eval

Method. The eval method evaluates a string of JavaScript code in the context of the specified object.

Syntax

[objectName.]eval(string)

Parameters

objectName is the object for which a string is to be evaluated. If omitted, the string is evaluated without regard to any object.

string is any string representing a JavaScript expression, statement, or sequence of statements. The expression can include variables and properties of existing objects.

Method of

eval is a method of all objects.

Implemented in

Navigator 2.0: a built-in JavaScript function, not associated with any object, but part of the language itself

Navigator 3.0: a method of every object

Description

The argument of the eval method is a string. If the string represents an expression, eval evaluates the expression. If the argument represents one or more JavaScript statements, eval performs the statements. Do not call eval to evaluate an arithmetic expression; JavaScript evaluates arithmetic expressions automatically.

If you construct an arithmetic expression as a string, you can use eval to evaluate it at a later time. For example, suppose you have a variable x. You can postpone evaluation of an expression involving x by assigning the string value of the expression, say "3 * x + 2", to a variable, and then calling eval at a later point in your script.

Examples

Example 1. Both of the write statements below display 42. The first evaluates the string "x + y + 1," and the second evaluates the string "42."

var x = 2
var y = 39
var z = "42"
document.write(eval("x + y + 1"), "<BR>") 
document.write(eval(z), "<BR>")

Example 2. In the following example, the getFieldName(n) function returns the name of the nth form element as a string. The first statement assigns the string value of the third form element to the variable field. The second statement uses eval to display the value of the form element.

var field = getFieldName(3) 
document.write("The field named ", field, " has value of ", eval(field + ".value"))

Example 3. The following example uses eval to evaluate the string str. This string consists of JavaScript statements that open an Alert dialog box and assigns z a value of 42 if x is five, and assigns zero to z otherwise. When the second statement is executed, eval will cause these statements to be performed, and it will also evaluate the set of statements and return the value that is assigned to z.

var str = "if (x == 5) {alert('z is 42'); z = 42;} else z = 0; "
document.write("<P>z is ", eval(str))

Example 4. In the following example, the setValue function uses eval to assign the value of the variable newValue to the text field textObject:

function setValue (textObject, newValue) {
   eval ("document.forms[0]." + textObject + ".value") = newValue
}

Example 5. The following example creates breed as a property of the object myDog, and also as a variable. The first write statement uses eval('breed') without specifying an object; the string "breed" is evaluated without regard to any object, and the write method displays "Shepherd", which is the value of the breed variable. The second write statement uses myDog.eval('breed') which specifies the object myDog; the string "breed" is evaluated with regard to the myDog object, and the write method displays "Lab", which is the value of the breed property of the myDog object.

function Dog(name,breed,color) {
   this.name=name
   this.breed=breed
   this.color=color
}
myDog = new Dog("Gabby")
myDog.breed="Lab"
var breed='Shepherd'
document.write("<P>" + eval('breed'))
document.write("<BR>" + myDog.eval('breed'))

Example 6. The following example uses eval within a function that defines an object type, flintstone. The statement fred = new flintstone("x=42") creates the object fred with the properties x, y, z, and z2. The write statements display the values of these properties as 42, 43, 44, and 45, respectively.

function flintstone(str) {
   this.eval("this."+str)
   this.eval("this.y=43")
   this.z=44
   this["z2"] = 45
}
fred = new flintstone("x=42")
document.write("<BR>fred.x is " + fred.x)
document.write("<BR>fred.y is " + fred.y)
document.write("<BR>fred.z is " + fred.z)
document.write("<BR>fred.z2 is " + fred.z2)