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

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

//------------------------------------------------------------------------------
// Rule Definition
//------------------------------------------------------------------------------
module.exports = {
    meta: {
        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"]
            }
        ]
    },

    create(context) {
        const sourceCode = context.getSourceCode();

        //----------------------------------------------------------------------
        // Helpers
        //----------------------------------------------------------------------
        /**
         * Gets the applicable preference for a particular keyword
         * @returns {string} The applicable option for the keyword, e.g. 'beside'
         */
        function getOption() {
            return context.options[0] || "beside";
        }

        /**
         * Validates the location of an arrow function body
         * @param {ASTNode} node The arrow function body
         * @param {string} keywordName The applicable keyword name for the arrow function body
         * @returns {void}
         */
        function validateExpression(node) {
            const option = getOption();

            let tokenBefore = sourceCode.getTokenBefore(node.body);
            const hasParens = tokenBefore.value === "(";

            if (node.type === "BlockStatement") {
                return;
            }

            let fixerTarget = node.body;

            if (hasParens) {

                // Gets the first token before the function body that is not an open paren
                tokenBefore = sourceCode.getTokenBefore(node.body, token => token.value !== "(");
                fixerTarget = sourceCode.getTokenAfter(tokenBefore);
            }

            if (tokenBefore.loc.end.line === fixerTarget.loc.start.line && option === "below") {
                context.report({
                    node: fixerTarget,
                    message: "Expected a linebreak before this expression.",
                    fix: fixer => fixer.insertTextBefore(fixerTarget, "\n")
                });
            } else if (tokenBefore.loc.end.line !== fixerTarget.loc.start.line && option === "beside") {
                context.report({
                    node: fixerTarget,
                    message: "Expected no linebreak before this expression.",
                    fix: fixer => fixer.replaceTextRange([tokenBefore.range[1], fixerTarget.range[0]], " ")
                });
            }
        }

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