@kuindji/conditional-string

Type-safe conditional string template parser with compile-time inference for TypeScript.

Features

• Compile-time type safety: Results are inferred at the type level based on condition values
• Static type override: Use .with<Data>() to specify static types for return type computation
• Simple syntax: Uses comment syntax /*if:condition*/.../*endif*/ that works with any string
• Negation support: Use ! prefix for negated conditions
• Dot notation: Access nested properties with user.isAdmin syntax
• Nested conditions: Supports arbitrary nesting of conditional blocks
• Zero runtime dependencies: Lightweight and fast

Installation

npm install @kuindji/conditional-string
# or
bun add @kuindji/conditional-string
# or
yarn add @kuindji/conditional-string

Usage

Basic Example

import { conditionalString } from "@kuindji/conditional-string";

const template = `
  Hello /*if:showName*/World/*endif*/!
  /*if:showGreeting*/Have a nice day!/*endif*/
`;

const result = conditionalString(
    template,
    {
        showName: true,
        showGreeting: false,
    } as const,
);

// Result: "Hello World!\n"
// TypeScript knows the exact string type at compile time!

SQL Example

import { conditionalString } from "@kuindji/conditional-string";

const sql = `
  SELECT * FROM users 
  WHERE 1=1 
  /*if:includeDeleted*/AND deleted = false/*endif*/
  /*if:filterActive*/AND active = true/*endif*/
`;

const result = conditionalString(
    sql,
    {
        includeDeleted: true,
        filterActive: false,
    } as const,
);

// Result: "SELECT * FROM users WHERE 1=1 AND deleted = false"

Negation

Use ! to negate a condition:

const template = `Welcome /*if:!isGuest*/back/*endif*/!`;

conditionalString(template, { isGuest: false } as const);
// Result: "Welcome back!"

conditionalString(template, { isGuest: true } as const);
// Result: "Welcome !"

Dot Notation for Nested Properties

Access nested object properties using dot notation:

const template =
    `User: /*if:user.isAdmin*/[ADMIN] /*endif*//*if:user.name*/John/*endif*/`;

conditionalString(template, { user: { isAdmin: true, name: true } } as const);
// Result: "User: [ADMIN] John"

Nested Conditions

Conditions can be nested arbitrarily:

const template = `Start /*if:a*/A /*if:b*/B/*endif*//*endif*/ End`;

conditionalString(template, { a: true, b: true } as const);
// Result: "Start A B End"

conditionalString(template, { a: true, b: false } as const);
// Result: "Start A  End"

conditionalString(template, { a: false, b: true } as const);
// Result: "Start  End"

Real-World SQL Example

const getUsersQuery = `
  SELECT u.id, u.name, u.email
  FROM users u
  WHERE 1=1
  /*if:searchTerm*/AND (u.name ILIKE $1 OR u.email ILIKE $1)/*endif*/
  /*if:roleFilter*/AND u.role = $2/*endif*/
  /*if:activeOnly*/AND u.active = true/*endif*/
  ORDER BY u.created_at DESC
`;

const query = conditionalString(
    getUsersQuery,
    {
        searchTerm: "john",
        roleFilter: null,
        activeOnly: true,
    } as const,
);

// Results in:
// SELECT u.id, u.name, u.email
// FROM users u
// WHERE 1=1
// AND (u.name ILIKE $1 OR u.email ILIKE $1)
// AND u.active = true
// ORDER BY u.created_at DESC

Type Safety

The library provides compile-time type inference. When using literal types (via as const), TypeScript will compute the exact resulting string type:

import { type ConditionalStringResult } from "@kuindji/conditional-string";

// The type is computed at compile time
type Result = ConditionalStringResult<
    "Hello /*if:flag*/World/*endif*/!",
    { flag: true; }
>;
// Result = "Hello World!"

type Result2 = ConditionalStringResult<
    "Hello /*if:flag*/World/*endif*/!",
    { flag: false; }
>;
// Result2 = "Hello !"

If the condition values are not literal types (e.g., widened boolean instead of true or false), the result type falls back to string.

Static Type Override with .with<Data>()

When you need to specify a static type for return type computation while allowing runtime values to vary, use the .with<Data>() method:

import { conditionalString } from "@kuindji/conditional-string";

// Runtime value determined dynamically
const showAdmin = Math.random() > 0.5;

const template = `User /*if:isAdmin*/[ADMIN]/*endif*/`;

// Using .with<Data>() to specify static type for return type computation
const result = conditionalString.with<{ isAdmin: true }>()(
    template,
    { isAdmin: showAdmin },  // Runtime value can be boolean
);

// Return type is computed as if isAdmin is always true: "User [ADMIN]"
// But runtime result depends on actual showAdmin value

This is useful when:

• You want predictable return types regardless of runtime values
• You're building type-safe query builders where the schema is known at compile time
• You need to decouple type computation from runtime data

The .with<Data>() method:

• Uses Data for return type computation
• Accepts runtime data with matching keys but unrestricted value types
• Template type is still inferred automatically

Truthy/Falsy Values

The library follows JavaScript truthy/falsy semantics:

• Falsy: false, 0, "", null, undefined
• Truthy: Everything else, including empty arrays [] and empty objects {}

Syntax

/*if:condition*/content/*endif*/

• condition: The property name to check (supports dot notation and ! prefix)
• content: The content to include when the condition is truthy

API

conditionalString<Template, Data>(template, data)

Processes a string template with conditional comments. Both Template and Data types are inferred.

Parameters:

• template: The string with conditional comments
• data: An object with condition values

Returns: ConditionalStringResult<Template, Data> - The processed string with conditionals resolved

---

conditionalString.with<StaticData>()(template, data)

Creates a typed version where StaticData is used for return type computation, but runtime data values are unrestricted.

Type Parameters:

• StaticData: The type used for computing the return type

Parameters:

• template: The string with conditional comments (type is inferred)
• data: An object with keys matching StaticData, but values can be any type

Returns: ConditionalStringResult<Template, StaticData> - The processed string with type computed using StaticData

Example:

// Static type says isAdmin is true, so return type includes "[ADMIN]"
// Runtime data can have isAdmin as any boolean
const result = conditionalString.with<{ isAdmin: true }>()(
    `User /*if:isAdmin*/[ADMIN]/*endif*/`,
    { isAdmin: someRuntimeBoolean },
);

---

ConditionalStringResult<Template, Data>

A utility type that computes the resulting string type based on the template and data types.

License

MIT