面试题与答案

Use the parseInt() function, that takes a string as the first parameter, and the base as a second parameter. So to convert hexadecimal 3F to decimal, use parseInt ("3F", 16);

Return true if the argument is not a number.

Since 2 and 5 are integers, this is number arithmetic, since 8 is a string, it's concatenation, so 78 is the result.

Since 1 is a string, everything is a string, so the result is 124.

arr[arr.length] = value;

It's a number in JavaScript, derived by dividing negative number by zero.

It refers to the current object.

A scripting language is a simple, interpreted programming language.
Scripts are embedded as plain text, interpreted by application.

Simpler execution model: don't need compiler or development environment.
Saves bandwidth: source code is downloaded, not compiled executable.
Platform-independence: code interpreted by any script-enabled browser.

But: slower than compiled code, not as powerful/full-featured.

Identifier� The name of a variable (or function)
Starts with a letter, can contains digits & underscores
Case Sensitive!!
Should be meaningful to someone reading your code
Good: accountBalance, amountDue

Bad: bal, due,

Just plain wrong: 2bOrNotToBe, +var, total-value

Must declare variables before they�re used in the program
Declare at the top of the program & terminate each statement with �;�
Intialize variables when appropriate
Local variables (declared within a function) destroyed after function exit.
Can only be accessed within the function

Example � Note Assignments
var candyBarPrice = 2.50;
var taxRate = .075;
var candyBarsPurchased;

Assignment �looks like� equal sign but does NOT behave like it

subTotal = subTotal + 1.50

subTotal �is assigned the value� that is currently in subTotal plus the value of 1.50

An expression is a statement that describes a computation.
Usually look like algebra formulas
total = subTotal * taxRate
Operators (+, -, *, /, etc.) have different levels of precedence, similar to algebra
Don�t rely on it! For clarity, use parentheses.

Strings are sequences of keyboard characters enclosed in quotes
�Hello World� or �Hello World�

Variables can hold strings
var greeting = �Hello World�

String can be empty, i.e., contain no characters
var myAnswer = ��

Use �\� (escape symbol) to �type� prohibited characters
\b for backspace, \n for newline, \t for tab, \� for double quote.

onsubmit - call when submit button is clicked
onclick - call when this button is clicked
onreset - call when the reset button is clicked
onload - call after page loads
onmouseover - call when mouse pointer enters image area
onmouseout - call when mouse pointer leaves image area
onfocus - call when control receives focus
onblur - call when a control loses focus
onchange - call when a control loses focus and the value of its contents has changed
onunload � call when a page is closed

If needed, you can declare local variables within a function.

local variable is visible only within the function body after it�s declared.

Commonly used to store results of an intermediate calculation.

function findMaxValue(num1, num2,num3) {
var tempMax; //local var

if (num1 >= num2) {
tempMax = num1;
}

else {
tempMax = num2;
}

if(num3 >= tempMax) {
tempMax = num3;
}

return tempMax;

} //end function

Global variables are those declared outside of functions
Global variables are �visible� from anywhere in the program, including inside functions

var globalHello = �Hello!�;

function writeHello() {
document.write(globalHello);
}
// outputs �Hello!�

Boolean, string and number.

Use document.forms[0].submit();
(0 refers to the index of the form � if you have more than one form in a page, then the first one has the index 0, second has index 1 and so on).

Return true if the argument is not a number.

GET: Parameters are passed in the querystring. Maximum amount of data that can be sent via the GET method is limited to about 2kb.
POST: Parameters are passed in the request body. There is no limit to the amount of data that can be transferred using POST. However, there are limits on the maximum amount of data that can be transferred in one name/value pair.

Use the "value" property.
var myValue = window.document.getElementById("MyTextBox").value;

var checkedP = window.document.getElementById("myCheckBox").checked;

<script> function setFocus() { if(focusElement != null) { document.forms[0].elements["myelementname"].focus(); } } </script>

An alert box displays only one button which is the OK button whereas the Confirm box displays two buttons namely OK and cancel.

Breaking is possible within a string statement by using a backslash \ at the end but not within any other javascript statement.
that is ,
document.write("Hello \ world");
is possible but not document.write \
("hello world");

<a href='javascript:window.close()' class='mainnav'> Close </a>

Use // for line comments and
/*

*/ for block comments

Number.MAX_VALUE
Number.MIN_VALUE

The null value is a unique value representing no value or no object.
It implies no object,or null string,no valid boolean value,no number and no array object.

document.getElementById("myObject").disabled = true;