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), both of which can accept a string of JavaScript code as their first argument. For example:

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:

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:

/*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:

/*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

Version

This rule was introduced in ESLint 0.0.7.

Resources

• Rule source
• Documentation source