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      Web technology reference for developers

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      Structure of content on the web

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      Code used to describe document style

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      General-purpose scripting language

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      Protocol for transmitting web resources

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  1. MDN Web Docs Glossary: Definitions of Web-related terms
  2. Nullish value
  1. MDN Web Docs Glossary
    1. Abstraction
    2. Accent
    3. Accessibility
    4. Accessibility tree
    5. Accessible name
    6. Adobe Flash
    7. Advance measure
    8. Ajax
    9. Algorithm
    10. Alignment container
    11. Alignment subject
    12. Alpha (alpha channel)
    13. ALPN
    14. API
    15. Apple Safari
    16. Application Context
    17. Argument
    18. ARIA
    19. ARPA
    20. ARPANET
    21. Array
    22. ASCII
    23. Asynchronous
    24. ATAG
    25. Attribute
    26. Bandwidth
    27. Base64
    28. Baseline
    29. Beacon
    30. Bézier curve
    31. BiDi
    32. BigInt
    33. Binding
    34. Bitwise flags
    35. Blink
    36. Block
    37. Block cipher mode of operation
    38. Block-level content
    39. Boolean
    40. Bounding Box
    41. Breadcrumb
    42. Brotli
    43. Browser
    44. Browsing context
    45. Buffer
    46. Cache
    47. Cacheable
    48. CalDAV
    49. Call stack
    50. Callback function
    51. Camel case
    52. Canonical order
    53. Canvas
    54. Card sorting
    55. CardDAV
    56. Caret
    57. CDN
    58. Certificate authority
    59. Certified
    60. Challenge-response authentication
    61. Character
    62. Character encoding
    63. Character set
    64. Chrome
    65. CIA
    66. Cipher
    67. Cipher suite
    68. Ciphertext
    69. Class
    70. Clickjacking
    71. Closure
    72. CMS
    73. Code point
    74. Code splitting
    75. Code unit
    76. Codec
    77. Color wheel
    78. Compile
    79. Compile time
    80. Composite operation
    81. Computer Programming
    82. Conditional
    83. Constant
    84. Constructor
    85. Continuous Media
    86. Control flow
    87. Cookie
    88. Copyleft
    89. CORS
    90. CORS-safelisted request header
    91. CORS-safelisted response header
    92. Crawler
    93. CRLF
    94. Cross Axis
    95. Cross-site scripting (XSS)
    96. CRUD
    97. Cryptanalysis
    98. Cryptographic hash function
    99. Cryptography
    100. CSP
    101. CSRF
    102. CSS
    103. CSS Object Model (CSSOM)
    104. CSS pixel
    105. CSS preprocessor
    106. Data structure
    107. Database
    108. Decryption
    109. Deep copy
    110. Delta
    111. Denial of Service
    112. Descriptor (CSS)
    113. Deserialization
    114. Developer Tools
    115. Digest
    116. Digital certificate
    117. Distributed Denial of Service
    118. DMZ
    119. DNS
    120. Doctype
    121. Document directive
    122. Document environment
    123. DOM (Document Object Model)
    124. Domain
    125. Domain name
    126. Domain sharding
    127. Dominator
    128. DoS attack
    129. DSL
    130. DTLS (Datagram Transport Layer Security)
    131. DTMF (Dual-Tone Multi-Frequency signaling)
    132. Dynamic typing
    133. ECMA
    134. ECMAScript
    135. Effective connection type
    136. Element
    137. Encapsulation
    138. Encryption
    139. Endianness
    140. Engine
    141. Entity
    142. Entity header
    143. Enumerated
    144. eTLD
    145. Event
    146. Exception
    147. EXIF
    148. Expando
    149. Fallback alignment
    150. Falsy
    151. Favicon
    152. Fetch directive
    153. Fetch metadata request header
    154. Fingerprinting
    155. Firefox OS
    156. Firewall
    157. First contentful paint
    158. First CPU idle
    159. First input delay
    160. First Meaningful Paint
    161. First paint
    162. First-class Function
    163. Flex
    164. Flex Container
    165. Flex Item
    166. Flexbox
    167. Forbidden header name
    168. Forbidden response header name
    169. Fork
    170. Fragmentainer
    171. Frame rate (FPS)
    172. FTP
    173. FTU
    174. Function
    175. Fuzz testing
    176. Gamut
    177. Garbage collection
    178. Gecko
    179. General header
    180. GIF
    181. Git
    182. Global object
    183. Global scope
    184. Global variable
    185. Glyph
    186. Google Chrome
    187. GPL
    188. GPU
    189. Graceful degradation
    190. Grid
    191. Grid Areas
    192. Grid Axis
    193. Grid Cell
    194. Grid Column
    195. Grid container
    196. Grid Lines
    197. Grid Row
    198. Grid Tracks
    199. Guard
    200. Gutters
    201. GZip compression
    202. Hash
    203. Head
    204. High-level programming language
    205. HMAC
    206. Hoisting
    207. Host
    208. Hotlink
    209. Houdini
    210. HPKP
    211. HSTS
    212. HTML
    213. HTML5
    214. HTTP
    215. HTTP header
    216. HTTP/2
    217. HTTP/3
    218. HTTPS
    219. HTTPS RR
    220. Hyperlink
    221. Hypertext
    222. IANA
    223. ICANN
    224. ICE
    225. IDE
    226. Idempotent
    227. Identifier
    228. IDL
    229. IETF
    230. IIFE
    231. IMAP
    232. Immutable
    233. IndexedDB
    234. Information architecture
    235. Inheritance
    236. Ink overflow
    237. Inline-level content
    238. Input method editor
    239. Instance
    240. Internationalization (i18n)
    241. Internet
    242. Interpolation
    243. Intrinsic size
    244. IP Address
    245. IPv4
    246. IPv6
    247. IRC
    248. ISO
    249. ISP
    250. ITU
    251. Jank
    252. Java
    253. JavaScript
    254. JPEG
    255. JSON
    256. JSON type representation
    257. Kebab case
    258. Key
    259. Keyword
    260. Largest contentful paint
    261. Latency
    262. Layout viewport
    263. Lazy load
    264. LGPL
    265. Ligature
    266. Literal
    267. Local scope
    268. Local variable
    269. Locale
    270. Localization
    271. Long task
    272. Loop
    273. Lossless compression
    274. Lossy compression
    275. LTR (Left To Right)
    276. Main Axis
    277. Main thread
    278. Markup
    279. MathML
    280. Media
    281. Metadata
    282. Method
    283. Microsoft Edge
    284. Microsoft Internet Explorer
    285. Middleware
    286. MIME
    287. MIME type
    288. Minification
    289. MitM
    290. Mixin
    291. Mobile First
    292. Modem
    293. Modularity
    294. Mozilla Firefox
    295. Mutable
    296. MVC
    297. Namespace
    298. NaN
    299. NAT
    300. Native
    301. Navigation directive
    302. Netscape Navigator
    303. Network throttling
    304. NNTP
    305. Node
    306. Node.js
    307. Non-normative
    308. Normative
    309. Null
    310. Nullish value
    311. Number
    312. Object
    313. Object reference
    314. OOP
    315. OpenGL
    316. OpenSSL
    317. Opera Browser
    318. Operand
    319. Operator
    320. Origin
    321. OTA
    322. OWASP
    323. P2P
    324. PAC
    325. Packet
    326. Page load time
    327. Page prediction
    328. Parameter
    329. Parent object
    330. Parse
    331. Parser
    332. Payload body
    333. Payload header
    334. PDF
    335. Perceived performance
    336. Percent-encoding
    337. PHP
    338. Pixel
    339. Placeholder names
    340. Plaintext
    341. Plugin
    342. PNG
    343. Polyfill
    344. Polymorphism
    345. POP3
    346. Port
    347. Prefetch
    348. Preflight request
    349. Prerender
    350. Presto
    351. Primitive
    352. Privileged
    353. Privileged code
    354. Progressive Enhancement
    355. Progressive web apps
    356. Promise
    357. Property
    358. Protocol
    359. Prototype
    360. Prototype-based programming
    361. Proxy server
    362. Pseudo-class
    363. Pseudo-element
    364. Pseudocode
    365. Public-key cryptography
    366. Python
    367. Quality values
    368. Quaternion
    369. QUIC
    370. RAIL
    371. Random Number Generator
    372. Raster image
    373. RDF
    374. Real User Monitoring (RUM)
    375. Recursion
    376. Reflow
    377. Regular expression
    378. Render-blocking
    379. Rendering engine
    380. Repaint
    381. Replay attack
    382. Repo
    383. Reporting directive
    384. Representation header
    385. Request header
    386. Resource Timing
    387. Response header
    388. Responsive web design
    389. REST
    390. RGB
    391. RIL
    392. Robots.txt
    393. Round Trip Time (RTT)
    394. Routers
    395. RSS
    396. Rsync
    397. RTCP (RTP Control Protocol)
    398. RTF
    399. RTL (Right to Left)
    400. RTP (Real-time Transport Protocol) and SRTP (Secure RTP)
    401. RTSP: Real-time streaming protocol
    402. Ruby
    403. Safe
    404. Same-origin policy
    405. SCM
    406. Scope
    407. Screen reader
    408. Script-supporting element
    409. Scroll container
    410. SCTP
    411. SDK (Software Development Kit)
    412. SDP
    413. Search engine
    414. Second-level Domain
    415. Secure Context
    416. Secure Sockets Layer (SSL)
    417. Selector (CSS)
    418. Self-Executing Anonymous Function
    419. Semantics
    420. SEO
    421. Serializable object
    422. Serialization
    423. Server
    424. Server Timing
    425. Session Hijacking
    426. SGML
    427. Shadow tree
    428. Shallow copy
    429. Shim
    430. Signature
    431. SIMD
    432. Simple header
    433. Simple response header
    434. SISD
    435. Site
    436. Site map
    437. SLD
    438. Sloppy mode
    439. Slug
    440. Smoke Test
    441. SMPTE (Society of Motion Picture and Television Engineers)
    442. SMTP
    443. Snake case
    444. Snap positions
    445. SOAP
    446. SPA (Single-page application)
    447. Specification
    448. Speculative parsing
    449. Speed index
    450. SQL
    451. SQL Injection
    452. SRI
    453. Stacking context
    454. State machine
    455. Statement
    456. Static method
    457. Static typing
    458. Sticky activation
    459. Strict mode
    460. String
    461. Stringifier
    462. STUN
    463. Style origin
    464. Stylesheet
    465. Submit button
    466. SVG
    467. SVN
    468. Symmetric-key cryptography
    469. Synchronous
    470. Syntax
    471. Syntax error
    472. Synthetic monitoring
    473. Table Grid Box
    474. Table Wrapper Box
    475. Tag
    476. TCP
    477. TCP handshake
    478. TCP slow start
    479. Telnet
    480. Texel
    481. The Khronos Group
    482. Thread
    483. Three js
    484. Time to first byte
    485. Time to interactive
    486. TLD
    487. TOFU
    488. Top layer
    489. Transient activation
    490. Transport Layer Security (TLS)
    491. Tree shaking
    492. Trident
    493. Truthy
    494. TTL
    495. TURN
    496. Type
    497. Type coercion
    498. Type conversion
    499. UDP (User Datagram Protocol)
    500. UI
    501. Undefined
    502. Unicode
    503. Unix time
    504. URI
    505. URL
    506. URN
    507. Usenet
    508. User agent
    509. UTF-8
    510. UUID
    511. UX
    512. Validator
    513. Value
    514. Variable
    515. Vendor Prefix
    516. Viewport
    517. Visual Viewport
    518. Void element
    519. VoIP
    520. W3C
    521. WAI
    522. WCAG
    523. Web performance
    524. Web server
    525. Web standards
    526. WebAssembly
    527. WebDAV
    528. WebExtensions
    529. WebGL
    530. WebIDL
    531. WebKit
    532. WebM
    533. WebP
    534. WebRTC
    535. WebSockets
    536. WebVTT
    537. WHATWG
    538. Whitespace
    539. WindowProxy
    540. World Wide Web
    541. Wrapper
    542. XForms Deprecated
    543. XHTML
    544. XInclude
    545. XLink
    546. XML
    547. XMLHttpRequest (XHR)
    548. XPath
    549. XQuery
    550. XSLT

Nullish value

In JavaScript, a nullish value is the value which is either null or undefined. Nullish values are always falsy.

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This page was last modified on Jun 8, 2023 by MDN contributors.

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