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Discussion on: Custom Type Guards in Typescript

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lioness100 profile image
Lioness100

I think I understand it. If a return type is described as x is y, then typescript assumes that if the function returns true x is y.

declare const x: y | z;
if (isY(x)) {
  // `x` is `y`;
}

// `x` is `y | z`
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However, with asserts x is y, typescript will assume that the function will throw if x isn't y.

declare const x: y | z;

// will throw if `x` isn't `y`
assertIsY(x);

// `x` is `y`
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Super cool!

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mainawycliffe profile image
Maina Wycliffe Author

Yeah, that is true, it's upto you to do an exhaustive check on whether an object is of that type before returning true.

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lioness100 profile image
Lioness100

Yeah, I understood the concept, I was just confused by the difference of asserts x is y vs x is y