add record extensions

records/hasField.test.ts

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+import { expect, test } from 'vitest'
+import { hasField } from './hasField.js'
+
+test('hasField infers that a field exists on an unknown object', () => {
+  const obj: unknown = { id: 1 }
+  expect(hasField(obj, 'id') ? obj.id : 0).toBe(1)
+})
+
+test.each([
+  { obj: { a: 1 }, key: 'a', expected: true },
+  { obj: { a: 1 }, key: 'b', expected: false },
+  { obj: {}, key: 'a', expected: false },
+  { obj: undefined, key: 'a', expected: false },
+])('hasField checks that a field exists on an unknown object', ({ obj, key, expected }) => {
+  expect(hasField(obj, key)).toBe(expected)
+})

records/hasField.ts

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+/**
+ * Checks if the given object has the specified field.
+ */
+export const hasField = <P extends PropertyKey>(obj: unknown, key: P): obj is Record<P, unknown> =>
+  obj != null && typeof obj === 'object' && key in obj
+
+/*
+ * Example:
+ *   const obj: { data: unknown } | { error: string } | unknown = ...
+ *   hasField(obj, 'data')
+ *     ? JSON.stringify(obj.data) // inferred that the data field exists
+ *     : obj?.error // inferred that it's nullable or has error field
+ */

records/index.ts

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+export * from './hasField.js'
+export * from './pickFields.js'
+export * from './omitFields.js'

records/omitFields.test.ts

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+import { expect, test } from 'vitest'
+import { omitFields } from './omitFields.js'
+
+test('omitFields omits a field from an object', () => {
+  expect(omitFields({ a: 1, b: 2, c: 3 }, 'b')).toMatchObject({ a: 1, c: 3 })
+})
+
+test('omitFields omits multiple fields from an object', () => {
+  expect(omitFields({ a: 1, b: 2, c: 3 }, 'b', 'c')).toMatchObject({ a: 1 })
+})
+
+test('omitFields returns same obj if no keys are specified', () => {
+  const obj = { a: 1, b: 2, c: 3 }
+  expect(omitFields(obj)).toBe(obj)
+})

records/omitFields.ts

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+/**
+ * Omits specified fields from an object and returns a copy without those fields.
+ */
+export const omitFields = <T extends Record<PropertyKey, unknown>, P extends keyof T>(obj: T, ...keys: P[]): Omit<T, P> => {
+  if (keys.length === 0) return obj
+
+  const copy = { ...obj }
+  for (const k of keys) {
+    // eslint-disable-next-line @typescript-eslint/no-dynamic-delete
+    delete copy[k]
+  }
+  return copy
+}
+
+/*
+ * Example:
+ *   const order = { id: ..., date: ..., customer: ..., status: ..., items: [...] }
+ *   const result = omitFields(order, 'customer', 'status') // { id: ..., date: ..., items: [...] }
+ */

records/pickFields.test.ts

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+import { expect, test } from 'vitest'
+import { pickFields } from './pickFields.js'
+
+test('pickFields picks a field from an object', () => {
+  expect(pickFields({ a: 1, b: 2, c: 3 }, 'b')).toMatchObject({ b: 2 })
+})
+
+test('pickFields picks multiple fields from an object', () => {
+  expect(pickFields({ a: 1, b: 2, c: 3 }, 'a', 'b')).toMatchObject({ a: 1, b: 2 })
+})
+
+test('pickFields returns same obj if no keys are specified', () => {
+  const obj = { a: 1, b: 2, c: 3 }
+  expect(pickFields(obj)).toBe(obj)
+})

records/pickFields.ts

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+/**
+ * Picks specific fields from an object and returns a copy with only those fields.
+ */
+export const pickFields = <T extends Record<PropertyKey, unknown>, P extends keyof T>(obj: T, ...keys: P[]): Pick<T, P> => {
+  if (keys.length === 0) return obj
+
+  const copy = {} as Pick<T, P>
+  for (const k of keys) {
+    copy[k] = obj[k]
+  }
+  return copy
+}
+
+/*
+ * Example:
+ *   const order = { id: ..., date: ..., customer: ..., status: ..., items: [...] }
+ *   const result = pickFields(order, 'id', 'date') // { id: ..., date: ... }
+ */