tsguarder
v1.0.1
Published
A simple tool for creating TypeScript guards including assertions
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tsguarder
A quite simple tool for creating TypeScript guards including assertions with several built-in guards for runtime type checking. A type guard can be used to check the runtime type of a variable and narrow its type accordingly.
Read more about guards and assertions from the official TypeScript documentation.
Installation
npm install tsguarder
Usage
Create a type guard:
import { createTypeGuard, TypeGuard } from "tsguarder";
const isPositiveNumber: TypeGuard<number> = createTypeGuard(
"must be a positive number", // optional
(value: unknown): value is number => {
return typeof value === "number" && value > 0;
}
);
Use as a type guard:
function doSomething(value: number | string) {
if (isPositiveNumber(value)) {
// do something with a number
const result = value.toFixed(2);
}
// do something with a number or a string
}
Use as an assertion:
function calculateFactorial(num: number) {
// the second argument is optional
isPositiveNumber.assert(num, "num argument");
}
calculateFactorial(100); // works
calculateFactorial("This is not a number"); // throws an error
console.log("This will not be printed");
If the type assertion fails, meaning the variable's type is not what was expected, TypeScript throws an error with the value name and the message provided in the guard.
TypeError: num argument: must be a positive number
An Explicit Type Annotation
⚠️ Note that we have used TypeGuard as the type annotation for the guard. This is because TypeScript requires an explicit type annotation to enable the use of type assertions. It concerns only assertions.
This does not work:
import { createTypeGuard } from "tsguarder";
const isString = createTypeGuard(
(value): value is string => typeof value === "string";
);
// ⛔️ TypeScript error:
// Assertions require every name in the call target to be
// declared with an explicit type annotation.ts(2775)
isString.assert('Hello, World!')
Read more about assertions in the TypeScript documentation.
Built-in guards
The tool comes with several built-in guards:
isString
Equivalent to typeof value === "string"
isNumber
Equivalent to typeof value === "number"
isBoolean
Equivalent to typeof value === "boolean"
isSymbol
Equivalent to typeof value === "symbol"
isUndefined
Equivalent to typeof value === "undefined"
isNull
Equivalent to value === null
isBigInt
Equivalent to typeof value === "bigint"
isFunction
Equivalent to typeof value === "function"
isObject
Equivalent to typeof value === "object"
isArray
Equivalent to Array.isArray(value)
isRecord
Equivalent to typeof value === "object" && value !== null && !Array.isArray(value)