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    • Overview / Web Technology

      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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      Interfaces for building web applications

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      Developing extensions for web browsers

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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. Adobe Flash
    6. Advance measure
    7. Ajax
    8. Algorithm
    9. Alignment container
    10. Alignment subject
    11. Alpha (alpha channel)
    12. ALPN
    13. API
    14. Apple Safari
    15. Application Context
    16. Argument
    17. ARIA
    18. ARPA
    19. ARPANET
    20. Array
    21. ASCII
    22. Asynchronous
    23. ATAG
    24. Attribute
    25. Bandwidth
    26. Base64
    27. Baseline
    28. Beacon
    29. Bézier curve
    30. BiDi
    31. BigInt
    32. Bitwise flags
    33. Blink
    34. Block
    35. Block cipher mode of operation
    36. Block-level content
    37. Boolean
    38. Bounding Box
    39. Breadcrumb
    40. Brotli
    41. Browser
    42. Browsing context
    43. Buffer
    44. Cache
    45. Cacheable
    46. CalDAV
    47. Call stack
    48. Callback function
    49. Canonical order
    50. Canvas
    51. Card sorting
    52. CardDAV
    53. Caret
    54. CDN
    55. Certificate authority
    56. Certified
    57. Challenge-response authentication
    58. Character
    59. Character encoding
    60. Character set
    61. Chrome
    62. CIA
    63. Cipher
    64. Cipher suite
    65. Ciphertext
    66. Class
    67. Clickjacking
    68. Closure
    69. CMS
    70. Code point
    71. Code splitting
    72. Code unit
    73. Codec
    74. Color wheel
    75. Compile
    76. Compile time
    77. Composite operation
    78. Computer Programming
    79. Conditional
    80. Constant
    81. Constructor
    82. Continuous Media
    83. Control flow
    84. Cookie
    85. Copyleft
    86. CORS
    87. CORS-safelisted request header
    88. CORS-safelisted response header
    89. Crawler
    90. CRLF
    91. Cross Axis
    92. Cross-site scripting (XSS)
    93. CRUD
    94. Cryptanalysis
    95. Cryptographic hash function
    96. Cryptography
    97. CSP
    98. CSRF
    99. CSS
    100. CSS Object Model (CSSOM)
    101. CSS pixel
    102. CSS preprocessor
    103. Data structure
    104. Database
    105. Decryption
    106. Deep copy
    107. Delta
    108. Denial of Service
    109. Descriptor (CSS)
    110. Deserialization
    111. Developer Tools
    112. Digest
    113. Digital certificate
    114. Distributed Denial of Service
    115. DMZ
    116. DNS
    117. Doctype
    118. Document directive
    119. Document environment
    120. DOM (Document Object Model)
    121. Domain
    122. Domain name
    123. Domain sharding
    124. Dominator
    125. DoS attack
    126. DSL
    127. DTLS (Datagram Transport Layer Security)
    128. DTMF (Dual-Tone Multi-Frequency signaling)
    129. Dynamic typing
    130. ECMA
    131. ECMAScript
    132. Effective connection type
    133. Element
    134. Encapsulation
    135. Encryption
    136. Endianness
    137. Engine
    138. Entity
    139. Entity header
    140. Enumerated
    141. eTLD
    142. Event
    143. Exception
    144. EXIF
    145. Expando
    146. Fallback alignment
    147. Falsy
    148. Favicon
    149. Fetch directive
    150. Fetch metadata request header
    151. Fingerprinting
    152. Firefox OS
    153. Firewall
    154. First contentful paint
    155. First CPU idle
    156. First input delay
    157. First interactive
    158. First Meaningful Paint
    159. First paint
    160. First-class Function
    161. Flex
    162. Flex Container
    163. Flex Item
    164. Flexbox
    165. Forbidden header name
    166. Forbidden response header name
    167. Fork
    168. Fragmentainer
    169. Frame rate (FPS)
    170. FTP
    171. FTU
    172. Function
    173. Fuzz testing
    174. Gamut
    175. Garbage collection
    176. Gecko
    177. General header
    178. GIF
    179. Git
    180. Global object
    181. Global scope
    182. Global variable
    183. Glyph
    184. Google Chrome
    185. GPL
    186. GPU
    187. Graceful degradation
    188. Grid
    189. Grid Areas
    190. Grid Axis
    191. Grid Cell
    192. Grid Column
    193. Grid container
    194. Grid Lines
    195. Grid Row
    196. Grid Tracks
    197. Guard
    198. Gutters
    199. GZip compression
    200. Hash
    201. Head
    202. High-level programming language
    203. HMAC
    204. Hoisting
    205. Host
    206. Hotlink
    207. Houdini
    208. HPKP
    209. HSTS
    210. HTML
    211. HTML5
    212. HTTP
    213. HTTP header
    214. HTTP/2
    215. HTTP/3
    216. HTTPS
    217. HTTPS RR
    218. Hyperlink
    219. Hypertext
    220. I18N
    221. IANA
    222. ICANN
    223. ICE
    224. IDE
    225. Idempotent
    226. Identifier
    227. IDL
    228. IETF
    229. IIFE
    230. IMAP
    231. Immutable
    232. IndexedDB
    233. Information architecture
    234. Inheritance
    235. Ink overflow
    236. Inline-level content
    237. Input method editor
    238. Instance
    239. Internationalization
    240. Internet
    241. Interpolation
    242. Intrinsic size
    243. IP Address
    244. IPv4
    245. IPv6
    246. IRC
    247. ISO
    248. ISP
    249. ITU
    250. Jank
    251. Java
    252. JavaScript
    253. JPEG
    254. JSON
    255. Key
    256. Keyword
    257. Largest contentful paint
    258. Latency
    259. Layout viewport
    260. Lazy load
    261. LGPL
    262. Ligature
    263. Literal
    264. Local scope
    265. Local variable
    266. Locale
    267. Localization
    268. Long task
    269. Loop
    270. Lossless compression
    271. Lossy compression
    272. LTR (Left To Right)
    273. Main Axis
    274. Main thread
    275. Markup
    276. MathML
    277. Media
    278. Metadata
    279. Method
    280. Microsoft Edge
    281. Microsoft Internet Explorer
    282. Middleware
    283. MIME
    284. MIME type
    285. Minification
    286. MitM
    287. Mixin
    288. Mobile First
    289. Modem
    290. Modularity
    291. Mozilla Firefox
    292. Mutable
    293. MVC
    294. Namespace
    295. NaN
    296. NAT
    297. Native
    298. Navigation directive
    299. Netscape Navigator
    300. Network throttling
    301. NNTP
    302. Node
    303. Node.js
    304. Non-normative
    305. Normative
    306. Null
    307. Nullish value
    308. Number
    309. Object
    310. Object reference
    311. OOP
    312. OpenGL
    313. OpenSSL
    314. Opera Browser
    315. Operand
    316. Operator
    317. Origin
    318. OTA
    319. OWASP
    320. P2P
    321. PAC
    322. Packet
    323. Page load time
    324. Page prediction
    325. Parameter
    326. Parent object
    327. Parse
    328. Parser
    329. Payload body
    330. Payload header
    331. PDF
    332. Perceived performance
    333. Percent-encoding
    334. PHP
    335. Pixel
    336. Placeholder names
    337. Plaintext
    338. Plugin
    339. PNG
    340. Polyfill
    341. Polymorphism
    342. POP3
    343. Port
    344. Prefetch
    345. Preflight request
    346. Prerender
    347. Presto
    348. Primitive
    349. Privileged
    350. Privileged code
    351. Progressive Enhancement
    352. Progressive web apps
    353. Promise
    354. Property
    355. Protocol
    356. Prototype
    357. Prototype-based programming
    358. Proxy server
    359. Pseudo-class
    360. Pseudo-element
    361. Pseudocode
    362. Public-key cryptography
    363. Python
    364. Quality values
    365. Quaternion
    366. QUIC
    367. RAIL
    368. Random Number Generator
    369. Raster image
    370. RDF
    371. Real User Monitoring (RUM)
    372. Recursion
    373. Reflow
    374. Regular expression
    375. Rendering engine
    376. Replay attack
    377. Repo
    378. Reporting directive
    379. Representation header
    380. Request header
    381. Resource Timing
    382. Response header
    383. Responsive web design
    384. REST
    385. RGB
    386. RIL
    387. Robots.txt
    388. Round Trip Time (RTT)
    389. Routers
    390. RSS
    391. Rsync
    392. RTCP (RTP Control Protocol)
    393. RTF
    394. RTL (Right to Left)
    395. RTP (Real-time Transport Protocol) and SRTP (Secure RTP)
    396. RTSP: Real-time streaming protocol
    397. Ruby
    398. Safe
    399. Same-origin policy
    400. SCM
    401. Scope
    402. Screen reader
    403. Script-supporting element
    404. Scroll container
    405. SCTP
    406. SDP
    407. Search engine
    408. Second-level Domain
    409. Secure Context
    410. Secure Sockets Layer (SSL)
    411. Selector (CSS)
    412. Self-Executing Anonymous Function
    413. Semantics
    414. SEO
    415. Serializable object
    416. Serialization
    417. Server
    418. Server Timing
    419. Session Hijacking
    420. SGML
    421. Shadow tree
    422. Shallow copy
    423. Shim
    424. Signature
    425. SIMD
    426. Simple header
    427. Simple response header
    428. SISD
    429. Site
    430. Site map
    431. SLD
    432. Sloppy mode
    433. Slug
    434. Smoke Test
    435. SMPTE (Society of Motion Picture and Television Engineers)
    436. SMTP
    437. Snap positions
    438. SOAP
    439. SPA (Single-page application)
    440. Specification
    441. Speculative parsing
    442. Speed index
    443. SQL
    444. SQL Injection
    445. SRI
    446. Stacking context
    447. State machine
    448. Statement
    449. Static method
    450. Static typing
    451. Sticky activation
    452. Strict mode
    453. String
    454. Stringifier
    455. STUN
    456. Style origin
    457. Stylesheet
    458. Submit button
    459. SVG
    460. SVN
    461. Symmetric-key cryptography
    462. Synchronous
    463. Syntax
    464. Syntax error
    465. Synthetic monitoring
    466. Table Grid Box
    467. Table Wrapper Box
    468. Tag
    469. TCP
    470. TCP handshake
    471. TCP slow start
    472. Telnet
    473. Texel
    474. The Khronos Group
    475. Thread
    476. Three js
    477. Time to first byte
    478. Time to interactive
    479. TLD
    480. TOFU
    481. Top layer
    482. Transient activation
    483. Transport Layer Security (TLS)
    484. Tree shaking
    485. Trident
    486. Truthy
    487. TTL
    488. TURN
    489. Type
    490. Type coercion
    491. Type conversion
    492. UDP (User Datagram Protocol)
    493. UI
    494. Undefined
    495. Unicode
    496. Unix time
    497. URI
    498. URL
    499. URN
    500. Usenet
    501. User agent
    502. UTF-8
    503. UUID
    504. UX
    505. Validator
    506. Value
    507. Variable
    508. Vendor Prefix
    509. Viewport
    510. Visual Viewport
    511. Void element
    512. VoIP
    513. W3C
    514. WAI
    515. WCAG
    516. Web performance
    517. Web server
    518. Web standards
    519. WebAssembly
    520. WebDAV
    521. WebExtensions
    522. WebGL
    523. WebIDL
    524. WebKit
    525. WebM
    526. WebP
    527. WebRTC
    528. WebSockets
    529. WebVTT
    530. WHATWG
    531. Whitespace
    532. WindowProxy
    533. World Wide Web
    534. Wrapper
    535. XForms Deprecated
    536. XHTML
    537. XInclude
    538. XLink
    539. XML
    540. XMLHttpRequest (XHR)
    541. XPath
    542. XQuery
    543. 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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