eslint/lib/rules/no-return-await.js

/**
 * @fileoverview Disallows unnecessary `return await`
 * @author Jordan Harband
 */
"use strict";

const astUtils = require("../ast-utils");

//------------------------------------------------------------------------------
// Rule Definition
//------------------------------------------------------------------------------

const message = "Redundant use of `await` on a return value.";

module.exports = {
    meta: {
        docs: {
            description: "disallow unnecessary `return await`",
            category: "Best Practices",

            // TODO: set to true
            recommended: false,

            url: "https://eslint.org/docs/rules/no-return-await"
        },
        fixable: null,
        schema: [
        ]
    },

    create(context) {

        /**
         * Reports a found unnecessary `await` expression.
         * @param {ASTNode} node The node representing the `await` expression to report
         * @returns {void}
         */
        function reportUnnecessaryAwait(node) {
            context.report({
                node: context.getSourceCode().getFirstToken(node),
                loc: node.loc,
                message
            });
        }

        /**
         * Determines whether a thrown error from this node will be caught/handled within this function rather than immediately halting
         * this function. For example, a statement in a `try` block will always have an error handler. A statement in
         * a `catch` block will only have an error handler if there is also a `finally` block.
         * @param {ASTNode} node A node representing a location where an could be thrown
         * @returns {boolean} `true` if a thrown error will be caught/handled in this function
         */
        function hasErrorHandler(node) {
            let ancestor = node;

            while (!astUtils.isFunction(ancestor) && ancestor.type !== "Program") {
                if (ancestor.parent.type === "TryStatement" && (ancestor === ancestor.parent.block || ancestor === ancestor.parent.handler && ancestor.parent.finalizer)) {
                    return true;
                }
                ancestor = ancestor.parent;
            }
            return false;
        }

        /**
         * Checks if a node is placed in tail call position. Once `return` arguments (or arrow function expressions) can be a complex expression,
         * an `await` expression could or could not be unnecessary by the definition of this rule. So we're looking for `await` expressions that are in tail position.
         * @param {ASTNode} node A node representing the `await` expression to check
         * @returns {boolean} The checking result
         */
        function isInTailCallPosition(node) {
            if (node.parent.type === "ArrowFunctionExpression") {
                return true;
            }
            if (node.parent.type === "ReturnStatement") {
                return !hasErrorHandler(node.parent);
            }
            if (node.parent.type === "ConditionalExpression" && (node === node.parent.consequent || node === node.parent.alternate)) {
                return isInTailCallPosition(node.parent);
            }
            if (node.parent.type === "LogicalExpression" && node === node.parent.right) {
                return isInTailCallPosition(node.parent);
            }
            if (node.parent.type === "SequenceExpression" && node === node.parent.expressions[node.parent.expressions.length - 1]) {
                return isInTailCallPosition(node.parent);
            }
            return false;
        }

        return {
            AwaitExpression(node) {
                if (isInTailCallPosition(node) && !hasErrorHandler(node)) {
                    reportUnnecessaryAwait(node);
                }
            }
        };
    }
};