JS: var Declaration

By Xah Lee. Date: . Last updated: .

Note: The use of var is deprecated since ES2015, due to its name hoisting complexity and name scope complexity [see JS: var Name Scope] , you should never use var. Use let instead. [see JS: let Declaration]

Var Declaration

var name; → declares a variable.

// declare variable
var x;
var y;

Multiple variables can be declared on the same line, separated by comma.

For example, var name1, name2, name3 …;

// declare multiple variables
var a, b, c;

Multiple variables can be declared and assigned at the same time.

// declare variable and assign
var x = 4;
// declare variable and assign multiple variables
var a = 1, b = 2, c = 3;
// declare multiple variables, some with value
var a, b = 2, c;

// all true
console.log(a === undefined);
console.log(b === 2);
console.log(c === undefined);

When a variable is declared but not assigned, the default value is undefined:

var x;
console.log(x === undefined); // true

When a variable is not declared nor assigned, and you try to access it, you get ReferenceError.

var x;
console.log(x === undefined); // true

// ReferenceError: y is not defined
console.log(y);

You should ALWAYS declare variables. It is harmless if you declared a variable redundantly multiple times.

var x;
console.log(x === undefined); // true
var x;
console.log(x === undefined); // true

var y = 3;
console.log(y === 3); // true
var y;
console.log(y === 3); // true

Variable Assignment

Variable assignment returns a value.

// variable assignment returns a value
var xx;
console.log(xx = 3);            // 3

So, variable assignment can be chained.

var xx;
var yy;

console.log(xx = yy = 3); // 3
console.log(xx === 3);    // true
console.log(yy === 3);    // true

var Declaration Order

JS: var Declaration Order

var Name Scope

JS: var Name Scope

JavaScript Basics

  1. JavaScript Basics
  2. Operators
  3. true, false
  4. Branch Control
  5. Loop
  6. Array Basics
  7. Object Basics

Value Types

  1. Value Types
  2. Primitive Value
  3. “typeof” Operator

Variable

  1. let
  2. const
  3. var 👎
  4. var Order 👎
  5. var Scope 👎
  6. Global Variable
  7. Destructuring Assignment

String

  1. String Overview
  2. Template String
  3. String Escape Sequence
  4. Unicode Escape Sequence
  5. Char, Code Unit, Codepoint

Function

  1. Define Function
  2. Arrow Function ⭐
  3. Function Params
  4. “arguments” Object
  5. Rest Params ⭐
  6. Arg Default Value ⭐
  7. Arg Destructure ⭐
  8. f Declaration vs Expression
  9. Closure
  10. f call apply bind
  11. Functional Programing

Object Property

  1. Property Overview
  2. Property Key
  3. Dot vs Bracket Notation
  4. Create/Delete Property
  5. Get/Set Property
  6. Property Existence
  7. Access Property
  8. List Properties
  9. Property Attributes
  10. Getter/Setter
  11. Property Descriptor
  12. Symbol

Object and Inheritance

  1. Object Overview
  2. Object Type
  3. Find Object's Type
  4. Prototype Chain
  5. Check is in Prototype Chain
  6. Get/Set Parent
  7. Show Prototype Chain
  8. Create Object
  9. Object Literal Expr
  10. Create Object with Parent X
  11. Prevent Adding Property
  12. Clone Object
  13. Test Object Equality
  14. Add Method to Prototype

Array

  1. Understand JS Array
  2. Create Array
  3. Sparse Array
  4. Array-Like Object
  5. Array How-To

Constructor/Class

  1. “this” Binding
  2. What's Constructor?
  3. Property Key "prototype"
  4. Operator “new”
  5. Operator “instanceof”
  6. Property Key “constructor”
  7. Class
  8. Keyword “extends”
  9. Keyword “super”

Iterable ⭐

  1. Interface
  2. Iterable
  3. Iterator
  4. Iterator Prototype
  5. for-of Loop
  6. Spread Operator
  7. Generator
  8. Generator Function

Regular Expression

  1. RegExp Basics
  2. RegExp Syntax
  3. Regex Find/Replace

Date

  1. Date Tutorial
  2. Compute Date Range

Set Object

  1. Set Object
  2. Union, Intersection, Diff

Map Object

  1. Map Object
  2. Iterate Over Map Object
  3. Convert Object to/from Map
  4. Map Filter
  5. Reverse Key/Value

Number

  1. String to/from Number
  2. Convert Decimal/Hexadecimal
  3. Format Number
  4. Random Number

Misc

  1. Import/Export
  2. Promise
  3. Proxy
  4. Try Catch Throw Finally
  5. ES2015 Features
  6. ES2016, 2017, 2018, 2019
  7. How to Convert ES5 to ES2015
  8. Style Guide
  9. Semicolon Rules
  10. Exclamation Before Function
  11. How to Determine Strict Mode?
  12. x.y.z Associativity
  13. Function Chain
  14. Random Number, Range, Integer, Element
  15. Timing JS Code
  16. Range Function for Array
  17. Tagged Template String
  18. Allowed Characters in Identifier
  19. Encode URL, Escape String

Object Reference

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