how to add a default value to a javascript function variable?

I can I do this on javascript (jQuery) function

function somename(variableone = "content"){
 return variableone;
}

Now to access that function:

alert(somename()) //this should alert "content"
alert(somename("hello world"); //this should return "hello world"

but I get this error Uncaught SyntaxError: Unexpected token =

If this is not possible, is there a way to achieve the same result? OR most importantly is this a good (correct) practice.

Answer

function somename(variableone){
    if(typeof variableone === "undefined")
        variableone = "content"
    return variableone;

}