eslint-plugin-criteo/lib/rules/no-spreading-accumulators.js

An opinionated collection of ESLint shared configs and rules used by Criteo.

/**
 * @fileoverview no-spreading-accumulators
 * @author Connor Ullman
 * @author Joe Pikowski
 */
'use strict';

const moreInfo = `More info: https://github.com/criteo/eslint-plugin-criteo#no-spreading-accumulators`;

export default {
  meta: {
    type: 'problem',
    fixable: 'code',
    schema: [],
    messages: {
      objectMessage: `Creating a new object by spreading the previous accumulator at every iteration of a .reduce() call has O(n^2) time & spatial complexity. If possible, make this O(n) by assigning to the existing accumulator or use mappedBy() or mappedByUnique() instead of .reduce(). ${moreInfo}`,
      arrayMessage: `Creating a new array by spreading the previous accumulator at every iteration of a .reduce() call has O(n^2) time & spatial complexity. If possible, make this O(n) by pushing to the existing accumulator instead. ${moreInfo}`,
    },
  },
  create: function (context) {
    return {
      'CallExpression:has([callee.property.name=reduce])[arguments] > ArrowFunctionExpression'(node) {
        // only fetch the source code if getNodeText is called, and only fetch it once if we do
        const sourceCode = {
          _source: null,
          getNodeText(node) {
            if (this._source == null) this._source = context.getSourceCode().text;
            if (node == null) return null;
            return this._source.slice(node.range[0], node.range[1]);
          },
        };

        const getReportConfig = (spreadNode, fix, messageId) => ({
          node: spreadNode.argument,
          loc: spreadNode.loc,
          messageId,
          fix,
        });

        const isSpreadNode = (spreadNode) =>
          spreadNode &&
          (spreadNode.type === 'ExperimentalSpreadProperty' ||
            spreadNode.type === 'SpreadProperty' ||
            spreadNode.type === 'SpreadElement');

        const arrowFn = node;
        const arrowFnFirstArg = arrowFn.params[0];
        if (!arrowFnFirstArg) return;

        const arrowFnFirstArgName = arrowFnFirstArg.name;
        const expression = arrowFn.body;
        if (!expression) return;

        if (expression.type === 'ObjectExpression') {
          let spreadIndex = null;
          let hasNonFirstArgumentSpread = true;
          for (let i = 0; i < expression.properties.length; i++) {
            const prop = expression.properties[i];
            if (isSpreadNode(prop)) {
              const isAccumulatorBeingSpread = prop.argument.name === arrowFnFirstArgName;
              if (isAccumulatorBeingSpread) {
                spreadIndex = i;
              } else {
                hasNonFirstArgumentSpread = false;
              }
            }
          }
          if (spreadIndex == null) return;

          const spreadNode = expression.properties[spreadIndex];
          if (!spreadNode) return;

          const getObjectFix = () => {
            // if something other than the accumulator is being spread, we won't automatically fix it
            if (!hasNonFirstArgumentSpread) return undefined;

            // if the spread isn't the first argument of the expression, then you can only assign to the accumulator if the entry
            // isn't already present without potentially changing behavior of the reduce
            if (spreadIndex !== 0) return undefined;

            if (arrowFn.params <= 1) return undefined;

            const paramsText = arrowFn.params.map((o) => sourceCode.getNodeText(o)).join(', ');
            const nonSpreadPropertyTexts = expression.properties
              .filter((o, i) => i != spreadIndex)
              .map(
                (o) => `${arrowFnFirstArgName}[${sourceCode.getNodeText(o.key)}] = ${sourceCode.getNodeText(o.value)}`,
              );
            if (nonSpreadPropertyTexts.length <= 0) return undefined;

            const bodyText = `${nonSpreadPropertyTexts.join('; ')}; return ${arrowFnFirstArgName};`;
            const newArrowFnText = `(${paramsText}) => { ${bodyText} }`;
            const fix = (fixer) => [fixer.replaceTextRange(arrowFn.range, newArrowFnText)];
            return fix;
          };
          const reportConfig = getReportConfig(spreadNode, getObjectFix(), 'objectMessage');
          return context.report(reportConfig);
        } else if (expression.type === 'ArrayExpression') {
          let spreadIndex = null;
          let hasNonFirstArgumentSpread = true;
          for (let i = 0; i < expression.elements.length; i++) {
            const prop = expression.elements[i];
            if (isSpreadNode(prop)) {
              const isAccumulatorBeingSpread = prop.argument.name === arrowFnFirstArgName;
              if (isAccumulatorBeingSpread) {
                spreadIndex = i;
              } else {
                hasNonFirstArgumentSpread = false;
              }
            }
          }
          if (spreadIndex == null) return;

          const spreadNode = expression.elements[spreadIndex];
          if (!spreadNode) return;

          const getArrayFix = () => {
            // if something other than the accumulator is being spread, we won't automatically fix it
            if (!hasNonFirstArgumentSpread) return undefined;

            // if the spread isn't the first argument of the expression, then you can only assign to the accumulator if the entry
            // isn't already present without potentially changing behavior of the reduce
            if (spreadIndex !== 0) return undefined;

            if (arrowFn.params <= 1) return undefined;

            const paramsText = arrowFn.params.map((o) => sourceCode.getNodeText(o)).join(', ');
            const nonSpreadElementTexts = expression.elements
              .filter((o, i) => i != spreadIndex)
              .map((o) => sourceCode.getNodeText(o));
            if (!nonSpreadElementTexts || nonSpreadElementTexts.length <= 0) return undefined;

            const bodyText = `${arrowFnFirstArgName}.push(${nonSpreadElementTexts.join(
              ', ',
            )}); return ${arrowFnFirstArgName};`;
            const newArrowFnText = `(${paramsText}) => { ${bodyText} }`;
            const fix = (fixer) => [fixer.replaceTextRange(arrowFn.range, newArrowFnText)];
            return fix;
          };
          const reportConfig = getReportConfig(spreadNode, getArrayFix(), 'arrayMessage');
          return context.report(reportConfig);
        }
      },
    };
  },
};