eslint-plugin-fp/rules/no-mutating-assign.js

ESLint rules for functional programming

'use strict';

const _ = require('lodash/fp');

const isObjectAssign = _.matches({
  type: 'MemberExpression',
  object: {
    type: 'Identifier',
    name: 'Object'
  },
  property: {
    type: 'Identifier',
    name: 'assign'
  }
});

const isObjectExpression = _.flow(
  _.property('type'),
  _.includes(_, ['ObjectExpression', 'ArrayExpression'])
);

const isFunctionExpression = _.flow(
  _.property('type'),
  _.includes(_, ['FunctionExpression', 'ArrowFunctionExpression'])
);

function isAllowedFirstArgument(arg) {
  return isObjectExpression(arg) || isFunctionExpression(arg);
}

const create = function (context) {
  return {
    CallExpression(node) {
      if (isObjectAssign(node.callee) && !isAllowedFirstArgument(node.arguments[0])) {
        context.report({
          node,
          message: 'Unallowed use of mutating `Object.assign`'
        });
      }
    }
  };
};

module.exports = {
  create,
  meta: {
    docs: {
      description: 'Forbid the use of [`Object.assign()`](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Object/assign) with a variable as first argument.',
      recommended: 'error'
    }
  }
};