The |default filter returns the value passed as the first argument if the filtered value is undefined or empty, otherwise the filtered value is returned.
{{ variable|default('The variable is not defined') }}
{{ variable.foo|default('The foo property on variable is not defined') }}
{{ variable['foo']|default('The foo key in variable is not defined') }}
{{ ''|default('The variable is empty') }}
When using the default filter on an expression that uses variables in some method calls, be sure to use the default filter whenever a variable can be undefined:
{{ variable.method(foo|default('bar'))|default('bar') }}