# Disallow Implied eval() (no-implied-eval)

It's considered a good practice to avoid using `eval()` in JavaScript. There are security and performance implications involved with doing so, which is why many linters (including ESLint) 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()` or `execScript()` (Internet Explorer only), all of which can accept a string of JavaScript code as their first argument. For example:

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

This is considered an implied `eval()` because a string of JavaScript code is
 passed in to be interpreted. The same can be done with `setInterval()` and `execScript()`. Both interpret the JavaScript code in  the global scope. For  both `setTimeout()` and `setInterval()`, the first argument can also be a function, and that is considered safer and is more performant:

```js
setTimeout(function() {
    alert("Hi!");
}, 100);
```

The best practice is to always use a function for the first argument of `setTimeout()` and `setInterval()` (and avoid `execScript()`).


## Rule Details

This rule aims to eliminate implied `eval()` through the use of `setTimeout()`, `setInterval()` or `execScript()`. As such, it will warn when either function is used with a string as the first argument.

Examples of **incorrect** code for this rule:

```js
/*eslint no-implied-eval: "error"*/

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

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

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

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

window.setInterval("foo = bar", 10);
```

Examples of **correct** code for this rule:

```js
/*eslint no-implied-eval: "error"*/

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

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

## When Not To Use It

If you want to allow `setTimeout()` and `setInterval()` with string arguments, then you can safely disable this rule.

## Related Rules

* [no-eval](no-eval.md)