no-property-in-node.md

Disallow using in to narrow node types instead of looking at properties (eslint-plugin/no-property-in-node)

💭 This rule requires type information.

When working with a node of type ESTree.Node or TSESTree.Node, it can be tempting to use the 'in' operator to narrow the node's type. 'in' narrowing is susceptible to confusing behavior from quirks of ASTs, such as node properties sometimes being omitted from nodes and other times explicitly being set to null or undefined.

Instead, checking a node's type property is generally considered preferable.

Rule Details

Examples of incorrect code for this rule:

/* eslint eslint-plugin/no-property-in-node: error */

/** @type {import('eslint').Rule.RuleModule} */
module.exports = {
  meta: {
    /* ... */
  },
  create(context) {
    return {
      'ClassDeclaration, FunctionDeclaration'(node) {
        if ('superClass' in node) {
          console.log('This is a class declaration:', node);
        }
      },
    };
  },
};

Examples of correct code for this rule:

/* eslint eslint-plugin/no-property-in-node: error */

/** @type {import('eslint').Rule.RuleModule} */
module.exports = {
  meta: {
    /* ... */
  },
  create(context) {
    return {
      'ClassDeclaration, FunctionDeclaration'(node) {
        if (node.type === 'ClassDeclaration') {
          console.log('This is a class declaration:', node);
        }
      },
    };
  },
};