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eval

Evaluates a string of JavaScript code without reference to a particular object.

Core function

Syntax

eval(string)

Parameters

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

Description

The argument of the eval function 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.

eval is also a method of all objects. This method is described for the Object class.

Examples

Example 1. Both of the print statements below display 42. The first evaluates the string "x + y + 1"; the second evaluates the string "42".

var x = 2
var y = 39
var z = "42"
print(eval("x + y + 1"))
print(eval(z))
Example 2. The following example uses eval to evaluate the string str. This string consists of JavaScript statements that assigns z a value of 42 if x is five, and assigns 0 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) {print('z is 42'); z = 42;} else z = 0; "
print("z is ", eval(str))
Example 3. The following example creates breed as a property of the object myDog, and also as a variable. The first print statement uses eval('breed') without specifying an object; the string "breed" is evaluated without regard to any object, and the print statement displays "Shepherd", which is the value of the breed variable. The second print statement uses myDog.eval('breed') which specifies the object myDog; the string "breed" is evaluated with regard to the myDog object, and the print statement 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'
print("eval('breed'))
print("myDog.eval('breed'))

See also

Object.eval method


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Last Updated: 10/31/97 16:38:00


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