When do I get this error?

The "'{a}' is not a function" error is thrown when JSLint, JSHint or ESLint encounters an attempt to invoke the Math object as a function. JSLint and ESLint (but not JSHint) will also raise this warning when they encounter an attempt to invoke the JSON object as a function. Here's an example:

var x = Math(),
    y = JSON();

Why do I get this error?

This error is raised to highlight what is most likely a misunderstanding of the language. The Math property of the global object is described in the specification as follows (ES5 §15.8):

The Math object does not have a [[Construct]] internal property; it is not possible to use the Math object as a constructor with the new operator.

The Math object does not have a [[Call]] internal property; it is not possible to invoke the Math object as a function.

This makes it very clear that the Math object cannot be invoked in the way you normally call a function or a constructor. Instead it is simply an object with a set of properties, some of which are functions.

The specification makes the same note about the JSON object (ES5 §15.12):

The JSON object does not have a [[Construct]] internal property; it is not possible to use the JSON object as a constructor with the new operator.

The JSON object does not have a [[Call]] internal property; it is not possible to invoke the JSON object as a function.

Both Math and JSON objects expose a number of properties that can be accessed in the normal way. If you're receiving this error the chances are you intended to invoke one of the function properties instead of the object itself. For example:

var x = Math.random(),
    y = JSON.stringify({});

In JSHint 1.0.0 and above you have the ability to ignore any warning with a special option syntax. The identifier of this warning is W063. This means you can tell JSHint to not issue this warning with the /*jshint -W063 */ directive.

In ESLint the rule that generates this warning is named no-obj-calls. You can disable it by setting it to 0, or enable it by setting it to 1.