Strongly-typed string functions for all!

😬 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.

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.

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 🤔

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.

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

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.

📖 API

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.

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.

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.

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.

import { concat } from 'string-ts'

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

endsWith

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

import { endsWith } from 'string-ts'

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

includes

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

import { includes } from 'string-ts'

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

join

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

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.

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.

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.

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.

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.

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.

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.

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.

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.

import { startsWith } from 'string-ts'

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

toLowerCase

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

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

import { upperCase } from 'string-ts'

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

reverse

This function reverses a string.

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 "...".

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.

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.

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.

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.

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.

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.

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.

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 to each of its keys at both runtime and type levels.

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.

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.

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.

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.

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.

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.

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.

import type * as St from 'string-ts'

Native TS type utilities

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

General type utilities from this library

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

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:

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

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

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

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.

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.