he/him 
Just got caught by this type cast semantic... > /[A-Za-z]/.test(undefined) === true Because it's essentially: > /[A-Za-z]/.test('undefined') === true Oh JavaScript.
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This is a great reason to use a type-checked version of JS (Flow or TypeScript): it renders an entire class of errors like this impossible.pic.twitter.com/EUL9GTbeLZ
See my follow up tweet with more context. TypeScript and Flow don’t require bounds checks for indexes.
