no-implied-eval

Disallow the use of eval()-like methods.

✅

Extending "plugin:@typescript-eslint/recommended-type-checked" in an ESLint configuration enables this rule.

💭

This rule requires type information to run.

It's considered a good practice to avoid using eval(). There are security and performance implications involved with doing so, which is why many linters recommend disallowing eval(). However, there are some other ways to pass a string and have it interpreted as JavaScript code that have similar concerns.

The first is using setTimeout(), setInterval(), setImmediate or execScript() (Internet Explorer only), all of which can accept a string of code as their first argument

setTimeout('alert(`Hi!`);', 100);

or using new Function()

const fn = new Function('a', 'b', 'return a + b');

This is considered an implied eval() because a string of code is passed in to be interpreted. The same can be done with setInterval(), setImmediate() and execScript(). All interpret the JavaScript code in the global scope.

The best practice is to avoid using new Function() or execScript() and always use a function for the first argument of setTimeout(), setInterval() and setImmediate().

Examples

This rule aims to eliminate implied eval() through the use of new Function(), setTimeout(), setInterval(), setImmediate() or execScript().

❌ Incorrect

setTimeout('alert(`Hi!`);', 100);

setInterval('alert(`Hi!`);', 100);

setImmediate('alert(`Hi!`)');

execScript('alert(`Hi!`)');

window.setTimeout('count = 5', 10);

window.setInterval('foo = bar', 10);

const fn = '() = {}';
setTimeout(fn, 100);

const fn = () => {
  return 'x = 10';
};
setTimeout(fn(), 100);

const fn = new Function('a', 'b', 'return a + b');

Open in Playground

✅ Correct

setTimeout(function () {
  alert('Hi!');
}, 100);

setInterval(function () {
  alert('Hi!');
}, 100);

setImmediate(function () {
  alert('Hi!');
});

execScript(function () {
  alert('Hi!');
});

const fn = () => {};
setTimeout(fn, 100);

const foo = {
  fn: function () {},
};
setTimeout(foo.fn, 100);
setTimeout(foo.fn.bind(this), 100);

class Foo {
  static fn = () => {};
}

setTimeout(Foo.fn, 100);

Open in Playground

How to Use

.eslintrc.cjs

module.exports = {
  "rules": {
    // Note: you must disable the base rule as it can report incorrect errors
    "no-implied-eval": "off",
    "@typescript-eslint/no-implied-eval": "error"
  }
};

Try this rule in the playground ↗

Options

See eslint/no-implied-eval options.

When Not To Use It

If your project is a rare one that needs to allow new Function() or setTimeout(), setInterval(), setImmediate() and execScript() with string arguments, then you can disable this rule. You might consider using ESLint disable comments for those specific situations instead of completely disabling this rule.

---

Type checked lint rules are more powerful than traditional lint rules, but also require configuring type checked linting. See Troubleshooting > Linting with Type Information > Performance if you experience performance degredations after enabling type checked rules.

Resources

• Rule source
• Test source

^{Taken with ❤️ from ESLint core.}