Manual Construction

import {randomKey} from '@sanity/util/content'

const key = randomKey(12) // e.g., "a1b2c3d4e5f6"

const randomKey = () => Math.random().toString(36).slice(2, 14)

{
  _type: 'block',
  _key: randomKey(),
  style: 'normal',
  children: [
    {_type: 'span', _key: randomKey(), text: 'Hello world', marks: []}
  ],
  markDefs: []
}

{
  _type: 'block',
  _key: randomKey(),
  style: 'h2', // h1, h2, h3, h4, h5, h6
  children: [
    {_type: 'span', _key: randomKey(), text: 'Section Title', marks: []}
  ],
  markDefs: []
}

{
  _type: 'block',
  _key: randomKey(),
  style: 'normal',
  children: [
    {_type: 'span', _key: randomKey(), text: 'This is ', marks: []},
    {_type: 'span', _key: randomKey(), text: 'bold', marks: ['strong']},
    {_type: 'span', _key: randomKey(), text: ' and ', marks: []},
    {_type: 'span', _key: randomKey(), text: 'italic', marks: ['em']},
    {_type: 'span', _key: randomKey(), text: ' text.', marks: []},
  ],
  markDefs: []
}

const linkKey = randomKey()

{
  _type: 'block',
  _key: randomKey(),
  style: 'normal',
  children: [
    {_type: 'span', _key: randomKey(), text: 'Visit ', marks: []},
    {_type: 'span', _key: randomKey(), text: 'Sanity', marks: [linkKey]},
    {_type: 'span', _key: randomKey(), text: ' for more.', marks: []},
  ],
  markDefs: [
    {_type: 'link', _key: linkKey, href: 'https://www.sanity.io'}
  ]
}

const linkKey = randomKey()

// "bold link" — both strong and linked
{_type: 'span', _key: randomKey(), text: 'bold link', marks: ['strong', linkKey]}

// Bullet list
[
  {
    _type: 'block', _key: randomKey(), style: 'normal',
    listItem: 'bullet', level: 1,
    children: [{_type: 'span', _key: randomKey(), text: 'First item', marks: []}],
    markDefs: []
  },
  {
    _type: 'block', _key: randomKey(), style: 'normal',
    listItem: 'bullet', level: 1,
    children: [{_type: 'span', _key: randomKey(), text: 'Second item', marks: []}],
    markDefs: []
  },
  {
    _type: 'block', _key: randomKey(), style: 'normal',
    listItem: 'bullet', level: 2, // nested
    children: [{_type: 'span', _key: randomKey(), text: 'Nested item', marks: []}],
    markDefs: []
  },
]

// Image block
{
  _type: 'image',
  _key: randomKey(),
  asset: {_type: 'reference', _ref: 'image-abc123-800x600-png'},
  alt: 'A description',
}

// Code block
{
  _type: 'code',
  _key: randomKey(),
  language: 'typescript',
  code: 'const x = 42',
}

// YouTube embed
{
  _type: 'youtube',
  _key: randomKey(),
  url: 'https://www.youtube.com/watch?v=dQw4w9WgXcQ',
}

function createBlock(
  text: string,
  style: string = 'normal',
  options?: {listItem?: string; level?: number}
) {
  return {
    _type: 'block',
    _key: randomKey(),
    style,
    ...(options?.listItem ? {listItem: options.listItem, level: options.level || 1} : {}),
    children: [{_type: 'span', _key: randomKey(), text, marks: []}],
    markDefs: [],
  }
}

// Usage
const blocks = [
  createBlock('Introduction', 'h2'),
  createBlock('This is a paragraph.'),
  createBlock('First point', 'normal', {listItem: 'bullet', level: 1}),
  createBlock('Second point', 'normal', {listItem: 'bullet', level: 1}),
]