eslint/lib/rules/implicit-arrow-linebreak.js

/**
 * @fileoverview enforce the location of arrow function bodies
 * @author Sharmila Jesupaul
 */
"use strict";

const { isCommentToken, isNotOpeningParenToken } = require("./utils/ast-utils");

//------------------------------------------------------------------------------
// Rule Definition
//------------------------------------------------------------------------------
module.exports = {
    meta: {
        type: "layout",

        docs: {
            description: "enforce the location of arrow function bodies",
            category: "Stylistic Issues",
            recommended: false,
            url: "https://eslint.org/docs/rules/implicit-arrow-linebreak"
        },

        fixable: "whitespace",

        schema: [
            {
                enum: ["beside", "below"]
            }
        ],
        messages: {
            expected: "Expected a linebreak before this expression.",
            unexpected: "Expected no linebreak before this expression."
        }
    },

    create(context) {
        const sourceCode = context.getSourceCode();
        const option = context.options[0] || "beside";

        /**
         * Validates the location of an arrow function body
         * @param {ASTNode} node The arrow function body
         * @returns {void}
         */
        function validateExpression(node) {
            if (node.body.type === "BlockStatement") {
                return;
            }

            const arrowToken = sourceCode.getTokenBefore(node.body, isNotOpeningParenToken);
            const firstTokenOfBody = sourceCode.getTokenAfter(arrowToken);

            if (arrowToken.loc.end.line === firstTokenOfBody.loc.start.line && option === "below") {
                context.report({
                    node: firstTokenOfBody,
                    messageId: "expected",
                    fix: fixer => fixer.insertTextBefore(firstTokenOfBody, "\n")
                });
            } else if (arrowToken.loc.end.line !== firstTokenOfBody.loc.start.line && option === "beside") {
                context.report({
                    node: firstTokenOfBody,
                    messageId: "unexpected",
                    fix(fixer) {
                        if (sourceCode.getFirstTokenBetween(arrowToken, firstTokenOfBody, { includeComments: true, filter: isCommentToken })) {
                            return null;
                        }

                        return fixer.replaceTextRange([arrowToken.range[1], firstTokenOfBody.range[0]], " ");
                    }
                });
            }
        }

        //----------------------------------------------------------------------
        // Public
        //----------------------------------------------------------------------
        return {
            ArrowFunctionExpression: node => validateExpression(node)
        };
    }
};