Condition Builder

Inspired by Flutter condition_builder

A tiny TypeScript/JavaScript utility for clean multi-way conditional logic — no if/else chains, no nested ternaries.

import { ConditionBuilder } from 'wi-condition-builder'

const result = new ConditionBuilder<string>()
    .on(() => role === 'admin', 'Dashboard')
    .on(() => role === 'editor', 'Editor Panel')
    .on(() => role === 'viewer', 'Read-only View')
    .build(() => 'Login Screen')

Features

• First-match wins — conditions are evaluated in declaration order
• Lazy evaluation — values can be passed as thunks (functions), evaluated only when matched
• Optional fallback — provide a default via .build(() => defaultValue)
• Null-safe — if no condition matches and no fallback is given, returns null

Install

npm install wi-condition-builder

Usage

Basic

const result = new ConditionBuilder<number>()
    .on(() => true, 1)
    .on(() => false, 2)
    .build(() => 3)

// result === 1

Lazy (thunk) values

Pass a function if the value is expensive to compute or has side effects:

const result = new ConditionBuilder<string>()
    .on(() => isLoading, () => fetchData())
    .on(() => isCached, () => readCache())
    .build(() => 'fallback')

API

new ConditionBuilder<T>()

Creates an empty builder.

.on(predicate, value): this

| Param | Type | Description | |-------------|---------------------|-----------------------------------------------------| | predicate | () => boolean | Called at build time to check this condition | | value | T \| (() => T) | Returned if predicate matches. Use () => T for lazy evaluation |

Returns this for chaining.

.build(fallback?)

build(): T | null
build(fallback: () => T): T

| Param | Type | Description | |------------|---------------|------------------------------------------| | fallback?| () => T | Called if no condition matched |

Returns T if a condition matched or fallback was used, or null if no fallback provided.

Real-world example

type Page = 'dashboard' | 'settings' | 'profile' | 'login'

const currentPage = new ConditionBuilder<Page>()
    .on(() => user.role === 'admin', 'dashboard')
    .on(() => !!user.session, 'profile')
    .on(() => true, 'login')
    .build()

Without ConditionBuilder:

const currentPage = user.role === 'admin'
    ? 'dashboard'
    : user.session
        ? 'profile'
        : 'login'

Comparison

Without this library, the same logic requires nested ternaries or mutable variables:

// Nested ternary — hard to read with 3+ conditions
const result = condition1 ? value1
    : condition2 ? value2
    : condition3 ? value3
    : fallback

// if/else — verbose, mutable variable needed
let result: string
if (condition1) {
    result = value1
} else if (condition2) {
    result = value2
} else {
    result = value3
}

License

MIT