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/**
* @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",
recommended: false // TODO: set to true
},
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);
}
}
};
}
};
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