No external ip for created VM

Hello,
We don't get an external IP assigned for our newly created VM. We create a VM with the following script;

var zone = gce.zone('europe-west1-b');
var name = 'testing';
var BUILDER_MACHINE_CONFIG = {
    machineType: "n1-standard-1",
    disks: [{
        autoDelete: true,
        boot: true,
        initializeParams: {
            sourceImage: 'https://www.googleapis.com/compute/v1/projects/PROJECT/global/images/custom-img'
        }
    }]
};
zone.createVM(name, BUILDER_MACHINE_CONFIG, function(err, vm, operation, apiResponse) {
        console.log(err);
        operation.on('complete', function(metadata) {
                console.log('completed');
        });
});

And it does create an instance successfully, but no IP is assigned and there is no option to SSH into it. Anyone able to shed some light on this?

So the IP is not ephemeral by default, and we don't know how to set it to be ephemeral when we create the instance.

Turns out I didn't have an accessConfig set, which prompted the instance to not be created with an ephemeral IP. Here's the functioning code;

var zone = gce.zone('europe-west1-b');
var name = 'testing';
var BUILDER_MACHINE_CONFIG = {
    machineType: "n1-standard-1",
    disks: [{
        autoDelete: true,
        boot: true,
        initializeParams: {
            sourceImage: 'https://www.googleapis.com/compute/v1/projects/PROJECT/global/images/custom-img'
        }
    }],
    networkInterfaces: [{
        network: 'global/networks/default',
        accessConfigs: [
        {
            kind: 'compute#accessConfig'
        }]
    }]
};
zone.createVM(name, BUILDER_MACHINE_CONFIG, function(err, vm, operation, apiResponse) {
        console.log(err);
        operation.on('complete', function(metadata) {
                console.log('completed');
        });
});