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/**
* @fileoverview Rule to replace assignment expressions with operator assignment
* @author Brandon Mills
*/
"use strict";
//------------------------------------------------------------------------------
// Requirements
//------------------------------------------------------------------------------
const astUtils = require("./utils/ast-utils");
//------------------------------------------------------------------------------
// Helpers
//------------------------------------------------------------------------------
/**
* Checks whether an operator is commutative and has an operator assignment
* shorthand form.
* @param {string} operator Operator to check.
* @returns {boolean} True if the operator is commutative and has a
* shorthand form.
*/
function isCommutativeOperatorWithShorthand(operator) {
return ["*", "&", "^", "|"].includes(operator);
}
/**
* Checks whether an operator is not commutative and has an operator assignment
* shorthand form.
* @param {string} operator Operator to check.
* @returns {boolean} True if the operator is not commutative and has
* a shorthand form.
*/
function isNonCommutativeOperatorWithShorthand(operator) {
return ["+", "-", "/", "%", "<<", ">>", ">>>", "**"].includes(operator);
}
//------------------------------------------------------------------------------
// Rule Definition
//------------------------------------------------------------------------------
/**
* Determines if the left side of a node can be safely fixed (i.e. if it activates the same getters/setters and)
* toString calls regardless of whether assignment shorthand is used)
* @param {ASTNode} node The node on the left side of the expression
* @returns {boolean} `true` if the node can be fixed
*/
function canBeFixed(node) {
return (
node.type === "Identifier" ||
(node.type === "MemberExpression" &&
(node.object.type === "Identifier" ||
node.object.type === "ThisExpression") &&
(!node.computed || node.property.type === "Literal"))
);
}
/** @type {import('../types').Rule.RuleModule} */
module.exports = {
meta: {
type: "suggestion",
defaultOptions: ["always"],
docs: {
description:
"Require or disallow assignment operator shorthand where possible",
recommended: false,
frozen: true,
url: "https://eslint.org/docs/latest/rules/operator-assignment",
},
schema: [
{
enum: ["always", "never"],
},
],
fixable: "code",
messages: {
replaced:
"Assignment (=) can be replaced with operator assignment ({{operator}}).",
unexpected:
"Unexpected operator assignment ({{operator}}) shorthand.",
},
},
create(context) {
const never = context.options[0] === "never";
const sourceCode = context.sourceCode;
/**
* Returns the operator token of an AssignmentExpression or BinaryExpression
* @param {ASTNode} node An AssignmentExpression or BinaryExpression node
* @returns {Token} The operator token in the node
*/
function getOperatorToken(node) {
return sourceCode.getFirstTokenBetween(
node.left,
node.right,
token => token.value === node.operator,
);
}
/**
* Ensures that an assignment uses the shorthand form where possible.
* @param {ASTNode} node An AssignmentExpression node.
* @returns {void}
*/
function verify(node) {
if (
node.operator !== "=" ||
node.right.type !== "BinaryExpression"
) {
return;
}
const left = node.left;
const expr = node.right;
const operator = expr.operator;
if (
isCommutativeOperatorWithShorthand(operator) ||
isNonCommutativeOperatorWithShorthand(operator)
) {
const replacementOperator = `${operator}=`;
if (astUtils.isSameReference(left, expr.left, true)) {
context.report({
node,
messageId: "replaced",
data: { operator: replacementOperator },
fix(fixer) {
if (canBeFixed(left) && canBeFixed(expr.left)) {
const equalsToken = getOperatorToken(node);
const operatorToken = getOperatorToken(expr);
const leftText = sourceCode
.getText()
.slice(node.range[0], equalsToken.range[0]);
const rightText = sourceCode
.getText()
.slice(
operatorToken.range[1],
node.right.range[1],
);
// Check for comments that would be removed.
if (
sourceCode.commentsExistBetween(
equalsToken,
operatorToken,
)
) {
return null;
}
return fixer.replaceText(
node,
`${leftText}${replacementOperator}${rightText}`,
);
}
return null;
},
});
} else if (
astUtils.isSameReference(left, expr.right, true) &&
isCommutativeOperatorWithShorthand(operator)
) {
/*
* This case can't be fixed safely.
* If `a` and `b` both have custom valueOf() behavior, then fixing `a = b * a` to `a *= b` would
* change the execution order of the valueOf() functions.
*/
context.report({
node,
messageId: "replaced",
data: { operator: replacementOperator },
});
}
}
}
/**
* Warns if an assignment expression uses operator assignment shorthand.
* @param {ASTNode} node An AssignmentExpression node.
* @returns {void}
*/
function prohibit(node) {
if (
node.operator !== "=" &&
!astUtils.isLogicalAssignmentOperator(node.operator)
) {
context.report({
node,
messageId: "unexpected",
data: { operator: node.operator },
fix(fixer) {
if (canBeFixed(node.left)) {
const firstToken = sourceCode.getFirstToken(node);
const operatorToken = getOperatorToken(node);
const leftText = sourceCode
.getText()
.slice(node.range[0], operatorToken.range[0]);
const newOperator = node.operator.slice(0, -1);
let rightText;
// Check for comments that would be duplicated.
if (
sourceCode.commentsExistBetween(
firstToken,
operatorToken,
)
) {
return null;
}
// If this change would modify precedence (e.g. `foo *= bar + 1` => `foo = foo * (bar + 1)`), parenthesize the right side.
if (
astUtils.getPrecedence(node.right) <=
astUtils.getPrecedence({
type: "BinaryExpression",
operator: newOperator,
}) &&
!astUtils.isParenthesised(
sourceCode,
node.right,
)
) {
rightText = `${sourceCode.text.slice(operatorToken.range[1], node.right.range[0])}(${sourceCode.getText(node.right)})`;
} else {
const tokenAfterOperator =
sourceCode.getTokenAfter(operatorToken, {
includeComments: true,
});
let rightTextPrefix = "";
if (
operatorToken.range[1] ===
tokenAfterOperator.range[0] &&
!astUtils.canTokensBeAdjacent(
{
type: "Punctuator",
value: newOperator,
},
tokenAfterOperator,
)
) {
rightTextPrefix = " "; // foo+=+bar -> foo= foo+ +bar
}
rightText = `${rightTextPrefix}${sourceCode.text.slice(operatorToken.range[1], node.range[1])}`;
}
return fixer.replaceText(
node,
`${leftText}= ${leftText}${newOperator}${rightText}`,
);
}
return null;
},
});
}
}
return {
AssignmentExpression: !never ? verify : prohibit,
};
},
};
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