no-new-func

Disallow new operators with the Function object

It’s possible to create functions in JavaScript from strings at runtime using the Function constructor, such as:

var x = new Function("a", "b", "return a + b");
var x = Function("a", "b", "return a + b");
var x = Function.call(null, "a", "b", "return a + b");
var x = Function.apply(null, ["a", "b", "return a + b"]);
var x = Function.bind(null, "a", "b", "return a + b")();
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This is considered by many to be a bad practice due to the difficulty in debugging and reading these types of functions. In addition, Content-Security-Policy (CSP) directives may disallow the use of eval() and similar methods for creating code from strings.

Rule Details

This error is raised to highlight the use of a bad practice. By passing a string to the Function constructor, you are requiring the engine to parse that string much in the way it has to when you call the eval function.

Examples of incorrect code for this rule:

/*eslint no-new-func: "error"*/

var x = new Function("a", "b", "return a + b");
var x = Function("a", "b", "return a + b");
var x = Function.call(null, "a", "b", "return a + b");
var x = Function.apply(null, ["a", "b", "return a + b"]);
var x = Function.bind(null, "a", "b", "return a + b")();
var f = Function.bind(null, "a", "b", "return a + b"); // assuming that the result of Function.bind(...) will be eventually called.
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Examples of correct code for this rule:

/*eslint no-new-func: "error"*/

var x = function (a, b) {
    return a + b;
};
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When Not To Use It

In more advanced cases where you really need to use the Function constructor.

Version

This rule was introduced in ESLint v0.0.7.

Resources

• Rule source
• Tests source