Within the source code of VSCode, you will find various functions that have a specific return type declaration, such as the following:
export function isString(str: any): str is string {
if (typeof (str) === _typeof.string || str instanceof String) {
return true;
}
return false;
}
This leads me to question the purpose of using "str is string" instead of simply stating "boolean."
Furthermore, can we apply the "str is string" pattern in other scenarios?
That technique is known as User-Defined Type Guards.
Standard type guards allow you to achieve the following:
function fn(obj: string | number) {
if (typeof obj === "string") {
console.log(obj.length); // obj is string here
} else {
console.log(obj); // obj is number here
}
}
While using typeof or instanceof, what about interfaces like this:
interface Point2D {
x: number;
y: number;
}
interface Point3D extends Point2D {
z: number;
}
function isPoint2D(obj: any): obj is Point2D {
return obj && typeof obj.x === "number" && typeof obj.y === "number";
}
function isPoint3D(obj: any): obj is Point2D {
return isPoint2D(obj) && typeof (obj as any).z === "number";
}
function fn(point: Point2D | Point3D) {
if (isPoint2D(point)) {
// point is Point2D
} else {
// point is Point3D
}
}
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