During development, use an assert statement— assert(<condition>, <optionalMessage>); —to disrupt normal execution if a boolean condition is false.
// Make sure the variable has a non-null value.
assert(text != null);
// Make sure the value is less than 100.
assert(number < 100);
// Make sure this is an https URL.
assert(urlString.startsWith(‘https’));
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To attach a message to an assertion, add a string as the second argument to assert (optionally with a trailing comma):
assert(urlString.startsWith(‘https’),
‘URL ($urlString) should start with “https”.’);
The first argument to assert can be any expression that resolves to a boolean value. If the expression’s value is true, the assertion succeeds and execution continues. If it’s false, the assertion fails and an exception (an AssertionError) is thrown.