📜var

🚧 under construction

JS ⟩ variable ⟩ var

Stop using var❗ ( 👉see why❗)

( ⭐ var is a statement, not a declaration❗)

declares a variable in function scope / global scope ( and does a lot of side effects❗).

var

is a reserved word❗
is a statement, not a declaration❗( 👉 var is a statement❗️❗)
doesn't have block scope. ( 👉 var has no block scope❗️❗)

👉 compare： function declaration

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❗

in global scope,

var / function are implemented as global object property❗
these properties cannot be deleted with delete❗

👉 global var / function is global object property❗️

Variable Statement

var statement declares variables that are scoped to the running execution context's VariableEnvironment.
var variables are created when their containing Environment Record is instantiated and are initialized to undefined when created.
Within the scope of any VariableEnvironment a common BindingIdentifier may appear in more than one VariableDeclaration but those declarations collectively define only one variable.
A variable defined by a VariableDeclaration with an Initializer is assigned the value of its Initializer's AssignmentExpression when the VariableDeclaration is executed, not when the variable is created.

📘 JS spec ⟩ Variable Statement

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Last updated 2 years ago