How does one provide default parameters in Javascript anonymous functions?

I am new to JS and I need to use an anonymous function, but when I provide default values to parameters as in named functions, I get the error "Uncaught SyntaxError: Unexpected token =".

Here's the code excerpt:

//some properties
initResize: function(isPlayerInitializing=true){ 
    //some execution
},
//some more properties

I would like to know how one would provide default values to parameters of anonymous functions in Javascript.