# string-ts

> Strongly-typed string functions.

Latest version **2.3.1** (published 2025-11-28) · MIT license · 0 weekly downloads

## Install

```sh
npm install string-ts
pnpm add string-ts
yarn add string-ts
bun add string-ts
```

## Health

**Score 60/100 (C)** — status: stable.

Positive: has types; esm support; no vulnerabilities; high quality score.

Warnings: low downloads.

## Facts

| | |
|---|---|
| Version | 2.3.1 |
| Published | 2025-11-28 |
| First published | 2023-06-23 |
| Weekly downloads | 0 |
| License | MIT |
| TypeScript types | bundled |
| Module format | ESM + CommonJS |
| Dependencies | 0 |
| Unpacked size | 131 KB |
| Known vulnerabilities | 0 |
| Install scripts | no |
| GitHub stars | 1256 |
| Author | Gustavo Guichard |
| Maintainers | guga.guichard |

## Links

- npm: https://www.npmjs.com/package/string-ts
- Repository: https://github.com/gustavoguichard/string-ts
- Homepage: https://github.com/gustavoguichard/string-ts#readme
- Issues: https://github.com/gustavoguichard/string-ts/issues
- npm.io page: https://npm.io/package/string-ts

## Recent versions

- 2.3.1 (latest) — 2025-11-28
- 2.3.0-experimental.4 (experimental) — 2025-05-05
- 2.3.0 — 2025-11-27
- 2.3.0-experimental.3 — 2025-05-05
- 2.3.0-experimental.2 — 2025-05-05
- 2.3.0-experimental.1 — 2025-05-02
- 2.3.0-experimental.0 — 2025-05-02
- 2.2.1 — 2025-02-03
- 2.2.0 — 2024-06-19
- 2.1.1 — 2024-04-25
- 2.1.0 — 2024-03-18
- 2.0.0 — 2023-12-19
- 1.3.3 — 2023-11-09
- 1.3.2 — 2023-10-13
- 1.3.1 — 2023-10-12
- … 14 more at https://npm.io/package/string-ts/versions

## README

<p align="center">
  <img alt="StringTs Banner" src="./docs/string-ts-banner.png" width="800" />
</p>

[![NPM](https://img.shields.io/npm/v/string-ts)](https://www.npmjs.org/package/string-ts)
![Library size](https://img.shields.io/bundlephobia/minzip/string-ts)
[![Code Coverage](https://img.shields.io/codecov/c/github/gustavoguichard/string-ts)](https://app.codecov.io/gh/gustavoguichard/string-ts)
[![All Contributors](https://img.shields.io/github/contributors/gustavoguichard/string-ts)](#-contributors)

## Strongly-typed string functions for all!

![A demonstration of string-ts](https://github.com/gustavoguichard/string-ts/assets/566971/0aa5603f-871d-4eb7-8ace-6a73466cec4d)

## 😬 The problem

When you are working with literal strings, the string manipulation functions only work at the runtime level and the types don't follow those transformations.
You end up losing type information and possibly having to cast the result.

```ts
const str = 'hello-world'
const result = str.replace('-', ' ') // you should use: as 'hello world'
//    ^? string
```

## 🤓 The solution

This library aims to solve this problem by providing a set of common functions that work with literal strings at both type and runtime level.

```ts
import { replace } from 'string-ts'

const str = 'hello-world'
const result = replace(str, '-', ' ')
//    ^ 'hello world'
```

## 🔍 Why this matters

TypeScript yields the best static analysis when types are highly specific.
Literals are more specific than type `string`.
This library preserves literals (and unions of literals) after transformations, unlike most existing utility libraries (and built-in string methods.)

[I still don't get the purpose of this library 🤔](#%EF%B8%8F-interview)

### In-depth example

In the below example, I want to get a strongly-typed, camel-case version of `process.env`.
One flow results in a loose type, and the other results in a more precise type.
This example should illustrate the highly-specific and flexible nature of `string-ts`.

```ts
import { deepCamelKeys } from 'string-ts'
import { camelCase, mapKeys } from 'lodash-es'
import z from 'zod'

const EnvSchema = z.object({
  NODE_ENV: z.string(),
})

function getEnvLoose() {
  const rawEnv = EnvSchema.parse(process.env)
  const env = mapKeys(rawEnv, (_v, k) => camelCase(k))
  //    ^? Dictionary<string>

  // `Dictionary<string>` is too loose
  // TypeScript is okay with this, 'abc' is expected to be of type `string`
  // This will have unexpected behavior at runtime
  console.log(env.abc)
}

function getEnvPrecise() {
  const rawEnv = EnvSchema.parse(process.env)
  const env = deepCamelKeys(rawEnv)
  //    ^? { nodeEnv: string }

  // Error: Property 'abc' does not exist on type '{ nodeEnv: string; }'
  // Our type is more specific, so TypeScript catches this error.
  // This mistake will be caught at compile time
  console.log(env.abc)
}

function main() {
  getEnvLoose()
  getEnvPrecise()
}

main()
```

## 📦 Installation

```bash
npm install string-ts
```

## 🌳 Tree shaking

`string-ts` has been designed with tree shaking in mind.
We have tested it with build tools like Webpack, Vite, Rollup, etc.

## 👌 Supported TypeScript versions

`string-ts` currently only works on TypeScript v5+.

It also only work with common ASCII characters characters. We don't plan to support international characters or emojis.

## 🪄 Native String Method Overrides

Extracting functions from `string-ts` is the recommended usage, but if you'd like to apply strongly-typed versions of native string methods project-wide, you can do so with a single import.

Inspired by [ts-reset](https://github.com/total-typescript/ts-reset), this import overrides the native `String.prototype` type declarations, giving you strong typing for supported methods like `charAt`, `startsWith`, `endsWith`, `replace`, `split`, and more.

```ts
import 'string-ts/native'

const str = 'hello-world' as const

str.replace('-', '_')
//  ^? 'hello_world'

str.charAt(6)
//  ^? 'w'

str.startsWith('hello')
//  ^? true

str.split('-')
//  ^? ['hello', 'world']
```

This has minimal impact on type-checker performance. In our tests, TSC went from 17.21s to 17.41s.

_Note: Only methods already supported by the library are typed. The `length` property cannot be strongly-typed as it's a read-only property, not a method._

---

# 📖 API

- [Runtime counterparts of native type utilities](#runtime-counterparts-of-native-type-utilities)
  - [capitalize](#capitalize)
  - [uncapitalize](#uncapitalize)
- [Strongly-typed alternatives to native runtime utilities](#strongly-typed-alternatives-to-native-runtime-utilities)
  - [charAt](#charat)
  - [concat](#concat)
  - [endsWith](#endsWith)
  - [includes](#includes)
  - [join](#join)
  - [length](#length)
  - [padEnd](#padend)
  - [padStart](#padstart)
  - [repeat](#repeat)
  - [replace](#replace)
  - [replaceAll](#replaceall)
  - [slice](#slice)
  - [split](#split)
  - [startsWith](#startsWith)
  - [toLowerCase](#tolowercase)
  - [toUpperCase](#touppercase)
  - [trim](#trim)
  - [trimEnd](#trimend)
  - [trimStart](#trimstart)
- [Strongly-typed alternatives to common loosely-typed functions](#strongly-typed-alternatives-to-common-loosely-typed-functions)
  - [camelCase](#camelcase)
  - [constantCase](#constantcase)
  - [delimiterCase](#delimitercase)
  - [kebabCase](#kebabcase)
  - [pascalCase](#pascalcase)
  - [reverse](#reverse)
  - [snakeCase](#snakecase)
  - [titleCase](#titlecase)
  - [truncate](#truncate)
  - [words](#words)
- [Strongly-typed shallow transformation of objects](#strongly-typed-shallow-transformation-of-objects)
  - [camelKeys](#camelkeys)
  - [constantKeys](#constantkeys)
  - [delimiterKeys](#delimiterkeys)
  - [kebabKeys](#kebabkeys)
  - [pascalKeys](#pascalkeys)
  - [replaceKeys](#replacekeys)
  - [snakeKeys](#snakekeys)
- [Strongly-typed deep transformation of objects](#strongly-typed-deep-transformation-of-objects)
  - [deepCamelKeys](#deepcamelkeys)
  - [deepConstantKeys](#deepconstantkeys)
  - [deepDelimiterKeys](#deepdelimiterkeys)
  - [deepKebabKeys](#deepkebabkeys)
  - [deepPascalKeys](#deeppascalkeys)
  - [deepSnakeKeys](#deepsnakekeys)
- [Type Utilities](#type-utilities)
  - [Native TS type utilities](#native-ts-type-utilities)
  - [General Type utilities from this library](#general-type-utilities-from-this-library)
  - [Casing type utilities](#casing-type-utilities)
  - [Other exported type utilities](#other-exported-type-utilities)
- [Runtime-only utilities](#runtime-only-utilities)
  - [deepTransformKeys](#deeptransformkeys)

---

## Runtime counterparts of native type utilities

### capitalize

Capitalizes the first letter of a string. This is a runtime counterpart of `Capitalize<T>` from `src/types.d.ts`.

```ts
import { capitalize } from 'string-ts'

const str = 'hello world'
const result = capitalize(str)
//    ^ 'Hello world'
```

### uncapitalize

Uncapitalizes the first letter of a string. This is a runtime counterpart of `Uncapitalize<T>` from `src/types.d.ts`.

```ts
import { uncapitalize } from 'string-ts'

const str = 'Hello world'
const result = uncapitalize(str)
//    ^ 'hello world'
```

## Strongly-typed alternatives to native runtime utilities

### charAt

This function is a strongly-typed counterpart of `String.prototype.charAt`.

```ts
import { charAt } from 'string-ts'

const str = 'hello world'
const result = charAt(str, 6)
//    ^ 'w'
```

### concat

This function is a strongly-typed counterpart of `String.prototype.concat`.

```ts
import { concat } from 'string-ts'

const result = concat('a', 'bc', 'def')
//    ^ 'abcdef'
```

### endsWith

This function is a strongly-typed counterpart of `String.prototype.endsWith`.

```ts
import { endsWith } from 'string-ts'

const result = endsWith('abc', 'c')
//    ^ true
```

### includes

This function is a strongly-typed counterpart of `String.prototype.includes`.

```ts
import { includes } from 'string-ts'

const result = includes('abcde', 'bcd')
//    ^ true
```

### join

This function is a strongly-typed counterpart of `Array.prototype.join`.

```ts
import { join } from 'string-ts'

const str = ['hello', 'world']
const result = join(str, ' ')
//    ^ 'hello world'
```

### length

This function is a strongly-typed counterpart of `String.prototype.length`.

```ts
import { length } from 'string-ts'

const str = 'hello'
const result = length(str)
//    ^ 5
```

### padEnd

This function is a strongly-typed counterpart of `String.prototype.padEnd`.

```ts
import { padEnd } from 'string-ts'

const str = 'hello'
const result = padEnd(str, 10, '=')
//    ^ 'hello====='
```

### padStart

This function is a strongly-typed counterpart of `String.prototype.padStart`.

```ts
import { padStart } from 'string-ts'

const str = 'hello'
const result = padStart(str, 10, '=')
//    ^ '=====hello'
```

### repeat

This function is a strongly-typed counterpart of `String.prototype.repeat`.

```ts
import { repeat } from 'string-ts'

const str = 'abc'
const result = repeat(str, 3)
//    ^ 'abcabcabc'
```

### replace

This function is a strongly-typed counterpart of `String.prototype.replace`.

_Warning: this is a partial implementation, as we don't fully support Regex. Using a RegExp lookup will result in a loose typing._

```ts
import { replace } from 'string-ts'

const str = 'hello-world-'
const result = replace(str, '-', ' ')
//    ^ 'hello world-'
const looselyTypedResult = replace(str, /-/, ' ')
//    ^ string
```

### replaceAll

This function is a strongly-typed counterpart of `String.prototype.replaceAll`.
It also has a polyfill for runtimes older than ES2021.

_Warning: this is a partial implementation, as we don't fully support Regex. Using a RegExp lookup will result in a loose typing._

```ts
import { replaceAll } from 'string-ts'

const str = 'hello-world-'
const result = replaceAll(str, '-', ' ')
//    ^ 'hello world '
const looselyTypedResult = replaceAll(str, /-/g, ' ')
//    ^ string
```

### slice

This function is a strongly-typed counterpart of `String.prototype.slice`.

```ts
import { slice } from 'string-ts'

const str = 'hello-world'
const result = slice(str, 6)
//    ^ 'world'
const result2 = slice(str, 1, 5)
//    ^ 'ello'
const result3 = slice(str, -5)
//    ^ 'world'
```

### split

This function is a strongly-typed counterpart of `String.prototype.split`.

```ts
import { split } from 'string-ts'

const str = 'hello-world'
const result = split(str, '-')
//    ^ ['hello', 'world']
```

### startsWith

This function is a strongly-typed counterpart of `String.prototype.startsWith`.

```ts
import { startsWith } from 'string-ts'

const result = startsWith('abc', 'a')
//    ^ true
```

### toLowerCase

This function is a strongly-typed counterpart of `String.prototype.toLowerCase`.

```ts
import { toLowerCase } from 'string-ts'

const str = 'HELLO WORLD'
const result = toLowerCase(str)
//    ^ 'hello world'
```

### toUpperCase

This function is a strongly-typed counterpart of `String.prototype.toUpperCase`.

```ts
import { toUpperCase } from 'string-ts'

const str = 'hello world'
const result = toUpperCase(str)
//    ^ 'HELLO WORLD'
```

### trim

This function is a strongly-typed counterpart of `String.prototype.trim`.

```ts
import { trim } from 'string-ts'

const str = '  hello world  '
const result = trim(str)
//    ^ 'hello world'
```

### trimEnd

This function is a strongly-typed counterpart of `String.prototype.trimEnd`.

```ts
import { trimEnd } from 'string-ts'

const str = '  hello world  '
const result = trimEnd(str)
//    ^ '  hello world'
```

### trimStart

This function is a strongly-typed counterpart of `String.prototype.trimStart`.

```ts
import { trimStart } from 'string-ts'

const str = '  hello world  '
const result = trimStart(str)
//    ^ 'hello world  '
```

## Strongly-typed alternatives to common loosely-typed functions

### lowerCase

This function converts a string to `lower case` at both runtime and type levels.
_NOTE: this function will split by words and join them with `" "`, unlike `toLowerCase`._

```ts
import { lowerCase } from 'string-ts'

const str = 'HELLO-WORLD'
const result = lowerCase(str)
//    ^ 'hello world'
```

### camelCase

This function converts a string to `camelCase` at both runtime and type levels.

```ts
import { camelCase } from 'string-ts'

const str = 'hello-world'
const result = camelCase(str)
//    ^ 'helloWorld'
```

### constantCase

This function converts a string to `CONSTANT_CASE` at both runtime and type levels.

```ts
import { constantCase } from 'string-ts'

const str = 'helloWorld'
const result = constantCase(str)
//    ^ 'HELLO_WORLD'
```

### delimiterCase

This function converts a string to a new case with a custom delimiter at both runtime and type levels.

```ts
import { delimiterCase } from 'string-ts'

const str = 'helloWorld'
const result = delimiterCase(str, '.')
//    ^ 'hello.World'
```

### kebabCase

This function converts a string to `kebab-case` at both runtime and type levels.

```ts
import { kebabCase } from 'string-ts'

const str = 'helloWorld'
const result = kebabCase(str)
//    ^ 'hello-world'
```

### pascalCase

This function converts a string to `PascalCase` at both runtime and type levels.

```ts
import { pascalCase } from 'string-ts'

const str = 'hello-world'
const result = pascalCase(str)
//    ^ 'HelloWorld'
```

### snakeCase

This function converts a string to `snake_case` at both runtime and type levels.

```ts
import { snakeCase } from 'string-ts'

const str = 'helloWorld'
const result = snakeCase(str)
//    ^ 'hello_world'
```

### titleCase

This function converts a string to `Title Case` at both runtime and type levels.

```ts
import { titleCase } from 'string-ts'

const str = 'helloWorld'
const result = titleCase(str)
//    ^ 'Hello World'
```

### upperCase

This function converts a string to `UPPER CASE` at both runtime and type levels.
_NOTE: this function will split by words and join them with `" "`, unlike `toUpperCase`._

```ts
import { upperCase } from 'string-ts'

const str = 'hello-world'
const result = upperCase(str)
//    ^ 'HELLO WORLD'
```

### reverse

This function reverses a string.

```ts
import { reverse } from 'string-ts'

const str = 'Hello StringTS!'
const result = reverse(str)
//    ^ '!TSgnirtS olleH'
```

### truncate

This function truncates string if it's longer than the given maximum string length. The last characters of the truncated string are replaced with the omission string which defaults to "...".

```ts
import { truncate } from 'string-ts'

const str = '-20someVery-weird String'
const result = truncate(str, 8)
//    ^ '-20so...'
```

### words

This function identifies the words in a string and returns a tuple of words split by separators, differences in casing, numbers, and etc.

```ts
import { words } from 'string-ts'

const str = '-20someVery-weird String'
const result = words(str)
//    ^ ['20', 'some', 'Very', 'weird', 'String']
```

## Strongly-typed shallow transformation of objects

### camelKeys

This function shallowly converts the keys of an object to `camelCase` at both runtime and type levels.

```ts
import { camelKeys } from 'string-ts'

const data = {
  'hello-world': {
    'foo-bar': 'baz',
  },
} as const
const result = camelKeys(data)
//    ^ { helloWorld: { 'foo-bar': 'baz' } }
```

### constantKeys

This function shallowly converts the keys of an object to `CONSTANT_CASE` at both runtime and type levels.

```ts
import { constantKeys } from 'string-ts'

const data = {
  helloWorld: {
    fooBar: 'baz',
  },
} as const
const result = constantKeys(data)
//    ^ { 'HELLO_WORLD': { 'fooBar': 'baz' } }
```

### delimiterKeys

This function shallowly converts the keys of an object to a new case with a custom delimiter at both runtime and type levels.

```ts
import { delimiterKeys } from 'string-ts'

const data = {
  'hello-world': {
    'foo-bar': 'baz',
  },
} as const
const result = delimiterKeys(data, '.')
//    ^ { 'hello.world': { 'foo-bar': 'baz' } }
```

### kebabKeys

This function shallowly converts the keys of an object to `kebab-case` at both runtime and type levels.

```ts
import { kebabKeys } from 'string-ts'

const data = {
  helloWorld: {
    fooBar: 'baz',
  },
} as const
const result = kebabKeys(data)
//    ^ { 'hello-world': { fooBar: 'baz' } }
```

### pascalKeys

This function shallowly converts the keys of an object to `PascalCase` at both runtime and type levels.

```ts
import { pascalKeys } from 'string-ts'

const data = {
  'hello-world': {
    'foo-bar': 'baz',
  },
} as const
const result = pascalKeys(data)
//    ^ { HelloWorld: { FooBar: 'baz' } }
```

### snakeKeys

This function shallowly converts the keys of an object to `snake_case` at both runtime and type levels.

```ts
import { snakeKeys } from 'string-ts'

const data = {
  helloWorld: {
    fooBar: 'baz',
  },
} as const
const result = snakeKeys(data)
//    ^ { 'hello_world': { 'fooBar': 'baz' } }
```

### replaceKeys

This function shallowly transforms the keys of an object by applying [`replace`](#replace) to each of its keys at both runtime and type levels.

```ts
import { replaceKeys } from 'string-ts'

const data = {
  helloWorld: {
    fooBar: 'baz',
  },
} as const
const result = replaceKeys(data, 'o', 'a')
//    ^ { 'hellaWorld': { 'fooBar': 'baz' } }
```

## Strongly-typed deep transformation of objects

### deepCamelKeys

This function recursively converts the keys of an object to `camelCase` at both runtime and type levels.

```ts
import { deepCamelKeys } from 'string-ts'

const data = {
  'hello-world': {
    'foo-bar': 'baz',
  },
} as const
const result = deepCamelKeys(data)
//    ^ { helloWorld: { fooBar: 'baz' } }
```

### deepConstantKeys

This function recursively converts the keys of an object to `CONSTANT_CASE` at both runtime and type levels.

```ts
import { deepConstantKeys } from 'string-ts'

const data = {
  helloWorld: {
    fooBar: 'baz',
  },
} as const
const result = deepConstantKeys(data)
//    ^ { 'HELLO_WORLD': { 'FOO_BAR': 'baz' } }
```

### deepDelimiterKeys

This function recursively converts the keys of an object to a new case with a custom delimiter at both runtime and type levels.

```ts
import { deepDelimiterKeys } from 'string-ts'

const data = {
  'hello-world': {
    'foo-bar': 'baz',
  },
} as const
const result = deepDelimiterKeys(data, '.')
//    ^ { 'hello.world': { 'foo.bar': 'baz' } }
```

### deepKebabKeys

This function recursively converts the keys of an object to `kebab-case` at both runtime and type levels.

```ts
import { deepKebabKeys } from 'string-ts'

const data = {
  helloWorld: {
    fooBar: 'baz',
  },
} as const
const result = deepKebabKeys(data)
//    ^ { 'hello-world': { 'foo-bar': 'baz' } }
```

### deepPascalKeys

This function recursively converts the keys of an object to `PascalCase` at both runtime and type levels.

```ts
import { deepPascalKeys } from 'string-ts'

const data = {
  'hello-world': {
    'foo-bar': 'baz',
  },
} as const
const result = deepPascalKeys(data)
//    ^ { HelloWorld: { FooBar: 'baz' } }
```

### deepSnakeKeys

This function recursively converts the keys of an object to `snake_case` at both runtime and type levels.

```ts
import { deepSnakeKeys } from 'string-ts'

const data = {
  helloWorld: {
    fooBar: 'baz',
  },
} as const
const result = deepSnakeKeys(data)
//    ^ { 'hello_world': { 'foo_bar': 'baz' } }
```

## Type utilities

All the functions presented in this API have associated type counterparts.

```ts
import type * as St from 'string-ts'
```

### Native TS type utilities

```ts
Capitalize<'hello world'> // 'Hello world'
Lowercase<'HELLO WORLD'> // 'hello world'
Uppercase<'hello world'> // 'HELLO WORLD'
```

### General type utilities from this library

```ts
St.CharAt<'hello world', 6> // 'w'
St.Concat<['a', 'bc', 'def']> // 'abcdef'
St.EndsWith<'abc', 'c'> // true
St.Includes<'abcde', 'bcd'> // true
St.Join<['hello', 'world'], '-'> // 'hello-world'
St.Length<'hello'> // 5
St.PadEnd<'hello', 10, '='> // 'hello====='
St.PadStart<'hello', 10, '='> // '=====hello'
St.Repeat<'abc', 3> // 'abcabcabc'
St.Replace<'hello-world', 'l', '1'> // 'he1lo-world'
St.ReplaceAll<'hello-world', 'l', '1'> // 'he11o-wor1d'
St.Reverse<'Hello World!'> // '!dlroW olleH'
St.Slice<'hello-world', -5> // 'world'
St.Split<'hello-world', '-'> // ['hello', 'world']
St.Trim<' hello world '> // 'hello world'
St.StartsWith<'abc', 'a'> // true
St.TrimEnd<' hello world '> // ' hello world'
St.TrimStart<' hello world '> // 'hello world '
St.Truncate<'hello world', 9, '[...]'> // 'hello[...]
St.Words<'hello-world'> // ['hello', 'world']
```

### Casing type utilities

#### Core

```ts
St.CamelCase<'hello-world'> // 'helloWorld'
St.ConstantCase<'helloWorld'> // 'HELLO_WORLD'
St.DelimiterCase<'hello world', '.'> // 'hello.world'
St.KebabCase<'helloWorld'> // 'hello-world'
St.PascalCase<'hello-world'> // 'HelloWorld'
St.SnakeCase<'helloWorld'> // 'hello_world'
St.TitleCase<'helloWorld'> // 'Hello World'
```

##### Missing types

_Note that we do not include `UpperCase` and `LowerCase` types. These would be too close to the existing TS types `Uppercase` and `Lowercase`._

One could create either by doing like so:

```ts
type LowerCase<T extends string> = Lowercase<DelimiterCase<T, ' '>>
type UpperCase<T extends string> = Uppercase<DelimiterCase<T, ' '>>
// or
type LowerCase<T extends string> = ReturnType<typeof lowerCase<T>>
type UpperCase<T extends string> = ReturnType<typeof upperCase<T>>
```

#### Shallow object key transformation

```ts
St.CamelKeys<{
  'hello-world': { 'foo-bar': 'baz' }
}> // { helloWorld: { 'foo-bar': 'baz' } }
St.ConstantKeys<{
  helloWorld: { fooBar: 'baz' }
}> // { 'HELLO_WORLD': { fooBar: 'baz' } }
St.DelimiterKeys<{ 'hello-world': { 'foo-bar': 'baz' } }, '.'>
// { 'hello.world': { 'foo-bar': 'baz' } }
St.KebabKeys<{
  helloWorld: { fooBar: 'baz' }
}> // { 'hello-world': { fooBar: 'baz' } }
St.PascalKeys<{
  'hello-world': { 'foo-bar': 'baz' }
}> // { HelloWorld: { 'foo-bar': 'baz' } }
St.SnakeKeys<{
  helloWorld: { fooBar: 'baz' }
}> // { 'hello_world': { fooBar: 'baz' } }
```

#### Deep object key transformation

```ts
St.DeepCamelKeys<{
  'hello-world': { 'foo-bar': 'baz' }
}> // { helloWorld: { fooBar: 'baz' } }
St.DeepConstantKeys<{
  helloWorld: { fooBar: 'baz' }
}> // { 'HELLO_WORLD': { 'FOO_BAR': 'baz' } }
St.DeepDelimiterKeys<
  {
    'hello-world': { 'foo-bar': 'baz' }
  },
  '.'
> // { 'hello.world': { 'foo.bar': 'baz' } }
St.DeepKebabKeys<{
  helloWorld: { fooBar: 'baz' }
}> // { 'hello-world': { 'foo-bar': 'baz' } }
St.DeepPascalKeys<{
  'hello-world': { 'foo-bar': 'baz' }
}> // { HelloWorld: { FooBar: 'baz' } }
St.DeepSnakeKeys<{
  helloWorld: { fooBar: 'baz' }
}> // { 'hello_world': { 'foo_bar': 'baz' } }
```

### Other exported type utilities

```ts
St.IsDigit<'a'> // false
St.IsDigit<'1'> // true
St.IsLetter<'a'> // true
St.IsLetter<'1'> // false
St.IsLower<'a'> // true
St.IsLower<'A'> // false
St.IsUpper<'a'> // false
St.IsUpper<'A'> // true
St.IsSeparator<' '> // true
St.IsSeparator<'-'> // true
St.IsSeparator<'a'> // false
St.IsSpecial<'a'> // false
St.IsSpecial<'!'> // true
St.IsSpecial<' '> // false
```

## Runtime-only utilities

### deepTransformKeys

This function recursively converts the keys of an object to a custom format, but only at runtime level.

```ts
import { deepTransformKeys, toUpperCase } from 'string-ts'

const data = { helloWorld: 'baz' } as const

type MyType<T> = { [K in keyof T as Uppercase<K>]: T[K] }
const result = deepTransformKeys(data, toUpperCase) as MyType<typeof data>
//    ^ { 'HELLOWORLD': 'baz' }
```

## 🎙️ Interview

For a deeper dive into the code, reasoning, and how this library came to be, check out this interview with the author of StringTs on the Michigan TypeScript Meetup.

[![StringTs on Michigan TS](https://img.youtube.com/vi/dOXpkAmmahw/0.jpg)](https://www.youtube.com/watch?v=dOXpkAmmahw)

## 🐝 Contributors

Thanks goes to these wonderful people ([emoji key](https://allcontributors.org/docs/en/emoji-key)):

<!-- ALL-CONTRIBUTORS-LIST:START - Do not remove or modify this section -->
<!-- markdownlint-disable -->
<table>
  <tbody>
    <tr>
      <td align="center" valign="top" width="14.28%"><a href="https://github.com/gustavoguichard"><img src="https://avatars.githubusercontent.com/u/566971?v=4?s=100" width="100px;" alt="Guga Guichard"/><br /><sub><b>Guga Guichard</b></sub></a><br /><a href="https://github.com/gustavoguichard/string-ts/commits?author=gustavoguichard" title="Code">💻</a> <a href="#projectManagement-gustavoguichard" title="Project Management">📆</a> <a href="#promotion-gustavoguichard" title="Promotion">📣</a> <a href="#maintenance-gustavoguichard" title="Maintenance">🚧</a> <a href="https://github.com/gustavoguichard/string-ts/commits?author=gustavoguichard" title="Documentation">📖</a> <a href="https://github.com/gustavoguichard/string-ts/issues?q=author%3Agustavoguichard" title="Bug reports">🐛</a> <a href="#infra-gustavoguichard" title="Infrastructure (Hosting, Build-Tools, etc)">🚇</a> <a href="#question-gustavoguichard" title="Answering Questions">💬</a> <a href="#research-gustavoguichard" title="Research">🔬</a> <a href="https://github.com/gustavoguichard/string-ts/pulls?q=is%3Apr+reviewed-by%3Agustavoguichard" title="Reviewed Pull Requests">👀</a> <a href="#ideas-gustavoguichard" title="Ideas, Planning, & Feedback">🤔</a> <a href="#example-gustavoguichard" title="Examples">💡</a></td>
      <td align="center" valign="top" width="14.28%"><a href="https://github.com/jly36963"><img src="https://avatars.githubusercontent.com/u/33426811?v=4?s=100" width="100px;" alt="Landon Yarrington"/><br /><sub><b>Landon Yarrington</b></sub></a><br /><a href="https://github.com/gustavoguichard/string-ts/commits?author=jly36963" title="Code">💻</a> <a href="#maintenance-jly36963" title="Maintenance">🚧</a> <a href="https://github.com/gustavoguichard/string-ts/commits?author=jly36963" title="Documentation">📖</a> <a href="https://github.com/gustavoguichard/string-ts/pulls?q=is%3Apr+reviewed-by%3Ajly36963" title="Reviewed Pull Requests">👀</a> <a href="#ideas-jly36963" title="Ideas, Planning, & Feedback">🤔</a> <a href="#example-jly36963" title="Examples">💡</a> <a href="#question-jly36963" title="Answering Questions">💬</a> <a href="https://github.com/gustavoguichard/string-ts/issues?q=author%3Ajly36963" title="Bug reports">🐛</a></td>
      <td align="center" valign="top" width="14.28%"><a href="https://github.com/p9f"><img src="https://avatars.githubusercontent.com/u/20539361?v=4?s=100" width="100px;" alt="Guillaume"/><br /><sub><b>Guillaume</b></sub></a><br /><a href="https://github.com/gustavoguichard/string-ts/commits?author=p9f" title="Code">💻</a> <a href="#maintenance-p9f" title="Maintenance">🚧</a> <a href="https://github.com/gustavoguichard/string-ts/commits?author=p9f" title="Documentation">📖</a> <a href="https://github.com/gustavoguichard/string-ts/issues?q=author%3Ap9f" title="Bug reports">🐛</a> <a href="#infra-p9f" title="Infrastructure (Hosting, Build-Tools, etc)">🚇</a> <a href="#question-p9f" title="Answering Questions">💬</a> <a href="#ideas-p9f" title="Ideas, Planning, & Feedback">🤔</a></td>
      <td align="center" valign="top" width="14.28%"><a href="https://totaltypescript.com"><img src="https://avatars.githubusercontent.com/u/28293365?v=4?s=100" width="100px;" alt="Matt Pocock"/><br /><sub><b>Matt Pocock</b></sub></a><br /><a href="https://github.com/gustavoguichard/string-ts/commits?author=mattpocock" title="Documentation">📖</a> <a href="https://github.com/gustavoguichard/string-ts/commits?author=mattpocock" title="Code">💻</a> <a href="#promotion-mattpocock" title="Promotion">📣</a></td>
      <td align="center" valign="top" width="14.28%"><a href="https://luca.md"><img src="https://avatars.githubusercontent.com/u/1881266?v=4?s=100" width="100px;" alt="Andrew Luca"/><br /><sub><b>Andrew Luca</b></sub></a><br /><a href="https://github.com/gustavoguichard/string-ts/commits?author=iamandrewluca" title="Documentation">📖</a> <a href="#promotion-iamandrewluca" title="Promotion">📣</a></td>
      <td align="center" valign="top" width="14.28%"><a href="https://github.com/mjuksel"><img src="https://avatars.githubusercontent.com/u/10691584?v=4?s=100" width="100px;" alt="Mjuksel"/><br /><sub><b>Mjuksel</b></sub></a><br /><a href="https://github.com/gustavoguichard/string-ts/commits?author=mjuksel" title="Code">💻</a> <a href="#ideas-mjuksel" title="Ideas, Planning, & Feedback">🤔</a></td>
      <td align="center" valign="top" width="14.28%"><a href="https://huguesverlin.fr"><img src="https://avatars.githubusercontent.com/u/9151470?v=4?s=100" width="100px;" alt="hverlin"/><br /><sub><b>hverlin</b></sub></a><br /><a href="https://github.com/gustavoguichard/string-ts/commits?author=hverlin" title="Code">💻</a></td>
    </tr>
  </tbody>
</table>

<!-- markdownlint-restore -->

<!-- ALL-CONTRIBUTORS-LIST:END -->

This project follows the [all-contributors](https://github.com/all-contributors/all-contributors) specification. Contributions of any kind welcome!

StringTs logo by [NUMI](https://github.com/numi-hq/open-design):

[<img src="https://raw.githubusercontent.com/numi-hq/open-design/main/assets/numi-lockup.png" alt="NUMI Logo" style="width: 200px;"/>](https://numi.tech/?ref=string-ts)

## 🫶 Acknowledgements

This library got a lot of inspiration from libraries such as [lodash](https://github.com/lodash/lodash), [ts-reset](https://github.com/total-typescript/ts-reset), [type-fest](https://github.com/sindresorhus/type-fest), [HOTScript](https://github.com/gvergnaud/hotscript), and many others.

---
_Source: https://npm.io/package/string-ts · Machine-readable twin of the npm.io package page. Health data is recomputed on every publish._