typescript / expert
Snippet
Assertion Signatures for Runtime Guarding
Assertion signatures use the 'asserts' keyword to tell the compiler that if a function returns normally, a certain condition must be true. This narrows types across the remainder of the containing scope, reducing the need for nested if-blocks.
snippet.ts
typescript
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function assertIsString(val: any): asserts val is string {if (typeof val !== "string") {throw new Error("Not a string!");}}function process(input: unknown) {assertIsString(input);// input is now narrowed to 'string' for the rest of the scopeconsole.log(input.toUpperCase());}
Breakdown
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asserts val is string
The signature indicating that the function narrows the 'val' parameter upon successful execution.
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input.toUpperCase()
Allowed because the compiler knows 'input' must be a string after the assertion call.