fix(aws-autoscaling): allow minSize to be set to 0 (#1015)

Previously, minSize was being set to `props.minSize || 1`. Since 0 is
falsey in javascript, this would mean 0 passed in through the ASG props
evaluate to false and the field would always be set to the default value
of 1. This change uses strict equality comparison to allow 0 to be set.

Author: SoManyHs

packages/@aws-cdk/aws-autoscaling/lib/auto-scaling-group.ts

@@ -212,9 +212,9 @@ export class AutoScalingGroup extends cdk.Construct implements cdk.ITaggable, el
 
     launchConfig.addDependency(this.role);
 
-    const minSize = props.minSize || 1;
-    const maxSize = props.maxSize || 1;
-    const desiredCapacity = props.desiredCapacity || 1;
+    const minSize = props.minSize !== undefined ? props.minSize : 1;
+    const maxSize = props.maxSize !== undefined ? props.maxSize : 1;
+    const desiredCapacity = props.desiredCapacity !== undefined ? props.desiredCapacity : 1;
 
     if (desiredCapacity < minSize || desiredCapacity > maxSize) {
       throw new Error(`Should have minSize (${minSize}) <= desiredCapacity (${desiredCapacity}) <= maxSize (${maxSize})`);

packages/@aws-cdk/aws-autoscaling/test/test.auto-scaling-group.ts

@@ -126,6 +126,29 @@ export = {
     test.done();
   },
 
+  'can set minSize, maxSize, desiredCapacity to 0'(test: Test) {
+    const stack = new cdk.Stack(undefined, 'MyStack', { env: { region: 'us-east-1', account: '1234' }});
+    const vpc = mockVpc(stack);
+
+    new autoscaling.AutoScalingGroup(stack, 'MyFleet', {
+      instanceType: new ec2.InstanceTypePair(ec2.InstanceClass.M4, ec2.InstanceSize.Micro),
+      machineImage: new ec2.AmazonLinuxImage(),
+      vpc,
+      minSize: 0,
+      maxSize: 0,
+      desiredCapacity: 0
+    });
+
+    expect(stack).to(haveResource("AWS::AutoScaling::AutoScalingGroup", {
+        MinSize: "0",
+        MaxSize: "0",
+        DesiredCapacity: "0",
+      }
+    ));
+
+    test.done();
+  },
+
   'addToRolePolicy can be used to add statements to the role policy'(test: Test) {
     const stack = new cdk.Stack(undefined, 'MyStack', { env: { region: 'us-east-1', account: '1234' }});
     const vpc = mockVpc(stack);