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📄 no-loop-func

📊 Analysis Summary

Metric	Count
🔧 Functions	7
📦 Imports	7
📊 Variables & Constants	1
📑 Type Aliases	2

📚 Table of Contents

• Default Export
• Imports
• Variables & Constants
• Functions
• Type Aliases

🛠️ File Location:

📂 packages/eslint-plugin/src/rules/no-loop-func.ts

📤 Default Export

export default createRule<Options, MessageIds>({ ... })

Property	Value
name	'no-loop-func'
meta.type	'suggestion'
meta.deprecated.deprecatedSince	'8.64.0'
meta.deprecated.replacedBy	[ { rule: { name: 'no-loop-func', url: 'https://eslint.org/docs/latest/rules/no-loop-func', }, }, ]
meta.deprecated.url	'https://github.com/typescript-eslint/typescript-eslint/issues/12496'
meta.docs.description	'Disallow function declarations that contain unsafe references inside loop statements'
meta.docs.extendsBaseRule	true
meta.hasSuggestions	baseRule.meta.hasSuggestions
meta.messages	baseRule.meta.messages
meta.schema	[]
defaultOptions	[]

Entry point: create — documented under Functions.

---

📦 Imports

Name	Source
TSESLint	@typescript-eslint/utils
TSESTree	@typescript-eslint/utils
AST_NODE_TYPES	@typescript-eslint/utils
InferMessageIdsTypeFromRule	../util
InferOptionsTypeFromRule	../util
createRule	../util
getESLintCoreRule	../util/getESLintCoreRule

---

Variables & Constants

Name	Type	Kind	Value	Exported
CONSTANT_BINDINGS	Set<string>	const	new Set(['await using', 'const', 'using'])	✗

---

Functions

create(context: any): { ArrowFunctionExpression: (node: TSESTree.ArrowFunctionExp…

Parameters:

• context any

Returns: { ArrowFunctionExpression: (node: TSESTree.ArrowFunctionExpression | TSESTree.FunctionDeclaration | TSESTree.FunctionExpression) => void; FunctionDeclaration: (node: TSESTree.ArrowFunctionExpression | TSESTree.FunctionDeclaration | TSESTree.FunctionExpression) => void; FunctionExpression: (node: TSESTree.ArrowFunctionExpression | TSESTree.FunctionDeclaration | TSESTree.FunctionExpression) => void; }

Calls:

• SKIPPED_IIFE_NODES.has
• getContainingLoopNode
• CONSTANT_BINDINGS.has
• getTopLoopNode
• upperRef.isWrite
• variable?.references.every
• context.sourceCode.getScope
• isIIFE
• references.some
• SKIPPED_IIFE_NODES.add
• references .filter(r => r.resolved && !isSafe(loopNode, r)) .map
• context.report
• unsafeRefs.join

Internal Comments:

/**
     * Gets the containing loop node of a specified node.
     *
     * We don't need to check nested functions, so this ignores those.
     * `Scope.through` contains references of nested functions.
     *
     * @param node An AST node to get.
     * @returns The containing loop node of the specified node, or `null`.
     */
// `init` is outside of the loop.
// `right` is outside of the loop.
// We don't need to check nested functions.
// We need to check nested functions only in case of IIFE.
/**
     * Gets the containing loop node of a given node.
     * If the loop was nested, this returns the most outer loop.
     * @param node A node to get. This is a loop node.
     * @param excludedNode A node that the result node should not include.
     * @returns The most outer loop node.
     */
/**
     * Checks whether a given reference which refers to an upper scope's variable is
     * safe or not.
     * @param loopNode A containing loop node.
     * @param reference A reference to check.
     * @returns `true` if the reference is safe or not.
     */
// type references are all safe
// this only really matters for global types that haven't been configured
// Variables which are declared by `const`, `using`, or `await using` are
// safe. They can't be reassigned, so each iteration captures a fresh one.
/*
       * Variables which are declared by `let` in the loop is safe.
       * It's a different instance from the next loop step's.
       */
/*
       * WriteReferences which exist after this border are unsafe because those
       * can modify the variable.
       */ (x2)
/**
       * Checks whether a given reference is safe or not.
       * The reference is every reference of the upper scope's variable we are
       * looking now.
       *
       * It's safe if the reference matches one of the following condition.
       * - is readonly.
       * - doesn't exist inside a local function and after the border.
       *
       * @param upperRef A reference to check.
       * @returns `true` if the reference is safe.
       */
/**
     * Reports functions which match the following condition:
     * - has a loop node in ancestors.
     * - has any references which refers to an unsafe variable.
     *
     * @param node The AST node to check.
     */
// Check if the function is referenced elsewhere in the code (x2)

Code

create(context) {
    const SKIPPED_IIFE_NODES = new Set<
      | TSESTree.ArrowFunctionExpression
      | TSESTree.FunctionDeclaration
      | TSESTree.FunctionExpression
    >();

    /**
     * Gets the containing loop node of a specified node.
     *
     * We don't need to check nested functions, so this ignores those.
     * `Scope.through` contains references of nested functions.
     *
     * @param node An AST node to get.
     * @returns The containing loop node of the specified node, or `null`.
     */
    function getContainingLoopNode(node: TSESTree.Node): TSESTree.Node | null {
      for (
        let currentNode = node;
        currentNode.parent;
        currentNode = currentNode.parent
      ) {
        const parent = currentNode.parent;

        switch (parent.type) {
          case AST_NODE_TYPES.WhileStatement:
          case AST_NODE_TYPES.DoWhileStatement:
            return parent;

          case AST_NODE_TYPES.ForStatement:
            // `init` is outside of the loop.
            if (parent.init !== currentNode) {
              return parent;
            }
            break;

          case AST_NODE_TYPES.ForInStatement:
          case AST_NODE_TYPES.ForOfStatement:
            // `right` is outside of the loop.
            if (parent.right !== currentNode) {
              return parent;
            }
            break;

          case AST_NODE_TYPES.ArrowFunctionExpression:
          case AST_NODE_TYPES.FunctionExpression:
          case AST_NODE_TYPES.FunctionDeclaration:
            // We don't need to check nested functions.

            // We need to check nested functions only in case of IIFE.
            if (SKIPPED_IIFE_NODES.has(parent)) {
              break;
            }
            return null;

          default:
            break;
        }
      }

      return null;
    }

    /**
     * Gets the containing loop node of a given node.
     * If the loop was nested, this returns the most outer loop.
     * @param node A node to get. This is a loop node.
     * @param excludedNode A node that the result node should not include.
     * @returns The most outer loop node.
     */
    function getTopLoopNode(
      node: TSESTree.Node,
      excludedNode: TSESTree.Node | null | undefined,
    ): TSESTree.Node {
      const border = excludedNode ? excludedNode.range[1] : 0;
      let retv = node;
      let containingLoopNode: TSESTree.Node | null = node;

      while (containingLoopNode && containingLoopNode.range[0] >= border) {
        retv = containingLoopNode;
        containingLoopNode = getContainingLoopNode(containingLoopNode);
      }

      return retv;
    }

    /**
     * Checks whether a given reference which refers to an upper scope's variable is
     * safe or not.
     * @param loopNode A containing loop node.
     * @param reference A reference to check.
     * @returns `true` if the reference is safe or not.
     */
    function isSafe(
      loopNode: TSESTree.Node,
      reference: TSESLint.Scope.Reference,
    ): boolean {
      const variable = reference.resolved;
      const definition = variable?.defs[0];
      const declaration = definition?.parent;
      const kind =
        declaration?.type === AST_NODE_TYPES.VariableDeclaration
          ? declaration.kind
          : '';

      // type references are all safe
      // this only really matters for global types that haven't been configured
      if (reference.isTypeReference) {
        return true;
      }

      // Variables which are declared by `const`, `using`, or `await using` are
      // safe. They can't be reassigned, so each iteration captures a fresh one.
      if (CONSTANT_BINDINGS.has(kind)) {
        return true;
      }

      /*
       * Variables which are declared by `let` in the loop is safe.
       * It's a different instance from the next loop step's.
       */
      if (
        kind === 'let' &&
        declaration &&
        declaration.range[0] > loopNode.range[0] &&
        declaration.range[1] < loopNode.range[1]
      ) {
        return true;
      }

      /*
       * WriteReferences which exist after this border are unsafe because those
       * can modify the variable.
       */
      const border = getTopLoopNode(
        loopNode,
        kind === 'let' ? declaration : null,
      ).range[0];

      /**
       * Checks whether a given reference is safe or not.
       * The reference is every reference of the upper scope's variable we are
       * looking now.
       *
       * It's safe if the reference matches one of the following condition.
       * - is readonly.
       * - doesn't exist inside a local function and after the border.
       *
       * @param upperRef A reference to check.
       * @returns `true` if the reference is safe.
       */
      function isSafeReference(upperRef: TSESLint.Scope.Reference): boolean {
        const id = upperRef.identifier;

        return (
          !upperRef.isWrite() ||
          (variable?.scope.variableScope === upperRef.from.variableScope &&
            id.range[0] < border)
        );
      }

      return variable?.references.every(isSafeReference) ?? false;
    }

    /**
     * Reports functions which match the following condition:
     * - has a loop node in ancestors.
     * - has any references which refers to an unsafe variable.
     *
     * @param node The AST node to check.
     */
    function checkForLoops(
      node:
        | TSESTree.ArrowFunctionExpression
        | TSESTree.FunctionDeclaration
        | TSESTree.FunctionExpression,
    ): void {
      const loopNode = getContainingLoopNode(node);

      if (!loopNode) {
        return;
      }

      const references = context.sourceCode.getScope(node).through;

      if (!(node.async || node.generator) && isIIFE(node)) {
        const isFunctionExpression =
          node.type === AST_NODE_TYPES.FunctionExpression;

        // Check if the function is referenced elsewhere in the code
        const isFunctionReferenced =
          isFunctionExpression && node.id
            ? references.some(r => r.identifier.name === node.id?.name)
            : false;

        if (!isFunctionReferenced) {
          SKIPPED_IIFE_NODES.add(node);
          return;
        }
      }

      const unsafeRefs = references
        .filter(r => r.resolved && !isSafe(loopNode, r))
        .map(r => r.identifier.name);

      if (unsafeRefs.length > 0) {
        context.report({
          node,
          messageId: 'unsafeRefs',
          data: { varNames: `'${unsafeRefs.join("', '")}'` },
        });
      }
    }

    return {
      ArrowFunctionExpression: checkForLoops,
      FunctionDeclaration: checkForLoops,
      FunctionExpression: checkForLoops,
    };
  }

isIIFE(node: | TSESTree.ArrowFunctionExpression | TS…): boolean

Parameters:

• node | TSESTree.ArrowFunctionExpression | TSESTree.FunctionDeclaration | TSESTree.FunctionExpression

Returns: boolean

Code

function isIIFE(
  node:
    | TSESTree.ArrowFunctionExpression
    | TSESTree.FunctionDeclaration
    | TSESTree.FunctionExpression,
): boolean {
  return (
    node.parent.type === AST_NODE_TYPES.CallExpression &&
    node.parent.callee === node
  );
}

Internal helpers

Declared inside another function in this file.

getContainingLoopNode(node: TSESTree.Node): TSESTree.Node | null

Gets the containing loop node of a specified node.

We don't need to check nested functions, so this ignores those. Scope.through contains references of nested functions.

Parameters:

• node any: An AST node to get.

Returns: undefined The containing loop node of the specified node, or null.

Raw JSDoc

/**
     * Gets the containing loop node of a specified node.
     *
     * We don't need to check nested functions, so this ignores those.
     * `Scope.through` contains references of nested functions.
     *
     * @param node An AST node to get.
     * @returns The containing loop node of the specified node, or `null`.
     */

Calls:

• SKIPPED_IIFE_NODES.has

Internal Comments:

// `init` is outside of the loop.
// `right` is outside of the loop.
// We don't need to check nested functions.
// We need to check nested functions only in case of IIFE.

Code

function getContainingLoopNode(node: TSESTree.Node): TSESTree.Node | null {
      for (
        let currentNode = node;
        currentNode.parent;
        currentNode = currentNode.parent
      ) {
        const parent = currentNode.parent;

        switch (parent.type) {
          case AST_NODE_TYPES.WhileStatement:
          case AST_NODE_TYPES.DoWhileStatement:
            return parent;

          case AST_NODE_TYPES.ForStatement:
            // `init` is outside of the loop.
            if (parent.init !== currentNode) {
              return parent;
            }
            break;

          case AST_NODE_TYPES.ForInStatement:
          case AST_NODE_TYPES.ForOfStatement:
            // `right` is outside of the loop.
            if (parent.right !== currentNode) {
              return parent;
            }
            break;

          case AST_NODE_TYPES.ArrowFunctionExpression:
          case AST_NODE_TYPES.FunctionExpression:
          case AST_NODE_TYPES.FunctionDeclaration:
            // We don't need to check nested functions.

            // We need to check nested functions only in case of IIFE.
            if (SKIPPED_IIFE_NODES.has(parent)) {
              break;
            }
            return null;

          default:
            break;
        }
      }

      return null;
    }

getTopLoopNode(node: TSESTree.Node, excludedNode: TSESTree.Node | null | undefined): TSESTree.Node

Gets the containing loop node of a given node. If the loop was nested, this returns the most outer loop.

Parameters:

• node any: A node to get. This is a loop node.
• excludedNode any: A node that the result node should not include.

Returns: undefined The most outer loop node.

Raw JSDoc

/**
     * Gets the containing loop node of a given node.
     * If the loop was nested, this returns the most outer loop.
     * @param node A node to get. This is a loop node.
     * @param excludedNode A node that the result node should not include.
     * @returns The most outer loop node.
     */

Calls:

• getContainingLoopNode

Code

function getTopLoopNode(
      node: TSESTree.Node,
      excludedNode: TSESTree.Node | null | undefined,
    ): TSESTree.Node {
      const border = excludedNode ? excludedNode.range[1] : 0;
      let retv = node;
      let containingLoopNode: TSESTree.Node | null = node;

      while (containingLoopNode && containingLoopNode.range[0] >= border) {
        retv = containingLoopNode;
        containingLoopNode = getContainingLoopNode(containingLoopNode);
      }

      return retv;
    }

isSafe(loopNode: TSESTree.Node, reference: TSESLint.Scope.Reference): boolean

Checks whether a given reference which refers to an upper scope's variable is safe or not.

Parameters:

• loopNode any: A containing loop node.
• reference any: A reference to check.

Returns: undefined true if the reference is safe or not.

Raw JSDoc

/**
     * Checks whether a given reference which refers to an upper scope's variable is
     * safe or not.
     * @param loopNode A containing loop node.
     * @param reference A reference to check.
     * @returns `true` if the reference is safe or not.
     */

Calls:

• CONSTANT_BINDINGS.has
• getTopLoopNode
• upperRef.isWrite
• variable?.references.every

Internal Comments:

// type references are all safe
// this only really matters for global types that haven't been configured
// Variables which are declared by `const`, `using`, or `await using` are
// safe. They can't be reassigned, so each iteration captures a fresh one.
/*
       * Variables which are declared by `let` in the loop is safe.
       * It's a different instance from the next loop step's.
       */
/*
       * WriteReferences which exist after this border are unsafe because those
       * can modify the variable.
       */ (x2)
/**
       * Checks whether a given reference is safe or not.
       * The reference is every reference of the upper scope's variable we are
       * looking now.
       *
       * It's safe if the reference matches one of the following condition.
       * - is readonly.
       * - doesn't exist inside a local function and after the border.
       *
       * @param upperRef A reference to check.
       * @returns `true` if the reference is safe.
       */

Code

function isSafe(
      loopNode: TSESTree.Node,
      reference: TSESLint.Scope.Reference,
    ): boolean {
      const variable = reference.resolved;
      const definition = variable?.defs[0];
      const declaration = definition?.parent;
      const kind =
        declaration?.type === AST_NODE_TYPES.VariableDeclaration
          ? declaration.kind
          : '';

      // type references are all safe
      // this only really matters for global types that haven't been configured
      if (reference.isTypeReference) {
        return true;
      }

      // Variables which are declared by `const`, `using`, or `await using` are
      // safe. They can't be reassigned, so each iteration captures a fresh one.
      if (CONSTANT_BINDINGS.has(kind)) {
        return true;
      }

      /*
       * Variables which are declared by `let` in the loop is safe.
       * It's a different instance from the next loop step's.
       */
      if (
        kind === 'let' &&
        declaration &&
        declaration.range[0] > loopNode.range[0] &&
        declaration.range[1] < loopNode.range[1]
      ) {
        return true;
      }

      /*
       * WriteReferences which exist after this border are unsafe because those
       * can modify the variable.
       */
      const border = getTopLoopNode(
        loopNode,
        kind === 'let' ? declaration : null,
      ).range[0];

      /**
       * Checks whether a given reference is safe or not.
       * The reference is every reference of the upper scope's variable we are
       * looking now.
       *
       * It's safe if the reference matches one of the following condition.
       * - is readonly.
       * - doesn't exist inside a local function and after the border.
       *
       * @param upperRef A reference to check.
       * @returns `true` if the reference is safe.
       */
      function isSafeReference(upperRef: TSESLint.Scope.Reference): boolean {
        const id = upperRef.identifier;

        return (
          !upperRef.isWrite() ||
          (variable?.scope.variableScope === upperRef.from.variableScope &&
            id.range[0] < border)
        );
      }

      return variable?.references.every(isSafeReference) ?? false;
    }

isSafeReference(upperRef: TSESLint.Scope.Reference): boolean

Checks whether a given reference is safe or not. The reference is every reference of the upper scope's variable we are looking now.

It's safe if the reference matches one of the following condition. - is readonly. - doesn't exist inside a local function and after the border.

Parameters:

• upperRef any: A reference to check.

Returns: undefined true if the reference is safe.

Raw JSDoc

/**
       * Checks whether a given reference is safe or not.
       * The reference is every reference of the upper scope's variable we are
       * looking now.
       *
       * It's safe if the reference matches one of the following condition.
       * - is readonly.
       * - doesn't exist inside a local function and after the border.
       *
       * @param upperRef A reference to check.
       * @returns `true` if the reference is safe.
       */

Calls:

• upperRef.isWrite

Code

function isSafeReference(upperRef: TSESLint.Scope.Reference): boolean {
        const id = upperRef.identifier;

        return (
          !upperRef.isWrite() ||
          (variable?.scope.variableScope === upperRef.from.variableScope &&
            id.range[0] < border)
        );
      }

checkForLoops(node: | TSESTree.ArrowFunctionExpression | TS…): void

Reports functions which match the following condition: - has a loop node in ancestors. - has any references which refers to an unsafe variable.

Parameters:

• node any: The AST node to check.

Raw JSDoc

/**
     * Reports functions which match the following condition:
     * - has a loop node in ancestors.
     * - has any references which refers to an unsafe variable.
     *
     * @param node The AST node to check.
     */

Calls:

• getContainingLoopNode
• context.sourceCode.getScope
• isIIFE
• references.some
• SKIPPED_IIFE_NODES.add
• references .filter(r => r.resolved && !isSafe(loopNode, r)) .map
• context.report
• unsafeRefs.join

Internal Comments:

// Check if the function is referenced elsewhere in the code (x2)

Code

function checkForLoops(
      node:
        | TSESTree.ArrowFunctionExpression
        | TSESTree.FunctionDeclaration
        | TSESTree.FunctionExpression,
    ): void {
      const loopNode = getContainingLoopNode(node);

      if (!loopNode) {
        return;
      }

      const references = context.sourceCode.getScope(node).through;

      if (!(node.async || node.generator) && isIIFE(node)) {
        const isFunctionExpression =
          node.type === AST_NODE_TYPES.FunctionExpression;

        // Check if the function is referenced elsewhere in the code
        const isFunctionReferenced =
          isFunctionExpression && node.id
            ? references.some(r => r.identifier.name === node.id?.name)
            : false;

        if (!isFunctionReferenced) {
          SKIPPED_IIFE_NODES.add(node);
          return;
        }
      }

      const unsafeRefs = references
        .filter(r => r.resolved && !isSafe(loopNode, r))
        .map(r => r.identifier.name);

      if (unsafeRefs.length > 0) {
        context.report({
          node,
          messageId: 'unsafeRefs',
          data: { varNames: `'${unsafeRefs.join("', '")}'` },
        });
      }
    }

---

Type Aliases

Options

type Options = InferOptionsTypeFromRule<typeof baseRule>;

MessageIds

type MessageIds = InferMessageIdsTypeFromRule<typeof baseRule>;

---

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