Unexpected parameter '{a}' in get {b} function

History

This warning has existed in two forms across the three main linters. It was introduced in the original version of JSLint and has remained (in a way) in all three tools ever since.

  • In JSLint and JSHint the warning given is "Unexpected parameter '{a}' in get {b} function"

  • In ESLint the message used is the more generic "Unexpected identifier"

The situations that produce the warning have not changed despite changes to the text of the warning itself.

When do I get this error?

The "Unexpected parameter '{a}' in get {b} function" error, and the alternative "Unexpected identifier", is thrown when JSLint, JSHint or ESLint encounters a named parameter in the signature of a property getter function. In the following example we create an object x with a getter and setter. The getter will always return half of the set value:

var x = {
    actual: 10,
    get x (value) {
        "use strict";
        return this.actual / 2;
    },
    set x (value) {
        "use strict";
        this.actual = value;
    }
};

Why do I get this error?

This error is raised to highlight a completely pointless and potentially confusing piece of code. Your code will run without error if you do not change it, but could be confusing to other developers and adds unnecessary bytes to the weight of your script. ECMAScript 5 added new syntax for object property getter and setter functions. The specification states the following in reference to getters (ES5 §8.6.1):

The function’s [[Call]] internal method... is called with an empty arguments list to return the property value each time a get access of the property is performed.

Since the runtime will never pass any arguments to the getter function, there is no need to provide any named parameters in the function signature. Simply remove them to fix the error:

var x = {
    actual: 10,
    get x () {
        "use strict";
        return this.actual / 2;
    },
    set x (value) {
        "use strict";
        this.actual = value;
    }
};

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 W076. This means you can tell JSHint to not issue this warning with the /*jshint -W076 */ directive.


About the author

James Allardice

This article was written by James Allardice, JavaScript engineer at Mammal in London. Passionate about AngularJS, Node and writing clean and maintainable JavaScript and uses JSHint every day to help achieve this.