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') }}