1 | /**
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2 | * @fileoverview Disallows unnecessary `return await`
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3 | * @author Jordan Harband
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4 | */
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5 | ;
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6 |
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7 | const astUtils = require("../util/ast-utils");
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8 |
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9 | //------------------------------------------------------------------------------
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10 | // Rule Definition
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11 | //------------------------------------------------------------------------------
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12 |
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13 | const message = "Redundant use of `await` on a return value.";
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14 |
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15 | module.exports = {
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16 | meta: {
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17 | type: "suggestion",
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18 |
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19 | docs: {
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20 | description: "disallow unnecessary `return await`",
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21 | category: "Best Practices",
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22 |
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23 | recommended: false,
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24 |
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25 | url: "https://eslint.org/docs/rules/no-return-await"
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26 | },
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27 |
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28 | fixable: null,
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29 |
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30 | schema: [
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31 | ]
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32 | },
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33 |
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34 | create(context) {
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35 |
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36 | /**
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37 | * Reports a found unnecessary `await` expression.
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38 | * @param {ASTNode} node The node representing the `await` expression to report
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39 | * @returns {void}
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40 | */
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41 | function reportUnnecessaryAwait(node) {
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42 | context.report({
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43 | node: context.getSourceCode().getFirstToken(node),
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44 | loc: node.loc,
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45 | message
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46 | });
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47 | }
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48 |
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49 | /**
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50 | * Determines whether a thrown error from this node will be caught/handled within this function rather than immediately halting
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51 | * this function. For example, a statement in a `try` block will always have an error handler. A statement in
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52 | * a `catch` block will only have an error handler if there is also a `finally` block.
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53 | * @param {ASTNode} node A node representing a location where an could be thrown
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54 | * @returns {boolean} `true` if a thrown error will be caught/handled in this function
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55 | */
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56 | function hasErrorHandler(node) {
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57 | let ancestor = node;
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58 |
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59 | while (!astUtils.isFunction(ancestor) && ancestor.type !== "Program") {
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60 | if (ancestor.parent.type === "TryStatement" && (ancestor === ancestor.parent.block || ancestor === ancestor.parent.handler && ancestor.parent.finalizer)) {
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61 | return true;
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62 | }
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63 | ancestor = ancestor.parent;
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64 | }
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65 | return false;
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66 | }
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67 |
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68 | /**
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69 | * 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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70 | * 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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71 | * @param {ASTNode} node A node representing the `await` expression to check
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72 | * @returns {boolean} The checking result
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73 | */
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74 | function isInTailCallPosition(node) {
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75 | if (node.parent.type === "ArrowFunctionExpression") {
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76 | return true;
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77 | }
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78 | if (node.parent.type === "ReturnStatement") {
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79 | return !hasErrorHandler(node.parent);
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80 | }
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81 | if (node.parent.type === "ConditionalExpression" && (node === node.parent.consequent || node === node.parent.alternate)) {
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82 | return isInTailCallPosition(node.parent);
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83 | }
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84 | if (node.parent.type === "LogicalExpression" && node === node.parent.right) {
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85 | return isInTailCallPosition(node.parent);
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86 | }
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87 | if (node.parent.type === "SequenceExpression" && node === node.parent.expressions[node.parent.expressions.length - 1]) {
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88 | return isInTailCallPosition(node.parent);
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89 | }
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90 | return false;
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91 | }
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92 |
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93 | return {
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94 | AwaitExpression(node) {
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95 | if (isInTailCallPosition(node) && !hasErrorHandler(node)) {
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96 | reportUnnecessaryAwait(node);
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97 | }
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98 | }
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99 | };
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100 | }
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101 | };
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