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🔰variable

JS ⟩ variable

a variable has a valueand an identifier as its name.

🈯 synonyms： "binding"

assignment to an undeclared identifier will result in an accidental global variable in sloppy mode❗️ in sloppy mode.

A "variable" is just a property of the environment record (associated with the currently executing block /function /module /script).

variables just been declared will be assigned undefined automatically (with some nuances).

var hoisting - declaration hoisted, value initialized to "undefined".
let/const/class hoisting - declaration hoisted, value "uninitialized"❗️
const requires initialization - must declare with initial value❗️
function hoisting - declaration hoisted, value initialized to a function.

(other than declaration) all occurrences of variables/identifiers serve in one of two "roles"：

the target of an assignment. (LHS reference)
the source of a value. (RHS reference)

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

a closure (or IIFE) can be used to create a scope to hide variables/functions.

// ⭐️ states of an identifier/variable (in a specific scope)
//    • undeclared identifier
//   -> uninitialized variable (declared but undefined) 
//   -> initialized variable (with value !== undefined)

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Last updated 3 years ago