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  • 🔰HTML
    • 🔰element
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        • type="range"
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  • 🔰CSS
    • ⭐box model
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        • center with "transform"
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    • 🔰values
      • 🔰units
        • 🔰相對單位
          • 🔸percentage (％)
          • 🔸rem
          • 🔸em
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        • radial-gradient()
    • 🔰variables
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      • 🔹calc()
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      • select direct children by JS
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  • 🍒JS
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    • 🧚technique
    • ⭐feature
      • ⭐️ ES5
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    • 🔰concept
      • 🔰execution context
        • 📘this
          • ❗"this" determined on call site❗️
        • 🔰running execution context
        • 🔰lexical environment
          • 🔰environment record
          • 🔸[[Environment]]
          • ❗lexical environment optimizaton
        • 🔰call stack
      • 🔰context
        • 🔰global context
        • 🔰class body context
      • 🔰environment
        • ⚙️JS engine
          • 🔰mode
            • 🛡️strict mode
              • 🛡️eval has its own scope in strict mode
              • ✅always use strict mode
            • ❗sloppy mode
              • ❗accidental global variable in sloppy mode❗️
              • ❗modify current scope at runtime❗️
            • ⚖️strict vs. sloppy
          • 🔰v8
        • 🔰Node.js
      • 🔰associated array
      • 🔰parser
    • 🔰compilation
      • 🔰lexing
      • 🔰parsing
      • 🔰code generation
      • 🔰JS is compiled
      • 🔰compile-time
      • 🔰runtime
    • 🔰grammar
      • 🚩directive
        • 📜'use strict'
      • *️token
        • 🗝️keyword
          • 🔐reserved word
            • ❗reserved word as property name is allowed❗️
          • 🔑contextual keyword
        • 🔣punctuator
          • *️hash (#)
          • 🔣semicolon (;)
            • 🔰automatic semicolon insertion
            • 🔰statements that must end with ";"
          • 🔣dot (.)
          • 🔣comma (,)
          • 🔣colon (:)
          • 🔣plus (+)
          • 🔣equal (=)
          • 🔣3 dots (...)
          • 🔣brackets []
          • 🔣braces {}
          • 🔣parentheses ()
          • 🔣question dot (?.)
          • 🔣question (?)
        • 🆔identifier
          • 🆔special identifier
          • ⚖️identifier vs. string
          • ⚖️identifier vs. keyword
          • ❗undeclared identifier
          • 🔰static binding
          • 🔰dynamic binding
        • ✡️literal
          • 🔰trailing comma
      • 🔰comment
      • 🈯declaration
        • ❗redeclaration
        • 🈯lexical declaration
      • 📜statement
        • 📦expression
          • 📦primary expression
          • 📦invocation expression：f()
            • 📦method invocation
          • 📦left-hand side expression
        • 🔁loop
          • 🔁while
          • 🔁do...while
          • 🔁for loops
            • 🔁for
              • 🔰"for-init" scope
              • 🔰"for-init" block
            • 🔁for-in
            • 🔁for-of
              • ❗arrays are iterated "live"❗️
              • 🔰for-of with objects
              • 🔰for-of with strings
              • ⚖️for-of vs. forEach
            • 🔁for-await
            • ⚖️for-in vs. for-of vs. in
            • 📘arr.forEach()
        • 🔀control flow
          • ⤴️jump
            • ⤴️return
            • ⤴️continue
            • ⤴️break
            • ⚖️continue vs. break
          • 🔀branch
            • 🔀if
            • 🔀switch
              • 🔀case clause
        • 📜other statement
          • 📜block
            • 🔰block scope
          • 📜label
          • 🛑with
          • 📜empty statement
        • ⛔statement expected❗️
      • 🚧operator
        • 🚧table of operators
        • 📖terms for operator
          • 🔰arity
          • 🔰associativity
          • 🔰lvalue
          • 🔰dividend
          • 🔰divisor
          • 🔰non-numeric operand
          • 🔰short-circuiting
        • 🔴assignment
          • ⭐destructuring assignment
            • 🔰destructuring array
            • 🔰destructuring iterable
            • 🔰object destructuring
            • 🔰nested destructuring
              • 🌟nested default values
            • 🔰destructuring arguments
          • ➕assignment (=)
        • 🔴arithmetic operator
          • ⭕unary arithmetic operator
            • 🚧unary plus (+)
          • ⭕binary arithmetic operator
            • ➕add/concate (+)
            • ➕division (/)
            • ➕remainder (%)
              • 💾modulo(a, b)
              • 💾mod(a, b)
              • ⚖️remainder vs. modulo vs. mod
            • ➕exponentiation (**)
          • ⭕bitwise operator
            • 🌟2's complement
            • 💾bitview()
            • ➕bitwise not (~)
              • 🔰double tilde (~~)
              • ⚖️(~~) vs. Math.trunc()
        • 🔴relational operator
          • 🔰equality & inequality operator
            • ➕sloppy equality (==)
            • ➕strict equality (===)
          • 🔰comparison operator
          • ➕in
          • ➕instanceof
        • 🚧logical operator
          • ➕nullish coalescing (??)
          • ⭕unary logical operator
          • ⭕binary logical operator
        • ⭕unary operator
          • ➕void
        • ⭕binary operator
          • ➕comma operator (,)
        • ⭕ternary operator
          • ➕conditional operator (?:)
        • 🔰spread operator (...)
        • ➕rest operator (...)
      • 🔰statement vs. declaration
    • 🔰scope
      • 🔰lexical scope
      • 🔰static scope
      • 🔰dynamic scope
      • ⚖️static vs. dynamic scoping
      • 🔰scope chain
      • ⚖️lexical environment vs. scope
      • 🔰top-level scope
      • 🌟global scope
        • 📘global object
          • 🏷️globalThis
          • 🍄window
            • 🔸window.name
            • 🔸window.navigator
          • 🍄global
          • 🔸eval
          • 🔸global object property
        • 💾isCurrentlyInGlobalScope()
        • 🔰global variable
          • 🔰global let
            • ❗global let shadows global object property
          • 🔰global var
        • 🔸DOM element with "id"
      • 🔰module scope
      • 🔰function scope
      • 🔰function name scope
      • 🔰parameter scope
      • 🔰hoisting
        • 🔰function hoisting
          • ❗function hoisting first❗️
        • 🔰variable hoisting
          • 🔰var hoisting
            • ❗function expression not hoisted
          • 🔰let/const/class hoisting
      • ⚖️context vs. scope
    • 💍value
      • 🔰type
        • 💍falsy
        • 📖terms for type
          • 🔰statically typed
          • 🔰dynamically typed
        • 🔰type conversion
        • 🔰type name
          • ➕typeof
          • 💾baseTypeName()
          • 💾typeName()
          • 💾custom type name
        • 🔰nested type
        • 👔type functions
          • 💾functionDefinition()
          • 💾isObject()
          • 💾isPrimitive()
          • 💾isFunction()
          • 💾isClass()
      • 🦠primitive
        • 💍nullish
        • 🔸undefined
        • 🔐null
        • 🔢Number
          • 🔰floating-point
          • 🔰integers
          • 🔴special number
            • 🔢Infinity
            • 🔢NaN
            • 🔢Number.EPSILON
            • 🔢Number.MAX_VALUE
            • 🔢Number.MAX_SAFE_INTEGER
          • 👔Number+ext
            • 💾n.toHex()
            • 💾n.isEqual()
            • 💾isNumber()
          • 👔Random
            • randomInt()
            • randomFloat()
          • 👔Quaternion
        • 🔢BigInt
        • 🦠Boolean
        • 🔤String
          • 🔰raw string
          • 🔰escape sequence
          • template literal
            • tagged template
            • tag function
          • 🔰String methods
            • str.map()
            • 💾str.capitalize()
            • str.countLetters()
            • 💾str.htmlToElement()
            • str.kebabToCamelCase()
            • 📘str.match()
              • ✨split into course names
            • 📘str.matchAll()
              • ✨matchAll() using named groups
              • ✨words in sentence
            • str.random()
            • str.removeWhitespaces()
            • 📘str.replace()
              • 🔰replacer function
              • ✨minus one
            • 💾str.replaceWhitespaces()
            • str.reverse()
            • str.shuffle()
            • 💾str.slice2()
            • 💾str.splice()
            • str.escapeHTML()
            • str.toDatasetPropName()
            • 💾str.words()
            • str.wordCounts()
            • String.fromCharCodes()
          • 👔String extension
            • 💾str.isKeyword
            • 💾str.isReservedWord
            • 💾str.isPunctuator
          • 🔰Unicode
            • 🔰code point
            • 🔰grapheme cluster
            • 🔰code plane
            • 🔰character encoding
              • 🔰UTF-16
                • 🔰code unit
                • 🔰surrogate pair
                • 🔰scalar
        • 🔺Symbol
          • 🔺Symbol.iterator
        • 🔰primitive wrapper
      • 🍄object
        • 🔸property
          • 🚩attribute
            • 🍄property descriptor
            • 💾ownPropertyFlags(obj)
          • 🔸getter/setter
          • 🔸property name
            • ⭐computed property name
          • ⭐shorthand property
          • 🔸internal property
          • 🔰property access
            • 📦property access expression
            • ➕dot notation (.)
              • ⚖️dot notation vs. decimal point
            • ➕bracket notation []
            • ➕optional chaining (?., ?.[])
              • ❓is Optional Chaining Supported?
              • 💾obj.prop(path)
            • ➕optional invocation ?.()
              • is Optional Chaining Supported?
              • obj.prop(path)
            • 🌟chaining rules
              • ✨jane.boyfriend?.money.more
          • 🔰property creation/deletion
            • 📘Object.defineProperty()
            • ➕delete
              • ❗unqualified identifier for delete
          • 🔰property testing
          • 🔰property enumeration
        • 🔰creating objects
          • ✡️object literal
            • ❗object literal is not a block❗️
          • ➕new
            • 🔸new.target
            • ⚖️callable vs. constructable
          • Ⓜ️Object.create()
            • ✨"pure" object
            • ✨guard against accidental modifications
        • 🔰extending objects
          • 📘Object.assign()
            • ⛔️ Object.assign causing TypeError
            • ❗️Object.assign copies with getter/setter
          • inheritance
            • 🔸__proto__
            • class A vs. class A extends Object
          • mixin
            • 💾mergeWithoutOverride()
            • 💾.assignDescriptors()
            • mixin inheritance
            • ✨mixin: HandleEvents
          • 💾 add default properties
          • extending built-in classes
        • 🔰converting objects
          • 🔰serializing objects
          • 🌟object -> primitive conversion
            • 🔘prefer-string
            • 🔘prefer-number
            • 🔘no preference
          • Ⓜ️.toString()
          • Ⓜ️.valueOf()
          • 💾valueToString()
        • Ⓜ️method
          • 💠[[HomeObject]]
          • 🔰polymorphism
        • 🔸prototype
          • 🔰prototype chain
            • 🚧printPrototypeChain()
      • 🍄function
        • 🔰creating functions
        • 🈯function declaration
          • ❗function in block (FiB)
          • ❗function redeclaration
          • 📘function declaration instantiation
        • 🔰function expression
          • ❗read-only identifier
          • 🔰named function expression
          • 🔰as variable
          • 🔰IIFE
        • 🔰arrow function
          • ❗arrow function as method❗️
          • 🔰arrow function as argument
          • ❗arrow function as class field❗️
          • ❗arrow function returning object literal needs (...)❗️
          • 🔰arrow function expression
        • 🔰closure
          • ❗scopes matter with closures
          • 💡private property by closure
          • ✨closure examples
            • ✨closure: to close over or not
            • ✨closure: manage grades
            • ✨closure as object
        • 🌿sorts of function
          • 🔰callback
          • 🔰higher-order function
            • 💾not(f)
            • 💾pipe(f, g, ...)
            • 💾partial(f, a, b ...)
            • 🧚decorator
              • 💾saveCallHistory(f)
              • 💾delay(f, s)
              • 💾benchmark(f)
              • 💾debounce(f, s)
              • 💾throttle(f, s)
              • ⚖️debounce vs. throttle
              • 🧚memoization
                • 🧚memoize by decorator
                  • 💾memoize(f)
                • 🧚memoize by closure
          • 🔰predicate
          • 🔰nested function
          • 🔰recursive function
            • ✨findSolution()
            • 💾deepEqual()
        • 🔰function boundary
        • 🔰function overloading
        • 🔰function forwarding
        • 🔸return value
        • 🔸function name
          • 🔰anonymous function
        • ⚖️parameter vs. argument
        • 🔸argument
          • ❗arguments are passed by value❗️
        • 🔸parameter
          • 🔸default parameter
          • 🔸rest parameters
            • 🔸rest parameters as object
          • 🔸destructured parameter
          • ⚖️simple vs. non-simple parameter
          • ❗deplicate parameters
        • 🔸prototype
        • 🔹constructor
      • 🍄class
        • 🔰initialization
          • 🔹static block
        • 🔸member
          • 🔸class field
          • 🔸private member
          • 🔸static member
            • 🔰init static property
          • private/protected members
          • 🔸getter/setter
          • 🔸method
          • 🔸bound method
        • 🔰creating class
          • 🔰delegation
        • 🔰inheritance
          • 📘super
          • derived class init
          • override constructor
          • override methods
          • override class fields
        • ✨examples
          • ✨Complex
          • ✨TypedMap
          • ✨Bag
      • 📘built-in objects
        • 📘Object
          • 🔸Object.prototype
          • 💾Object extension
            • 💾obj.mergeWith()
        • 📘Function
        • 📘Array
          • 🔸element
            • 🔸element index
              • ❗element index is string❗️
          • 🔸array length
          • 🔸array object property
          • 🍄sparse array
            • 🔰how array methods deal with "holes" ?
          • 🔰array-like
            • 👔Object+arrayLike
          • 🔰creating arrays
            • ✡️array literal
              • 🔰undefined element
          • 🔰accessing elements
          • 🔰iterating elements
          • 📘array methods
            • 📘arr.entries
            • 📘arr.filter()
            • 📘arr.flatMap()
            • 📘arr.reduce()
              • early break reduce()
            • 📘arr.slice()
            • 📘arr.splice()
          • 👔Array extension
            • 💾arr.containsSubarray()
            • 💾arr.copy()
            • 💾arr.isEmpty
            • 💾arr.last
            • 💾arr.none()
            • 💾arr.reversed()
            • 💾arr.shuffle()
            • 💾arr.randomElement
            • 💾arr.removeDuplicates()
            • 💾arr.uniqueMerge()
            • 💾arr.removeUndefined()
            • 💾arr.removeValue()
            • 💾arr.sum(), .average()
            • 💾arr.topN()
            • 💾arr.rank()
            • 💾arr.padEndSpaces()
            • 💾arr.max()
            • 💾arr.isMatrix
            • 💾arr.indexDictionaryForValues()
            • Array extensions (archive)
          • ✨array examples
            • 💾objects to HTML table
          • array & matrix methods
          • static methods
            • 📍Array(n) vs. Array(n).fill()
            • Array.from()
            • 🚧Array.list(n)
          • matrix methods
            • mat.matrixMap()
            • mat.transpose()
            • mat.maxElementInEachColumn()
          • Matrix methods
            • 🌀 Array.matrixFill()
        • 📘Set
          • 👔Set extension
            • 💾set.isEqualTo()
        • 📘Map
          • Don't use object properties
        • 📘RegExp
          • 🚩regex flag
            • 🚩flag /u
            • 🚩flag /d
          • 🔸lastIndex
          • 🔰creating regex
          • 🔰using regex
          • 🔰pattern
            • 🔰special characters
            • 🔰anchor
            • 🔰repeat
              • 🔰greedy vs. lazy
            • 🔰character set
            • 🔰alternation (or)
            • 🔰group
              • 🔰capturing group
                • 🔰named group
            • 🔰lookaround
            • 🔰inline modifier
            • 🔰replace pattern
            • 😀emoji
          • ✨regex examples
            • ✨parse .ini file
            • ✨quick brown fox
            • ✨JS-style numbers
        • 📘Proxy
          • 🔹handler method
          • 📘Reflect
          • ✨Proxy examples
            • ✨identity (proxy)
            • ✨read-only proxy
            • ✨observe()
        • 📘Date
    • 🔰variable
      • 🈯variable declaration
        • 📜var
          • ✅stop using var❗️
          • ❗var is a statement❗️
          • ❗var has no block scope❗️
          • ❗accessing var before declaration gets undefined❗️
          • ❗global var / function is global object property❗️
          • ❗var redeclaration applied even in strict mode❗️
          • ❗var in block can't shadow outer let❗️
          • ❗var can shadow parameter even in strict mode❗️
        • 🈯const
          • ⛔const requires initialization
          • ⛔const can't reassign
          • ⛔no const for classic "for"
        • 🈯let
          • ✅let redeclaration not allowed even in sloppy mode❗️
          • ✅let can't shadow parameter even in sloppy mode❗️
        • 🔰initial value
      • ❗variable redeclaration
        • ❗var redeclaration
        • ❗let redeclaration
      • 🔴accessing variables
        • ⛔temporal dead zone
          • ⛔TDZ
          • ⛔typeof let/const/class in TDZ gets an error❗️
        • ⛔uninitialized variable
        • 🔰LHS reference
        • 🔰RHS reference
        • ⚖️LHS vs. RHS reference
      • ❗variable shadowing
      • ❗can't delete variable/function❗️
    • 🔰module
      • ⭐ES module
        • 🈯import
          • ✨import ES module
          • dynamic import()
        • 🈯export
          • 📘default export
          • 📘named export
      • 🔰CommonJS
        • 📜package.json
      • ⚖️ES vs CommonJS
      • 🔰import.meta
      • 🔰package
      • 🔰bundler
      • 🔰minifier
      • 🔰browser-specific features
      • 🔰module pattern
    • 🔰iteration
      • 👔Iterable+ext
        • 💾range()
        • 💾obj.isIterable
      • 🔰iterable
        • 🔸make-iterator method
        • ❗only-iterate-once iterable
        • 🔰make iterables
        • ✨iterable examples
          • ✨ClosedRange
          • ✨Sequence
      • 🔰iterator
        • 🔸next()
        • 🔸return()
        • ❗iterators only iterate once❗️
        • 🔰iterable iterator
        • 🔰infinite iterator
        • 🔰make iterator iterable
        • 💾IteratorPrototype
        • 💾Iterator
      • 🔰iteration result
      • 📘Generator
        • 🔰generator function
          • 📘function*
          • 📦yield
          • 📘yield*
          • 🔰composition
          • 🔰generator function as ...
        • ✨generator examples
          • 💾*list()
          • 💾*integers()
          • 💾*closedRange()
          • 💾*fibonacci()
          • 💾*zip()
          • 💾*sequence()
          • 💾*interleave()
    • 🔰async code
      • 🔰thread
      • 🔰event loop
      • 🍄Promise
        • 👔custom promises
          • 💾getJSON()
          • 💾Promise.wait()
          • 💾futureValue()
        • 🔰using Promises
        • 🔰error handling
          • ⭐finally()
          • 🔰recoverable errors
        • 🔰chaining Promises
          • 💾Promise.inSeries()
        • 🔰Promises in parallel
      • 🆔async
        • 🔰async function
        • 🔰async method
        • 🔰async arrow function
        • 🔰async IIFE
        • 💾measureTime()
      • ➕await
        • 🔰await promises
          • 🔰await sequentially
          • 🔰await in parallel
        • 🔰await "thenable"
        • 🔰error handling
    • 🔰debugging
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  1. JS
  2. variable

variable redeclaration

🚧 under construction

Previousinitial valueNextvar redeclaration

Last updated 2 years ago

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⟩ ⟩ redeclaration

🚧 (redeclaration)

(redeclaration ) JS is lenient with var, strict with let.

  • var redeclaration applied even in strict mode❗️, even in strict mode

  • let redeclaration not allowed even in sloppy mode❗️, even in sloppy mode

  • var redeclaration

    • var redeclaration applied even in strict mode❗️ (even in strict mode)

    • var can shadow parameter even in strict mode❗️ - parameters can be shadowed by "var"

  • let redeclaration

    • let redeclaration not allowed even in sloppy mode❗️ (even in sloppy mode)

    • var can shadow parameter even in strict mode❗️ - parameters can't be shadowed by "let"

  • variable declaration

  • function redeclaration

  • variable shadowing

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