no-param-reassign.js
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/**
* @fileoverview Disallow reassignment of function parameters.
* @author Nat Burns
*/
"use strict";
//------------------------------------------------------------------------------
// Rule Definition
//------------------------------------------------------------------------------
const stopNodePattern = /(?:Statement|Declaration|Function(?:Expression)?|Program)$/;
module.exports = {
meta: {
docs: {
description: "disallow reassigning `function` parameters",
category: "Best Practices",
recommended: false
},
schema: [
{
type: "object",
properties: {
props: { type: "boolean" }
},
additionalProperties: false
}
]
},
create(context) {
const props = context.options[0] && Boolean(context.options[0].props);
/**
* Checks whether or not the reference modifies properties of its variable.
* @param {Reference} reference - A reference to check.
* @returns {boolean} Whether or not the reference modifies properties of its variable.
*/
function isModifyingProp(reference) {
let node = reference.identifier;
let parent = node.parent;
while (parent && !stopNodePattern.test(parent.type)) {
switch (parent.type) {
// e.g. foo.a = 0;
case "AssignmentExpression":
return parent.left === node;
// e.g. ++foo.a;
case "UpdateExpression":
return true;
// e.g. delete foo.a;
case "UnaryExpression":
if (parent.operator === "delete") {
return true;
}
break;
// EXCLUDES: e.g. cache.get(foo.a).b = 0;
case "CallExpression":
if (parent.callee !== node) {
return false;
}
break;
// EXCLUDES: e.g. cache[foo.a] = 0;
case "MemberExpression":
if (parent.property === node) {
return false;
}
break;
default:
break;
}
node = parent;
parent = node.parent;
}
return false;
}
/**
* Reports a reference if is non initializer and writable.
* @param {Reference} reference - A reference to check.
* @param {int} index - The index of the reference in the references.
* @param {Reference[]} references - The array that the reference belongs to.
* @returns {void}
*/
function checkReference(reference, index, references) {
const identifier = reference.identifier;
if (identifier &&
!reference.init &&
// Destructuring assignments can have multiple default value,
// so possibly there are multiple writeable references for the same identifier.
(index === 0 || references[index - 1].identifier !== identifier)
) {
if (reference.isWrite()) {
context.report({ node: identifier, message: "Assignment to function parameter '{{name}}'.", data: { name: identifier.name } });
} else if (props && isModifyingProp(reference)) {
context.report({ node: identifier, message: "Assignment to property of function parameter '{{name}}'.", data: { name: identifier.name } });
}
}
}
/**
* Finds and reports references that are non initializer and writable.
* @param {Variable} variable - A variable to check.
* @returns {void}
*/
function checkVariable(variable) {
if (variable.defs[0].type === "Parameter") {
variable.references.forEach(checkReference);
}
}
/**
* Checks parameters of a given function node.
* @param {ASTNode} node - A function node to check.
* @returns {void}
*/
function checkForFunction(node) {
context.getDeclaredVariables(node).forEach(checkVariable);
}
return {
// `:exit` is needed for the `node.parent` property of identifier nodes.
"FunctionDeclaration:exit": checkForFunction,
"FunctionExpression:exit": checkForFunction,
"ArrowFunctionExpression:exit": checkForFunction
};
}
};