Initial

.gitignore

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+*.orig
+*.pyc
+*.swp
+
+/.idea
+/.vscode
+gclient_config.py_entries
+/gh-pages
+/target
+
+# Files that help ensure VSCode can work but we don't want checked into the
+# repo
+/node_modules
+/tsconfig.json
+package.json
+package-lock.json
+
+# yarn creates this in error.
+tools/node_modules/
+
+# MacOS generated files
+.DS_Store
+.DS_Store?

Validator.test.ts

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+import { assertEquals, assertNotEquals } from "https://deno.land/std@0.53.0/testing/asserts.ts";
+import {
+  validate, Validatable, ArraySymbol,
+  isString
+} from "./mod.ts";
+
+Deno.test("validate schema (match)", async () => {
+  const values: [any, Validatable][] = [
+    ["string", isString],
+    ["string", [isString]],
+    [["arr", "ay"], {[ArraySymbol]: isString}],
+    [{foo: "bar", lorem: "ipsum"}, {foo: isString, lorem: [isString]}],
+  ];
+  for (const [value, constraints] of values) {
+    assertEquals([], await validate(value, constraints));
+  }
+});
+
+Deno.test("validate schema (no match)", async () => {
+  const values: [any, Validatable][] = [
+    [6, isString],
+    [false, [isString]],
+    [["arr", ["ay"]], {[ArraySymbol]: isString}],
+    [{foo: {}, lorem: "ipsum"}, {foo: isString, lorem: [isString]}],
+  ];
+  for (const [value, constraints] of values) {
+    assertNotEquals([], await validate(value, constraints));
+  }
+});

Validator.ts

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+export type Args = {[_: string]: any};
+
+export interface Validator {
+  type: string;
+  check: (value: any) => Promise<Args|undefined>|Args|undefined;
+  message: (value: any, args?: Args) => Promise<string|undefined>|string|undefined;
+}
+
+export interface ValidationError {
+  type: string;
+  param?: string[];
+  message?: string|null;
+  args?: Args;
+}
+
+export type Validatable = Schema|Validator|Validator[];
+
+export const ArraySymbol: unique symbol = Symbol("ArraySymbol");
+export type Schema = {[key: string]: Validatable}|{[ArraySymbol]: Validatable};
+
+export async function validate(value: any, validators: Validatable): Promise<ValidationError[]|undefined> {
+  if (instanceofValidatorArray(validators) || instanceofValidator(validators)) {
+    return validateValue(value, validators);
+  } else {
+    return validateSchema(value, validators);
+  }
+}
+
+function instanceofValidator(value: any): value is Validator {
+  return value &&
+    value.hasOwnProperty("type") && typeof value.type === "string" &&
+    value.hasOwnProperty("check") && typeof value.check === "function" &&
+    value.hasOwnProperty("message") && typeof value.message === "function";
+}
+
+function instanceofValidatorArray(value: any): value is Validator[] {
+  return Array.isArray(value) && value.every(x => instanceofValidator(x));
+}
+
+async function validateValue(value: any, validators: Validator|Validator[]): Promise<ValidationError[]> {
+  if (!Array.isArray(validators)) {
+    validators = [validators];
+  }
+  const result: ValidationError[] = [];
+  for (const validator of validators) {
+    const args = await validator.check(value);
+    if (args !== undefined) {
+      const message = await validator.message(value, args);
+      result.push({
+        type: validator.type,
+        args,
+        message
+      });
+    }
+  }
+  return result;
+}
+
+async function validateSchema(value: any, validators: Schema): Promise<ValidationError[]> {
+  if (validators.hasOwnProperty(ArraySymbol)) {
+    const v = validators as {[ArraySymbol]: Validatable};
+    if (Array.isArray(value)) {
+      const arr = await Promise.all(value.map(val => validate(val, v[ArraySymbol])));
+      const errors = arr.flatMap((val, idx) => (val ?? []).map(error => ({
+        ...error,
+        param: [`[${idx}]`, ...(error.param || [])]
+      })));
+      return errors;
+    } else {
+      return [{
+        type: "array",
+        param: ["[]"],
+        message: "Array expected!",
+        args: {}
+      }];
+    }
+  }
+  const v = validators as {[key: string]: Validatable};
+  const valErrors: ValidationError[] = [];
+  for (const prop in validators) {
+    if (!validators.hasOwnProperty(prop)) {
+      continue;
+    }
+    if (value.hasOwnProperty(prop)) {
+      const errors = await validate(value[prop], v[prop]);
+      valErrors.push(...(errors ?? []));
+    } else {
+      valErrors.push({
+        type: "property",
+        param: [prop],
+        message: `Property '${prop}' expected but not found!`,
+        args: {property: prop}
+      });
+    }
+  }
+  return valErrors;
+}

mod.ts

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+export { Args, Validator, ValidationError, validate, Validatable, ArraySymbol } from "./Validator.ts";
+export * from "./validators/string.ts";

validators/string.test.ts

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+import { isString } from "./string.ts";
+import { validate } from "../mod.ts";
+import { assertEquals, assertNotEquals } from "https://deno.land/std@0.53.0/testing/asserts.ts";
+
+Deno.test("isString (match)", async () => {
+  const values = [
+    "",
+    "foo",
+    new String(),
+    new String("bar")
+  ];
+  for (const value of values) {
+    assertEquals([], await validate(value, isString));
+  }
+});
+
+Deno.test("isString (no match)", async () => {
+  const values = [
+    undefined,
+    null,
+    0,
+    1,
+    true,
+    false,
+    () => {},
+    function named() {},
+    new Object(),
+    Symbol()
+  ];
+  for (const value of values) {
+    assertNotEquals([], await validate(value, isString));
+  }
+});

validators/string.ts

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+import { Validator, Args } from "../mod.ts";
+
+export const isString: Validator = {
+  type: "isString",
+  check: (value: any) => {
+    if (typeof value !== 'string' && !(value instanceof String)) {
+      return {};
+    }
+  },
+  message: (value: any, args?: Args) => {
+    return `The value '${value && value.toString()}' has to be a string.`
+  }
+}