no-implied-eval

Disallow the use of eval()-like methods.

🧠

Extending "plugin:@typescript-eslint/recommended-requiring-type-checking" 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

/* eslint @typescript-eslint/no-implied-eval: "error" */

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');

✅ Correct

/* eslint @typescript-eslint/no-implied-eval: "error" */

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);

When Not To Use It

Options

.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 ↗

How to Use

See eslint/no-implied-eval options.

If you want to allow new Function() or setTimeout(), setInterval(), setImmediate() and execScript() with string arguments, then you can safely disable this rule.

Resources

• Rule source
• Test source

^{Taken with ❤️ from ESLint core}