GitBook: [#206] deep update example

operators/alternative-default-value.md

@@ -0,0 +1,104 @@
+# Alternative (Default value)
+
+This operator is used to provide alternative (or default) values when a particular expression is either null or false.
+
+## LHS is defined
+
+Given a sample.yml file of:
+
+```yaml
+a: bridge
+```
+
+then
+
+```bash
+yq eval '.a // "hello"' sample.yml
+```
+
+will output
+
+```yaml
+bridge
+```
+
+## LHS is not defined
+
+Given a sample.yml file of:
+
+```yaml
+{}
+```
+
+then
+
+```bash
+yq eval '.a // "hello"' sample.yml
+```
+
+will output
+
+```yaml
+hello
+```
+
+## LHS is null
+
+Given a sample.yml file of:
+
+```yaml
+a: ~
+```
+
+then
+
+```bash
+yq eval '.a // "hello"' sample.yml
+```
+
+will output
+
+```yaml
+hello
+```
+
+## LHS is false
+
+Given a sample.yml file of:
+
+```yaml
+a: false
+```
+
+then
+
+```bash
+yq eval '.a // "hello"' sample.yml
+```
+
+will output
+
+```yaml
+hello
+```
+
+## RHS is an expression
+
+Given a sample.yml file of:
+
+```yaml
+a: false
+b: cat
+```
+
+then
+
+```bash
+yq eval '.a // .b' sample.yml
+```
+
+will output
+
+```yaml
+cat
+```