When do I get this error?

The "Expected parameter (value) in set '{a}' function" error is thrown when JSLint encounters property setter function in which the first parameter is not named value. 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 y() {
        "use strict";
        return this.actual / 2;
    },
    set y() {
        "use strict";
        this.actual = val;
    }
};

Why do I get this error?

This error is raised to highlight a lack of convention. Your code will run without error if you do not change it, but could be confusing to other developers. ECMAScript 5 added new syntax for object property getter and setter functions. The specification states the following in reference to setters (ES5 §8.6.1):

The function’s [[Call]] internal method... is called with an arguments list containing the assigned value as its sole argument each time a set access of the property is performed.

By convention, this single argument should be named value, since it will be used to set the value of something. To fix this error simply rename the parameter accordingly:

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