JavaScript 2.0 Core Language Variables

Thursday, November 11, 1999

Variable Definition

The general syntax for defining variables is:

A variable defined with var can be modified, while one defined with const cannot. Identifier is the name of the variable and TypeExpression is its type. Identifier can be any non-reserved identifier. TypeExpression is evaluated at the time the variable definition is evaluated and should evaluate to a type t.

If provided, AssignmentExpression gives the variable's initial value v. If not, undefined is assumed; an error occurs if undefined cannot be coerced to type t. AssignmentExpression is evaluated just after the TypeExpression is evaluated. The value v is then coerced to the variable's type t and stored in the variable. If the variable is defined using var, any values subsequently assigned to the variable are also coerced to type t at the time of each such assignment.

Multiple variables separated by commas can be defined in the same VariableDefinition. The values of earlier variables are available in the TypeExpressions and AssignmentExpressions of later variables.

If omitted, TypeExpression defaults to type any. Thus, the definition

var a, b=3, c:integer=7, d, e:type=boolean, f:number, g:e, h:int;

is equivalent to:

var a:Any=undefined;
var b:Any=3;
var c:integer=7;
var d:integer=undefined;  // coerced to +0
var e:type=boolean;
var f:number=undefined;   // coerced to +0
var g:boolean=undefined;  // coerced to false
var h:int=undefined;      // coerced to int(0)

const Definitions

const means that Identifier cannot be written after it is defined. It does not mean that Identifier will have the same value the next time it is bound. For example, the following is legal; a new j binding is created each time through the loop:

var k = 0;
for (var i = 0; i < 10; i++) {
  local const j = i;
  k += j;
}

Waldemar Horwat Last modified Thursday, November 11, 1999