he/him 
No JS tool (that I know of) will warn you about these conditions being impossible. But they should. const foo = 'FOO'; if (foo[0] === 'B') {} // impossible if (foo === 'BAR') {} // impossible
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For arrays of unknown length, the type should be `?T`. You can support refinements of `arr.length` to avoid a majority of these.
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Also refinements should work with function calls: function isString(str) { return typeof str === 'string'; } declare var foo: string | number; if (isString(foo)) { foo; // foo has been refined to string here } else { foo; // foo has been refined to number here }
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And it should work across files, and shouldn't require a %checks annotation like in Flow.
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TS can do this with user-defined type guards: https://www.typescriptlang.org/docs/handbook/advanced-types.html …
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Same as Flow's %checks then. It's not a very ergonomic way to define refinements, and you're limited to the simple syntax that it lets you define.
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This is possible with TS...you need to add a return type to your helper function though : str is string Try it here: https://goo.gl/sWewNu
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In TS is something like User-Defined Type Guards. Example
function isString(foo: string | number): foo is string {
return typeof foo === 'string'
}
Usage:
if (isString(s)) {
// here TS knows it is string
} else {
// here TS knows it is number
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