E property
Euler's constant and the base of natural logarithms, approximately 2.718.
语法Math.E
Property of
描述
Because E is a constant, it is a read-only property of Math.
例子
The following example displays Euler's constant: document.write("Euler's constant is " + Math.E)
See also
LN2, LN10, LOG2E, LOG10E, PI, SQRT1_2, SQRT2 properties
elements array
An array of objects corresponding to form elements (such as checkbox, radio, and text objects) in source order.
语法1. formName.elements[index]
2. formName.elements.length
formName is either the name of a form or an element in the forms array.
index is an integer representing an object on a form.
Property of
描述
You can reference a form's elements in your code by using the elements array. This array contains an entry for each object (button, checkbox, hidden, password, radio, reset, select, submit, text, or textarea object) in a form in source order. For example, if a form has a text field and two checkboxes, these elements are reflected as formName.elements[0], formName.elements[1], and formName.elements[2].
Although you can also reference a form's elements by using the element's name (from the NAME attribute), the elements array provides a way to reference form objects programatically without using their names. For example, if the first object on the userInfo form is the userName text object, you can evaluate it in either of the following ways: userInfo.userName.value
userInfo.elements[0].value
To obtain the number of elements on a form, use the length property: formName.elements.length. Each radio button in a radio object appears as a separate element in the elements array.
Elements in the elements array are read-only. For example, the statement formName.elements[0]="music" has no effect.
The value of each element in the elements array is the full htm statement for the object.
Properties
length reflects the number of elements on a form
例子
See the 例子 for the name property.
See also
form object
elements property
An array of objects corresponding to form elements (such as checkbox, radio, and text objects) in source order. See elements array.
encoding property
A string specifying the MIME encoding of the form.
语法formName.encoding
formName is either the name of a form or an element in the forms array.
Property of
描述
The encoding property initially reflects the ENCTYPE attribute of the <FORM> tag; however, setting encoding overrides the ENCTYPE attribute.
You can set the encoding property at any time.
Certain values of the encoding property may require specific values for other form properties. See RFC 1867 for more information.
例子
The following function returns the value of the musicForm encoding property: function getEncoding() {
return document.musicForm.encoding
}
See also
action, method, target properties
escape function
Returns the ASCII encoding of an argument in the ISO Latin-1 character set.
语法escape("string")
string is a non-alphanumeric string in the ISO Latin-1 character set, or a property of an existing object.
描述
The escape function is not a method associated with any object, but is part of the language itself.
The value returned by the escape function is a string of the form "%xx", where xx is the ASCII encoding of a character in the argument. If you pass the escape function an alphanumeric character, the escape function returns the same character.
例子
The following example returns "%26" escape("&")
The following example returns "%21%23" escape("!#")
See also
unescape function
eval function
The eval function evaluates a string and returns a value.
语法eval(string)
string is any string representing a JavaScript expression, statement, or sequence of statements. The expression can include variables and properties of existing objects.
描述
The eval function is a built-in JavaScript function. It is not a method associated with any object, but is part of the language itself.
The argument of the eval function is a string. Do not call eval to evaluate an arithmetic expression. JavaScript evaluates arithmetic expressions automatically. If the argument represents an expression, eval evaluates the expression. If the argument represents one or more JavaScript statements, eval performs the statements.
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.
例子
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 opens 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
}
exp method
Returns enumber, where number is the argument, and e is Euler's constant, the base of the natural logarithms.
语法Math.exp(number)
number is any numeric expression or a property of an existing object.
Method of
例子//Displays the value 2.718281828459045
document.write("The value of e<SUP>1</SUP> is " + Math.exp(1))
See also
