# TypeScript `unknown` type

## Like `any`, all types are assignable to unknown

let value: unknown;

value = true; // OK
value = 42; // OK
value = "Hello World"; // OK
value = []; // OK
value = {}; // OK
value = Math.random; // OK
value = null; // OK
value = undefined; // OK
value = new TypeError(); // OK
value = Symbol("type"); // OK

## What if assigning `unknown` to variables of other types

let value: unknown;
let value1: unknown = value; // OK
let value2: any = value; // OK
let value3: boolean = value; // Error
let value4: number = value; // Error
let value5: string = value; // Error
let value6: object = value; // Error
let value7: any[] = value; // Error
let value8: Function = value; // Error

## Performing operations on `unknown` type value

let value: unknown;

value.foo.bar; // Error
value.trim(); // Error
value(); // Error
new value(); // Error
value[0][1]; // Error

## Need to narrow down before using

let isUnknown: unknown = 123;

// Error: Type 'unknown' is not assignable to type 'number'
let value: number = isUnknown;

// After narrowing, PASS!
if(typeof isUnknown === 'number'){
    value = isUnknown
}

function stringifyForLogging(value: unknown): string {
  if (typeof value === "function") {
    // Within this branch, `value` has type `Function`,
    // so we can access the function's `name` property
    const functionName = value.name || "(anonymous)";
    return `[function ${functionName}]`;
  }

  if (value instanceof Date) {
    // Within this branch, `value` has type `Date`,
    // so we can call the `toISOString` method
    return value.toISOString();
  }

  return String(value);
}