JavaScript: var Declaration
Note: var deprecated
Note: The use of var is deprecated since ES2015, due to its name hoisting complexity and name scope complexity [see var Name Scope] , you should never use var. Use let Declaration instead.
Var Declaration
var name;-
declare a variable.
// declare variable var x; var y;
var name1, name2, name3 etc;-
declare multiple variables.
// declare multiple variables var a, b, c;
var name = val;- declare and assign.
var name1 = val1, name2 = val2, name3 = val3 etc;-
declare and assign multiple variables
// declare variable and assign var x = 4;
// declare variable and assign multiple variables var a = 1, b = 2, c = 3;
var name_or_assign_1, name_or_assign_2 etc;-
declare variables, some with assignment.
// 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);
var with no value
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);
Same var declared multiple times
It is harmless if you declared a variable redundantly multiple times.
var x; console.log(x === undefined); var x; console.log(x === undefined); var y = 3; console.log(y === 3); var y; console.log(y === 3);
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); console.log(yy === 3);