/**
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* @fileoverview Disallows unnecessary `return await`
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* @author Jordan Harband
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*/
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"use strict";
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const astUtils = require("./utils/ast-utils");
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//------------------------------------------------------------------------------
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// Rule Definition
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//------------------------------------------------------------------------------
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const message = "Redundant use of `await` on a return value.";
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module.exports = {
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meta: {
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type: "suggestion",
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docs: {
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description: "disallow unnecessary `return await`",
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category: "Best Practices",
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recommended: false,
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url: "https://eslint.org/docs/rules/no-return-await"
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},
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fixable: null,
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schema: [
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]
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},
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create(context) {
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/**
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* Reports a found unnecessary `await` expression.
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* @param {ASTNode} node The node representing the `await` expression to report
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* @returns {void}
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*/
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function reportUnnecessaryAwait(node) {
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context.report({
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node: context.getSourceCode().getFirstToken(node),
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loc: node.loc,
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message
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});
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}
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/**
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* Determines whether a thrown error from this node will be caught/handled within this function rather than immediately halting
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* this function. For example, a statement in a `try` block will always have an error handler. A statement in
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* a `catch` block will only have an error handler if there is also a `finally` block.
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* @param {ASTNode} node A node representing a location where an could be thrown
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* @returns {boolean} `true` if a thrown error will be caught/handled in this function
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*/
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function hasErrorHandler(node) {
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let ancestor = node;
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while (!astUtils.isFunction(ancestor) && ancestor.type !== "Program") {
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if (ancestor.parent.type === "TryStatement" && (ancestor === ancestor.parent.block || ancestor === ancestor.parent.handler && ancestor.parent.finalizer)) {
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return true;
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}
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ancestor = ancestor.parent;
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}
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return false;
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}
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/**
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* Checks if a node is placed in tail call position. Once `return` arguments (or arrow function expressions) can be a complex expression,
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* 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.
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* @param {ASTNode} node A node representing the `await` expression to check
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* @returns {boolean} The checking result
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*/
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function isInTailCallPosition(node) {
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if (node.parent.type === "ArrowFunctionExpression") {
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return true;
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}
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if (node.parent.type === "ReturnStatement") {
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return !hasErrorHandler(node.parent);
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}
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if (node.parent.type === "ConditionalExpression" && (node === node.parent.consequent || node === node.parent.alternate)) {
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return isInTailCallPosition(node.parent);
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}
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if (node.parent.type === "LogicalExpression" && node === node.parent.right) {
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return isInTailCallPosition(node.parent);
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}
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if (node.parent.type === "SequenceExpression" && node === node.parent.expressions[node.parent.expressions.length - 1]) {
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return isInTailCallPosition(node.parent);
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}
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return false;
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}
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return {
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AwaitExpression(node) {
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if (isInTailCallPosition(node) && !hasErrorHandler(node)) {
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reportUnnecessaryAwait(node);
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}
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}
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};
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}
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};
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