+1 for Flow. I've loved the fact that it's not a build step, so it was really easy to add Flow into my Babel+Browserify projects. I looked at Typescript before,

+1 for Flow. I've loved the fact that it's not a build step, so it was really easy to add Flow into my Babel+Browserify projects. I looked at Typescript before, but I didn't want to leave Browserify, and Typescript+Browserify did not look like it was a well-supported path. Also its strictness about nulls has saved me so many times and has forced me to make better APIs that carefully consider or prohibit them, instead of treating null as a boogeyman that I don't think about until it inevitably bites me.