Try and understand this block from JavaScript you don’t know:

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if (!Promise.wrap) {
    Promise.wrap = function(fn) {
        return function() {
            var args = [].slice.call( arguments ); // [].slice.call(arguments) creates a new copy of the array arguments.

            return new Promise( function(resolve,reject){
                fn.apply(
                    null,
                    args.concat( function(err,v){ // in addition to the arguments passed to the returned promisory, also create a callback function to the original callback-based fn. This callback is created to convert the callback invocation to Promise resolution.
                        if (err) {
                            reject( err );
                        }
                        else {
                            resolve( v );
                        }
                    } )
                );
            } );
        };
    };
}

fn.apply is a function call, fn.bind is creating a new function.