What does flex: 1 mean?

As we all know, the flex property is a shorthand for the flex-grow, flex-shrink, and the flex-basis properties. Its default value is 0 1 auto, which means

flex-grow: 0;flex-shrink: 1;flex-basis: auto;

but I've noticed, in many places flex: 1 is used. Is it shorthand for 1 1 auto or 1 0 auto? I can't understand what it means and I get nothing when I google.