no-eval.md

Disallow eval() (no-eval)

JavaScript's eval() function is potentially dangerous and is often misused. Using eval() on untrusted code can open a program up to several different injection attacks. The use of eval() in most contexts can be substituted for a better, alternative approach to a problem.

var obj = { x: "foo" },
    key = "x",
    value = eval("obj." + key);

Rule Details

This rule is aimed at preventing potentially dangerous, unnecessary, and slow code by disallowing the use of the eval() function. As such, it will warn whenever the eval() function is used.

Examples of incorrect code for this rule:

/*eslint no-eval: "error"*/

var obj = { x: "foo" },
    key = "x",
    value = eval("obj." + key);

(0, eval)("var a = 0");

var foo = eval;
foo("var a = 0");

// This `this` is the global object.
this.eval("var a = 0");

Examples of incorrect code for this rule with browser environment:

/*eslint no-eval: "error"*/
/*eslint-env browser*/

window.eval("var a = 0");

Examples of incorrect code for this rule with node environment:

/*eslint no-eval: "error"*/
/*eslint-env node*/

global.eval("var a = 0");

Examples of correct code for this rule:

/*eslint no-eval: "error"*/
/*eslint-env es6*/

var obj = { x: "foo" },
    key = "x",
    value = obj[key];

class A {
    foo() {
        // This is a user-defined method.
        this.eval("var a = 0");
    }

    eval() {
    }
}

Options

This rule has an option to allow indirect calls to eval. Indirect calls to eval are less dangerous than direct calls to eval because they cannot dynamically change the scope. Because of this, they also will not negatively impact performance to the degree of direct eval.

{
    "no-eval": ["error", {"allowIndirect": true}] // default is false
}

With this option the following patterns are considered problems:

/*eslint no-eval: "error"*/

var obj = { x: "foo" },
    key = "x",
    value = eval("obj." + key);

With this option the following patterns are not considered problems:

/*eslint no-eval: "error"*/

(0, eval)("var a = 0");

var foo = eval;
foo("var a = 0");

this.eval("var a = 0");

/*eslint no-eval: "error"*/
/*eslint-env browser*/

window.eval("var a = 0");

/*eslint no-eval: "error"*/
/*eslint-env node*/

global.eval("var a = 0");

Known Limitations

• This rule is warning every eval() even if the eval is not global's. This behavior is in order to detect calls of direct eval. Such as:

  module.exports = function(eval) {
      // If the value of this `eval` is built-in `eval` function, this is a
      // call of direct `eval`.
      eval("var a = 0");
  };

• This rule cannot catch renaming the global object. Such as:

  var foo = window;
  foo.eval("var a = 0");

Further Reading

• Eval is Evil, Part One
• How evil is eval

Related Rules

• no-implied-eval