> For the complete documentation index, see [llms.txt](https://openai.gitbook.io/code-cheatsheets/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://openai.gitbook.io/code-cheatsheets/js/typescript/narrowing.md).

# Narrowing Types

Unions will cause errors if all types don't have a property

**Error**

```ts
function padLeft(padding: number | string, input: string) {
  return " ".repeat(padding) + input;
}
/*
	Argument of type 'string | number' is not assignable to parameter of type 'number'.
  Type 'string' is not assignable to type 'number'.
*/
```

**Correct**

```ts
function padLeft(padding: number | string, input: string) {
  if (typeof padding === "number") {
    return " ".repeat(padding) + input;
  }
  return padding + input;
}
```

## Guards

#### Typeof

TypeScript expects this to return a certain set of strings:

* `"string"`
* `"number"`
* `"bigint"`
* `"boolean"`
* `"symbol"`
* `"undefined"`
* `"object"`
* `"function"`

#### Boolean

```ts
function printAll(strs: string | string[] | null) {
  if (strs && typeof strs === "object") {
    for (const s of strs) {
      console.log(s);
    }
  } else if (typeof strs === "string") {
    console.log(strs);
  }
}
```

Coerced to false

* `0`
* `NaN`
* `""` (the empty string)
* `0n` (the `bigint` version of zero)
* `null`
* `undefined`

#### instanceof

x instanceof Foo checks whether the *prototype chain* of `x` contains Foo.prototype

```ts
unction logValue(x: Date | string) {
  if (x instanceof Date) {
    console.log(x.toUTCString());
  } else {
    console.log(x.toUpperCase());
  }
}
```

### Advanced Types => Type predicates

**Way 1**

```ts
if ("swim" in pet) {
	return pet.swim();
}
```

**Way 2**

```ts
function isFish(pet: Fish | Bird): pet is Fish {
  return (pet as Fish).swim !== undefined;
}

// Both calls to 'swim' and 'fly' are now okay.
let pet = getSmallPet();
 
if (isFish(pet)) {
  pet.swim();
} else {
  pet.fly();
}
```