When do I get this error?

The "It is not necessary to initialize '{a}' to 'undefined'" error is thrown when JSLint, JSHint or ESLint encounters a variable statement in which the variable is explicitly initialised to undefined. Here's an example in which we attempt to declare a variable x and assign undefined to it:

var x = undefined;

Why do I get this error?

This error is raised to highlight a completely pointless piece of code. Your code will run without error if you do not change it, but you're needlessly increasing the size of your script. Since variable declarations are hoisted to the top of the scope in which they occur, and assignments happen in the expected place, variables are always initialized to undefined implicitly. The following is what happens when you enter a new scope (ES5 §10.5):

‌8. For each VariableDeclaration... d in source text order do
    a. Let dn be the Identifier in d.
    b. Let varAlreadyDeclared be the result of calling env's HasBinding concrete method passing dn as the argument.
    c. If varAlreadyDeclared is false, then
        i. Call env's CreateMutableBinding concrete method passing dn and configurableBindings as the arguments.
        ii. Call env's SetMutableBinding concrete method passing dn, undefined, and strict as the arguments.

It's the last line that's interesting. Effectively, it creates a binding in the current scope between the given identifier and the value undefined. This shows that every variable has the value undefined when it is created. When the variable statement is actually executed, it is assigned a value if an assignment expression is present as part of the statement (as it is in our example above). This is further clarified by the following (ES5 §12.2):

Variables are initialised to undefined when created. A variable with an Initialiser is assigned the value of its AssignmentExpression when the VariableStatement is executed, not when the variable is created.

You can fix the error by simply removing the assignment expression from the variable statement. The variable will still have the same value:

var x;

If, for whatever reason, you have to assign the undefined value to the variable, you can replace undefined with something that returns the undefined value. The simplest example of that is the void operator (although as of September 2013 JSLint will raise a new error for this):

var x = void 0;

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 W080. This means you can tell JSHint to not issue this warning with the /*jshint -W080 */ directive.

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