add json validator

src/json.ts

@@ -0,0 +1,53 @@
+export type ObjectType = { [key: string]: Schema };
+export type PrimitiveType = "null" | "boolean" | "number" | "string";
+export type Schema = PrimitiveType | ObjectType | Array<Schema>;
+
+export function validate<T>(json: any, schema: Schema): T {
+    if (typeof schema === "string") {
+        return validatePrimitive(json, schema) as T;
+    }
+
+    if (Array.isArray(schema)) {
+        if (schema.length !== 1) {
+            throw new SyntaxError("Invalid schema: Array types must only contain one element");
+        }
+        const type = schema[0];
+
+        if (!Array.isArray(json)) {
+            throw new TypeError(`Expected array, got ${typeof json} instead`);
+        }
+
+        const a = [];
+        for (let e of json) {
+            a.push(validate(e, type));
+        }
+        return a as T;
+    }
+
+    const o: any = {};
+    for (let prop in schema) {
+        if (schema.hasOwnProperty(prop)) {
+            if (!json.hasOwnProperty(prop)) {
+                throw new TypeError(`Property ${schema[prop]} is missing`);
+            }
+            o[prop] = validate(json[prop], schema[prop]) as any;
+        }
+    }
+    return o;
+}
+
+function validatePrimitive<T extends PrimitiveType>(json: any, type: T):
+PrimitiveToJSType<T> {
+    if (typeof json === type) {
+        return json;
+    } else {
+        throw new TypeError(`Expected type ${type}, got ${typeof json} instead`);
+    }
+}
+
+type PrimitiveToJSType<T extends PrimitiveType> =
+    T extends "null" ? null :
+    T extends "boolean" ? boolean :
+    T extends "number" ? number :
+    T extends "string" ? string :
+    never;