When do I get this error?

The "Move 'var' declarations to the top of the function" error is thrown when JSLint encounters a variable declaration in a for or for-in statement initialiser. Here's an example in which we have a simple for loop that declares a variable:

/*jslint plusplus: true */
function demo(arr, callback) {
    "use strict";
    for (var i = 0; i < arr.length; i++) {
        callback(a[i]);
    }
}

Why do I get this error?

This error is raised to highlight a possible lack of convention and a possible misunderstanding of the language. Your code will most likely work as expected if you do not resolve this issue, but you may have misunderstood how JavaScript handles variable declarations.

All declarations are hoisted to the top of the scope in which they appear. Variables in JavaScript have function scope, not block scope. In many other languages, the variable i in the above example would be scoped to the for loop body and would not be accessible to the containing scope. In JavaScript this is not the case and the variable is actually accessible in the containing scope, although it won't have a value until the loop initialiser has been executed.

To fix this issue, simply move the variable declaration out of the loop initialiser. This has the advantage of making the code read the way it is actually interpreted:

/*jslint plusplus: true */
function demo(arr, callback) {
    "use strict";
    var i;
    for (i = 0; i < arr.length; i++) {
        callback(arr[i]);
    }
}

This approach also prevents you from redeclaring i in every loop within a function. You can simply reuse the reference to the i declared at the top of the function. In this example we have two loops using the same variable. When the first loop completes the value of i will be equal to arr.length - 1. When the second loop is executed i is reset to 0:

/*jslint plusplus: true */
function demo(arr, arr2, callback) {
    "use strict";
    var i;
    for (i = 0; i < arr.length; i++) {
        callback(arr[i]);
    }
    for (i = 0; i < arr2.length; i++) {
        callback(arr2[i]);
    }
}