an object literal the first argument, this rule requires using the object spread syntax instead.
+
+**Please note:** This rule can only be used when using an `ecmaVersion` of 2018 or higher, 9 or higher, or when using an `ecmaVersion` of 2015-2017 or 5-8 with the `experimentalObjectRestSpread` parser option enabled.
+
+## Rule Details
+
+The following patterns are considered errors:
+
+```js
+
+Object.assign({}, foo)
+
+Object.assign({}, {foo: 'bar'})
+
+Object.assign({ foo: 'bar'}, baz)
+
+Object.assign({ foo: 'bar' }, Object.assign({ bar: 'foo' }))
+
+Object.assign({}, { foo, bar, baz })
+
+Object.assign({}, { ...baz })
+
+```
+
+The following patterns are not errors:
+
+```js
+
+Object.assign(...foo);
+
+// Any Object.assign call without an object literal as the first argument
+Object.assign(foo, { bar: baz });
+
+Object.assign(foo, Object.assign({ bar: 'foo' }));
+
+Object.assign(foo, { bar, baz })
+
+Object.assign(foo, { ...baz });
+
+// Object.assign with a single argument that is an object literal
+Object.assign({});
+
+Object.assign({ foo: bar });
+```
+
+## When Not To Use It
+
+When you don't care about syntactic sugar added by the object spread property.

lib/rules/prefer-object-spread.js

@@ -0,0 +1,143 @@
+/**
+ * @fileoverview Prefers object spread property over Object.assign
+ * @author Sharmila Jesupaul
+ * See LICENSE file in root directory for full license.
+ */
+
+"use strict";
+
+// Helpers
+/**
+ * Helper that returns the last element in an array
+ * @param {array} arr - array that you are searching in
+ * @returns {string} - Returns the last element in an array of string arguments
+ */
+function tail(arr) {
+    return arr[arr.length - 1];
+}
+
+/**
+ * Helper that strips the curlie braces from a stringified object, returning only the contents
+ * @param {string} objectString - Source code of an object literal
+ * @returns {string} - Returns the object with the curly braces stripped
+ */
+function stripCurlies(objectString) {
+    return objectString.slice(1, -1);
+}
+
+/**
+ * Helper that checks if the node is an Object.assign call
+ * @param {ASTNode} node - The node that the rule warns on
+ * @returns {boolean} - Returns true if the node is an Object.assign call
+ */
+function isObjectAssign(node) {
+    return (
+        node.callee &&
+        node.callee.type === "MemberExpression" &&
+        node.callee.object.name === "Object" &&
+        node.callee.property.name === "assign"
+    );
+}
+
+/**
+ * Autofixer that parses arguments that are passed into the Object.assign call, and returns a formatted object spread.
+ * @param {array} args - The node that the rule warns on, i.e. the Object.assign call
+ * @param {string} sourceCode - sourceCode of the Object.assign call
+ * @returns {array} formatted args - replaces the Object.assign with a spread object.
+ */
+function parseArgs(args, sourceCode) {
+    const mapped = args.map((arg, i) => {
+
+        // If the the argument is an empty object
+        if (arg.type === "ObjectExpression" && arg.properties.length === 0) {
+            return "";
+        }
+
+        // if the argument is another object.assign call run this function for all of it's arguments
+        if (isObjectAssign(arg)) {
+            return parseArgs(arg.arguments, sourceCode);
+        }
+
+        const next = args[i + 1] || {};
+
+        // if the argument is an object with properties
+        if (arg.type === "ObjectExpression") {
+            const trimmedObject = stripCurlies(sourceCode.getText(arg)).trim();
+
+            return next.range && next.range[0] ? `${trimmedObject}, ` : trimmedObject;
+        }
+
+        return next.range && next.range[0]
+            ? `...${sourceCode.getText(arg)}, `
+            : `...${sourceCode.getText(arg)}`;
+    });
+
+    return [].concat.apply([], mapped);
+}
+
+/**
+ * Autofixes the Object.assign call to use an object spread instead.
+ * @param {ASTNode|null} node - The node that the rule warns on, i.e. the Object.assign call
+ * @param {string} sourceCode - sourceCode of the Object.assign call
+ * @returns {Function} autofixer - replaces the Object.assign with a spread object.
+ */
+function autofixSpread(node, sourceCode) {
+    return fixer => {
+        const args = node.arguments;
+
+        const processedArgs = parseArgs(args, sourceCode).join("");
+
+        const lastArg = tail(args);
+        const firstArg = args[0];
+
+        const funcEnd = sourceCode.text
+            .slice(lastArg.range[1], node.range[1])
+            .split(")")[0];
+        const funcStart = tail(
+            sourceCode.text.slice(node.range[0], firstArg.range[0]).split("(")
+        );
+
+        return fixer.replaceText(
+            node,
+            `{${funcStart}${processedArgs}${funcEnd}}`
+        );
+    };
+}
+
+module.exports = {
+    meta: {
+        docs: {
+            description:
+                "disallow using Object.assign with an object literal as the first argument and prefer the use of object spread instead.",
+            category: "ECMAScript 6",
+            recommended: false,
+            url: "https://eslint.org/docs/rules/prefer-object-spread"
+        },
+        schema: [],
+        fixable: "code"
+    },
+
+    create: function rule(context) {
+        return {
+            CallExpression: node => {
+                const message = "Use an object spread instead of `Object.assign()` eg: `{ ...foo }`";
+                const sourceCode = context.getSourceCode();
+                const hasSpreadElement = node.arguments.length &&
+                    node.arguments.some(x => x.type === "SpreadElement");
+
+                if (
+                    node.arguments.length > 1 &&
+                    node.arguments[0].type === "ObjectExpression" &&
+                    isObjectAssign &&
+                    !hasSpreadElement
+                ) {
+                    context.report({
+                        node,
+                        message,
+                        fix: autofixSpread(node, sourceCode)
+                    });
+                }
+            }
+        };
+    }
+};

tests/lib/rules/prefer-object-spread.js

@@ -0,0 +1,195 @@
+/**
+ * @fileoverview Prefers object spread property over Object.assign
+ * @author Sharmila Jesupaul
+ * See LICENSE file in root directory for full license.
+ */
+
+"use strict";
+
+const rule = require("../../../lib/rules/prefer-object-spread");
+
+const RuleTester = require("../../../lib/testers/rule-tester");
+
+const parserOptions = {
+    ecmaVersion: 6,
+    ecmaFeatures: {
+        experimentalObjectRestSpread: true
+    }
+};
+
+const defaultErrorMessage =
+    "Use an object spread instead of `Object.assign()` eg: `{ ...foo }`";
+const ruleTester = new RuleTester();
+
+ruleTester.run("prefer-object-spread", rule, {
+    valid: [
+        {
+            code: "const bar = { ...foo }",
+            parserOptions
+        },
+        {
+            code: "Object.assign(...foo)",
+            parserOptions
+        },
+        {
+            code: "Object.assign(foo, { bar: baz })",
+            parserOptions
+        }
+    ],
+
+    invalid: [
+        {
+            code: "Object.assign({}, foo)",
+            output: "{...foo}",
+            errors: [
+                {
+                    message: defaultErrorMessage,
+                    type: "CallExpression"
+                }
+            ]
+        },
+        {
+            code: "Object.assign({}, { foo: 'bar' })",
+            output: "{foo: 'bar'}",
+            errors: [
+                {
+                    message: defaultErrorMessage,
+                    type: "CallExpression"
+                }
+            ]
+        },
+        {
+            code: "Object.assign({}, baz, { foo: 'bar' })",
+            output: "{...baz, foo: 'bar'}",
+            errors: [
+                {
+                    message: defaultErrorMessage,
+                    type: "CallExpression"
+                }
+            ]
+        },
+        {
+            code: "Object.assign({}, { foo: 'bar', baz: 'foo' })",
+            output: "{foo: 'bar', baz: 'foo'}",
+            errors: [
+                {
+                    message: defaultErrorMessage,
+                    type: "CallExpression"
+                }
+            ]
+        },
+        {
+            code: "Object.assign({ foo: 'bar' }, baz)",
+            output: "{foo: 'bar', ...baz}",
+            errors: [
+                {
+                    message: defaultErrorMessage,
+                    type: "CallExpression"
+                }
+            ]
+        },
+
+        // Many args
+        {
+            code: "Object.assign({ foo: 'bar' }, cats, dogs, trees, birds)",
+            output: "{foo: 'bar', ...cats, ...dogs, ...trees, ...birds}",
+            parserOptions,
+            errors: [
+                {
+                    message: defaultErrorMessage,
+                    type: "CallExpression"
+                }
+            ]
+        },
+
+        // Nested Object.assign calls
+        {
+            code:
+                "Object.assign({ foo: 'bar' }, Object.assign({ bar: 'foo' }, baz))",
+            output: "{foo: 'bar', bar: 'foo', ...baz}",
+            parserOptions,
+            errors: [
+                {
+                    message: defaultErrorMessage,
+                    type: "CallExpression"
+                },
+                {
+                    message: defaultErrorMessage,
+                    type: "CallExpression"
+                }
+            ]
+        },
+        {
+            code:
+                "Object.assign({ foo: 'bar' }, Object.assign({ bar: 'foo' }, Object.assign({}, { superNested: 'butwhy' })))",
+            output: "{foo: 'bar', bar: 'foo', superNested: 'butwhy'}",
+            parserOptions,
+            errors: [
+                {
+                    message: defaultErrorMessage,
+                    type: "CallExpression"
+                },
+                {
+                    message: defaultErrorMessage,
+                    type: "CallExpression"
+                },
+                {
+                    message: defaultErrorMessage,
+                    type: "CallExpression"
+                }
+            ]
+        },
+
+        // Mix spread in argument
+        {
+            code: "Object.assign({ foo: 'bar', ...bar }, baz)",
+            output: "{foo: 'bar', ...bar, ...baz}",
+            parserOptions,
+            errors: [
+                {
+                    message: defaultErrorMessage,
+                    type: "CallExpression"
+                }
+            ]
+        },
+
+        // Object shorthand
+        {
+            code: "Object.assign({}, { foo, bar, baz })",
+            output: "{foo, bar, baz}",
+            parserOptions,
+            errors: [
+                {
+                    message: defaultErrorMessage,
+                    type: "CallExpression"
+                }
+            ]
+        },
+
+        // Objects with computed properties
+        {
+            code: "Object.assign({}, { [bar]: 'foo' })",
+            output: "{[bar]: 'foo'}",
+            parserOptions,
+            errors: [
+                {
+                    message: defaultErrorMessage,
+                    type: "CallExpression"
+                }
+            ]
+        },
+
+        // Objects with spread properties
+        {
+            code: "Object.assign({ ...bar }, { ...baz })",
+            output: "{...bar, ...baz}",
+            parserOptions,
+            errors: [
+                {
+                    message: defaultErrorMessage,
+                    type: "CallExpression"
+                }
+            ]
+        }
+    ]
+});