Get all fields from a library Javascript

 // Open current SPWeb: _spPageContextInfo.webServerRelativeUrl

var ctx = new SP.ClientContext(_spPageContextInfo.webServerRelativeUrl);

// Get Documents library

var list = ctx.get_web().get_lists().getByTitle('TEST');

//get_fields() returns SP.FieldCollection object --- contains all SP.Field object properties > https://msdn.microsoft.com/en-us/library/office/jj246815.aspx

var fieldCollection = list.get_fields();

ctx.load(fieldCollection,'Include(InternalName,StaticName)');

var view = list.get_views().getByTitle('All Documents');

//get_viewFields() returns SP.ViewFieldCollection object --- only field names (Internal Names), but not a SP.Field object > https://msdn.microsoft.com/en-us/library/office/jj244841.aspx

var viewFieldCollection = view.get_viewFields();

ctx.load(viewFieldCollection);

ctx.executeQueryAsync(function (){

        var cont=0;

        var fields = 'SP.FieldCollection from list.get_fields()'

        fields += 'Internal Name - Static Name \n';

        fields += '--------------------------- \n';

        var listEnumerator = fieldCollection.getEnumerator();

        while (listEnumerator.moveNext()) {

            fields += listEnumerator.get_current().get_internalName() + ' - ' + listEnumerator.get_current().get_staticName() + ";\n "; //

            cont++;

        }

        console.log(fields + '-------------------------- \n Number of Fields: ' + cont);

        

        var cont=0;

        var viewfields = '\nSP.ViewFieldCollection from view.get_viewFields() \n';

        viewfields += 'Internal Name \n';

        viewfields += '--------------------------- \n';

        var listEnumerator = viewFieldCollection.getEnumerator();

        while (listEnumerator.moveNext()) {

            viewfields += listEnumerator.get_current() + "\n";

            cont++;

            // If we need more data about view fields we can call again to server using

            // var field = fieldCollection.getByInternalNameOrTitle(listEnumerator.get_current());

            // ctx.load(field);

            // ctx.executeQueryAsync(.....)

            // Or, If we would like to save a server query, we could iterate again

            // the fieldCollection.getEnumerator(); checking the Internal Name of the field

        }

        console.log(viewfields + '-------------------------- \n Number of Fields: ' + cont);

    },

    function(sender, args){

        console.log('Request collListItem failed. ' + args.get_message() + '\n' + args.get_stackTrace());

    }

);