partition()

partition<T, U>(array, predicate): [U[], Exclude<T, U>[]]

partition<T>(array, predicate): [T[], T[]]

Splits an array into two groups based on a predicate function.

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Type Parameters

T: T

The type of elements in the array.

U: U

The type being filtered for (when using type guard).

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Parameters

Overload 1:

array: readonly T[]

The array to partition.

predicate: (value, index, array) => value is U

A function that returns true for elements in the first group.

Overload 2:

array: readonly T[]

The array to partition.

predicate: (value, index, array) => boolean

A function that returns true for elements in the first group.

---

Returns: [U[], Exclude<T, U>[]]

A tuple of [matching, non-matching] elements.

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Since

2.0.0

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Performance

O(n) — single pass through the array.

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Also known as

fork (Radashi) · partition (Lodash, es-toolkit, Remeda, Ramda, Effect, Antfu) · ❌ (Modern Dash)

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Example

partition([1, 2, 3, 4], (n) => n % 2 === 0);
// => [[2, 4], [1, 3]]

partition(
  [{ active: true }, { active: false }, { active: true }],
  (u) => u.active
);
// => [[{ active: true }, { active: true }], [{ active: false }]]

---

How it works?

Splits an array into two groups: elements matching the predicate and elements that don't. Single pass through the array.

Element routing

partition vs filter

Function	Returns	Use case
filter	Matching only	Don't need rejected
partition	[matching, rejected]	Need both groups

---

Use Cases

Separate active and inactive users 📌

Split user lists based on status for targeted actions. Perfect for CRM systems, email campaigns, or user analytics.

#partition#users#active#inactive#CRM

const users = [
  { name: "Alice", isActive: true },
  { name: "Bob", isActive: false },
  { name: "Charlie", isActive: true },
  { name: "Diana", isActive: false },
];

const [activeUsers, inactiveUsers] = partition(users, (u) => u.isActive);
// activeUsers => [Alice, Charlie]
// inactiveUsers => [Bob, Diana]

Split valid and invalid form submissions 📌

Separate successful entries from errors for batch handling. Ideal for form validation, data imports, or API request processing.

#validation#form#submissions#errors#batch processing

const submissions = [
  { id: "S1", email: "user1@mail.com", age: 25 },
  { id: "S2", email: "invalid-email", age: 30 },
  { id: "S3", email: "user3@mail.com", age: 17 },
  { id: "S4", email: "user4@mail.com", age: 22 },
];

const isValid = (s) => s.email.includes("@") && s.age >= 18;

const [valid, invalid] = partition(submissions, isValid);
// valid => [S1, S4]
// invalid => [S2, S3]

Categorize transactions as income vs expenses

Split financial data for budgeting and reporting. Useful for banking apps, expense trackers, or accounting systems.

#transactions#income#expenses#financial#budgeting

const transactions = [
  { description: "Salary", amount: 5000 },
  { description: "Rent", amount: -1500 },
  { description: "Freelance", amount: 800 },
  { description: "Groceries", amount: -250 },
];

const [income, expenses] = partition(transactions, (t) => t.amount > 0);
// income => [Salary, Freelance]
// expenses => [Rent, Groceries]

const totalIncome = income.reduce((sum, t) => sum + t.amount, 0);
// => 5800

Separate approved vs rejected insurance claims

Split insurance claims for streamlined processing workflows. Essential for insurance companies managing claim approvals and appeals.

#insurance#claims#approved#rejected#assurance#sinistre#processing

const claims = [
  { id: "CLM-001", type: "auto", amount: 5000, status: "approved", riskScore: 0.2 },
  { id: "CLM-002", type: "home", amount: 15000, status: "rejected", riskScore: 0.9 },
  { id: "CLM-003", type: "health", amount: 3200, status: "approved", riskScore: 0.3 },
  { id: "CLM-004", type: "auto", amount: 8500, status: "pending", riskScore: 0.6 },
  { id: "CLM-005", type: "home", amount: 25000, status: "rejected", riskScore: 0.85 },
];

// Separate approved claims for payment processing
const [approved, needsReview] = partition(claims, (c) => c.status === "approved");
// approved => [CLM-001, CLM-003] - Ready for payout
// needsReview => [CLM-002, CLM-004, CLM-005] - Requires further action

// Separate high-risk claims for fraud investigation
const [highRisk, standard] = partition(claims, (c) => c.riskScore > 0.7);
// highRisk => [CLM-002, CLM-005] - Flag for investigation
// standard => [CLM-001, CLM-003, CLM-004] - Normal processing