Predefined Core Objects

이번 챕터에서는 자바스크립트(core JavaScript)에 '사전에 정의된 객체'를 설명한다. 자바스크립트에서 Array, Boolean, Date, Function, Math, Number, ReqExp, String 객체를 정의하고 있다.

배열 객체(Array Object)

JavaScript does not have an explicit array data type. However, you can use the predefined Array object and its methods to work with arrays in your applications. The Array object has methods for manipulating arrays in various ways, such as joining, reversing, and sorting them. It has a property for determining the array length and other properties for use with regular expressions.

An array is an ordered set of values that you refer to with a name and an index. For example, you could have an array called emp that contains employees' names indexed by their employee number. So emp[1] would be employee number one, emp[2] employee number two, and so on.

Creating an Array

The following statements create equivalent arrays:

var arr = new Array(element0, element1, ..., elementN);
var arr = Array(element0, element1, ..., elementN);
var arr = [element0, element1, ..., elementN];

element0, element1, ..., elementN is a list of values for the array's elements. When these values are specified, the array is initialized with them as the array's elements. The array's length property is set to the number of arguments.

The bracket syntax is called an "array literal" or "array initializer." It's shorter than other forms of array creation, and so is generally preferred. See Array Literals for details.

To create an Array with non-zero length, but without any items, either of the following can be used:

var arr = new Array(arrayLength);
var arr = Array(arrayLength);

// This has exactly the same effect
var arr = [];
arr.length = arrayLength;

Note: in the above code, arrayLength must be a Number. Otherwise, an array with a single element (the provided value) will be created. Calling arr.length will return arrayLength, but the array actually contains empty (undefined) elements. Running a for...in loop on the array will return none of the array's elements.

In addition to a newly defined variable as shown above, Arrays can also be assigned as a property of a new or an existing object:

var obj = {};
// ...
obj.prop = [element0, element1, ..., elementN];

// OR
var obj = {prop: [element0, element1, ...., elementN]}

If you wish to initialize an array with a single element, and the element happens to be a Number, you must use the bracket syntax. When a single Number value is passed to the Array() constructor or function, it is interpreted as an arrayLength, not as a single element.

var arr = [42];
var arr = Array(42); // Creates an array with no element, but with arr.length set to 42

// The above code is equivalent to
var arr = [];
arr.length = 42;

Calling Array(N) results in a RangeError, if N is a non-whole number whose fractional portion is non-zero. The following example illustrates this behavior.

var arr = Array(9.3);  // RangeError: Invalid array length

If your code needs to create arrays with single elements of an arbitrary data type, it is safer to use array literals. Or, create an empty array first before adding the single element to it.

Populating an Array

You can populate an array by assigning values to its elements. For example,

var emp = [];
emp[0] = "Casey Jones";
emp[1] = "Phil Lesh";
emp[2] = "August West";

Note: if you supply a non-integer value to the array operator in the code above, a property will be created in the object representing the array, instead of an array element.

 var arr = [];
arr[3.4] = "Oranges";
console.log(arr.length);                // 0
console.log(arr.hasOwnProperty(3.4));   // true

You can also populate an array when you create it:

var myArray = new Array("Hello", myVar, 3.14159);
var myArray = ["Mango", "Apple", "Orange"]

Referring to Array Elements

You refer to an array's elements by using the element's ordinal number. For example, suppose you define the following array:

var myArray = ["Wind", "Rain", "Fire"];

You then refer to the first element of the array as myArray[0] and the second element of the array as myArray[1]. The index of the elements begins with zero.

Note: the array operator (square brackets) is also used for accessing the array's properties (arrays are also objects in JavaScript). For example,

 var arr = ["one", "two", "three"];
arr[2];  // three
arr["length"];  // 3

Understanding length

At the implementation level, JavaScript's arrays actually store their elements as standard object properties, using the array index as the property name. The length property is special; it always returns the index of the last element plus one (in following example Dusty is indexed at 30 so cats.length returns 30 + 1). Remember, Javascript Array indexes are 0-based: they start at 0, not 1. This means that the length property will be one more than the highest index stored in the array:

var cats = [];
cats[30] = ['Dusty'];
print(cats.length); // 31

You can also assign to the length property. Writing a value that is shorter than the number of stored items truncates the array; writing 0 empties it entirely:

var cats = ['Dusty', 'Misty', 'Twiggy'];
console.log(cats.length); // 3

cats.length = 2;
console.log(cats); // prints "Dusty,Misty" - Twiggy has been removed

cats.length = 0;
console.log(cats); // prints nothing; the cats array is empty

cats.length = 3;
console.log(cats); // [undefined, undefined, undefined]

Iterating over arrays

A common operation is to iterate over the values of an array, processing each one in some way. The simplest way to do this is as follows:

var colors = ['red', 'green', 'blue'];
for (var i = 0; i < colors.length; i++) {
  console.log(colors[i]);
}

If you know that none of the elements in your array evaluate to false in a boolean context — if your array consists only of DOM nodes, for example, you can use a more efficient idiom:

var divs = document.getElementsByTagName('div');
for (var i = 0, div; div = divs[i]; i++) {
  /* Process div in some way */
}

This avoids the overhead of checking the length of the array, and ensures that the div variable is reassigned to the current item each time around the loop for added convenience.

The forEach() method, introduced in JavaScript 1.6, provides another way of iterating over an array:

var colors = ['red', 'green', 'blue'];
colors.forEach(function(color) {
  console.log(color);
});

The function passed to forEach is executed once for every item in the array, with the array item passed as the argument to the function. Unassigned values are not iterated in a forEach loop.

Note that the elements of array omitted when the array is defined are not listed when iterating by forEach, but are listed when undefined has been manually assigned to the element:

var array = ['first', 'second', , 'fourth'];

// returns ['first', 'second', 'fourth'];
array.forEach(function(element) {
  console.log(element);
})

if(array[2] === undefined) { console.log('array[2] is undefined'); } // true

var array = ['first', 'second', undefined, 'fourth'];

// returns ['first', 'second', undefined, 'fourth'];
array.forEach(function(element) {
  console.log(element);
})

Since JavaScript elements are saved as standard object properties, it is not advisable to iterate through JavaScript arrays using for...in loops because normal elements and all enumerable properties will be listed.

Array Methods

The Array object has the following methods:

• concat() joins two arrays and returns a new array.

  var myArray = new Array("1", "2", "3");
  myArray = myArray.concat("a", "b", "c"); // myArray is now ["1", "2", "3", "a", "b", "c"]
  

• join(deliminator = ",") joins all elements of an array into a string.

  var myArray = new Array("Wind", "Rain", "Fire");
  var list = myArray.join(" - "); // list is "Wind - Rain - Fire"
  

• push() adds one or more elements to the end of an array and returns the resulting length of the array.

  var myArray = new Array("1", "2");
  myArray.push("3"); // myArray is now ["1", "2", "3"]
  

• pop() removes the last element from an array and returns that element.

  var myArray = new Array("1", "2", "3");
  var last = myArray.pop(); // myArray is now ["1", "2"], last = "3"
  

• shift() removes the first element from an array and returns that element.

  var myArray = new Array ("1", "2", "3");
  var first = myArray.shift(); // myArray is now ["2", "3"], first is "1"
  

• unshift() adds one or more elements to the front of an array and returns the new length of the array.

  var myArray = new Array ("1", "2", "3");
  myArray.unshift("4", "5"); // myArray becomes ["4", "5", "1", "2", "3"]

• slice(start_index, upto_index) extracts a section of an array and returns a new array.

  var myArray = new Array ("a", "b", "c", "d", "e");
  myArray = myArray.slice(1, 4); /* starts at index 1 and extracts all elements
    until index 3, returning [ "b", "c", "d"] */
  

• splice(index, count_to_remove, addelement1, addelement2, ...) removes elements from an array and (optionally) replaces them.

  var myArray = new Array ("1", "2", "3", "4", "5");
  myArray.splice(1, 3, "a", "b", "c", "d"); // myArray is now ["1", "a", "b", "c", "d", "5"]
    // This code started at index one (or where the "2" was), removed 3 elements there, 
    // and then inserted all consecutive elements in its place.
  

• reverse() transposes the elements of an array: the first array element becomes the last and the last becomes the first.

  var myArray = new Array ("1", "2", "3");
  myArray.reverse(); // transposes the array so that myArray = [ "3", "2", "1" ]
  

• sort() sorts the elements of an array.

  var myArray = new Array("Wind", "Rain", "Fire");
  myArray.sort(); // sorts the array so that myArrray = [ "Fire", "Rain", "Wind" ]
  

  sort() can also take a callback function to determine how array elements are compared. The function compares two values and returns one of three values:

  • if a is less than b by the sorting system, return -1 (or any negative number)
  • if a is greater than b by the sorting system, return 1 (or any positive number)
  • if a and b are considered equivalent, return 0.

  For instance, the following will sort by the last letter of an array:

  var sortFn = function(a, b){
    if (a[a.length - 1] < b[b.length - 1]) return -1;
    if (a[a.length - 1] > b[b.length - 1]) return 1;
    if (a[a.length - 1] == b[b.length - 1]) return 0;
  }
  myArray.sort(sortFn); // sorts the array so that myArray = ["Wind","Fire","Rain"]

Compatibility code for older browsers can be found for each of these functions on the individual pages. Native browser support for these features in various browsers can be found here.

• indexOf(searchElement[, fromIndex]) searches the array for searchElement and returns the index of the first match.

  var a = ['a', 'b', 'a', 'b', 'a'];
  alert(a.indexOf('b')); // Alerts 1
  // Now try again, starting from after the last match
  alert(a.indexOf('b', 2)); // Alerts 3
  alert(a.indexOf('z')); // Alerts -1, because 'z' was not found
  

• lastIndexOf(searchElement[, fromIndex]) works like indexOf, but starts at the end and searches backwards.

  var a = ['a', 'b', 'c', 'd', 'a', 'b'];
  alert(a.lastIndexOf('b')); // Alerts 5
  // Now try again, starting from before the last match
  alert(a.lastIndexOf('b', 4)); // Alerts 1
  alert(a.lastIndexOf('z')); // Alerts -1
  

• forEach(callback[, thisObject]) executes callback on every array item.

  var a = ['a', 'b', 'c'];
  a.forEach(alert); // Alerts each item in turn
  

• map(callback[, thisObject]) returns a new array of the return value from executing callback on every array item.

  var a1 = ['a', 'b', 'c'];
  var a2 = a1.map(function(item) { return item.toUpperCase(); });
  alert(a2); // Alerts A,B,C
  

• filter(callback[, thisObject]) returns a new array containing the items for which callback returned true.

  var a1 = ['a', 10, 'b', 20, 'c', 30];
  var a2 = a1.filter(function(item) { return typeof item == 'number'; });
  alert(a2); // Alerts 10,20,30
  

• every(callback[, thisObject]) returns true if callback returns true for every item in the array.

  function isNumber(value){
    return typeof value == 'number';
  }
  var a1 = [1, 2, 3];
  alert(a1.every(isNumber)); // Alerts true
  var a2 = [1, '2', 3];
  alert(a2.every(isNumber)); // Alerts false
  

• some(callback[, thisObject]) returns true if callback returns true for at least one item in the array.

  function isNumber(value){
    return typeof value == 'number';
  }
  var a1 = [1, 2, 3];
  alert(a1.some(isNumber)); // Alerts true
  var a2 = [1, '2', 3];
  alert(a2.some(isNumber)); // Alerts true
  var a3 = ['1', '2', '3'];
  alert(a3.some(isNumber)); // Alerts false
  

The methods above that take a callback are known as iterative methods, because they iterate over the entire array in some fashion. Each one takes an optional second argument called thisObject. If provided, thisObject becomes the value of the this keyword inside the body of the callback function. If not provided, as with other cases where a function is invoked outside of an explicit object context, this will refer to the global object (window).

The callback function is actually called with three arguments. The first is the value of the current item, the second is its array index, and the third is a reference to the array itself. JavaScript functions ignore any arguments that are not named in the parameter list so it is safe to provide a callback function that only takes a single argument, such as alert.

• reduce(callback[, initialValue]) applies callback(firstValue, secondValue) to reduce the list of items down to a single value.

  var a = [10, 20, 30];
  var total = a.reduce(function(first, second) { return first + second; }, 0);
  alert(total) // Alerts 60
  

• reduceRight(callback[, initialValue]) works like reduce(), but starts with the last element.

reduce and reduceRight are the least obvious of the iterative array methods. They should be used for algorithms that combine two values recursively in order to reduce a sequence down to a single value.

Multi-Dimensional Arrays

Arrays can be nested, meaning that an array can contain another array as an element. Using this characteristic of JavaScript arrays, multi-dimensional arrays can be created.

The following code creates a two-dimensional array.

var a = new Array(4);
for (i = 0; i < 4; i++) {
  a[i] = new Array(4);
  for (j = 0; j < 4; j++) {
    a[i][j] = "[" + i + "," + j + "]";
  }
}

This example creates an array with the following rows:

Row 0: [0,0] [0,1] [0,2] [0,3]
Row 1: [1,0] [1,1] [1,2] [1,3]
Row 2: [2,0] [2,1] [2,2] [2,3]
Row 3: [3,0] [3,1] [3,2] [3,3]

Arrays and Regular Expressions

When an array is the result of a match between a regular expression and a string, the array returns properties and elements that provide information about the match. An array is the return value of RegExp.exec(), String.match(), and String.split(). For information on using arrays with regular expressions, see Regular Expressions.

Working with Array-like objects

Some JavaScript objects, such as the NodeList returned by document.getElementsByTagName() or the arguments object made available within the body of a function, look and behave like arrays on the surface but do not share all of their methods. The arguments object provides a length attribute but does not implement the forEach() method, for example.

Array generics, introduced in JavaScript 1.6, provide a way of running Array methods against other array-like objects. Each standard array method has a corresponding method on the Array object itself; for example:

 function alertArguments() {
   Array.forEach(arguments, function(item) {
     alert(item);
   });
 }

These generic methods can be emulated more verbosely in older versions of JavaScript using the call method provided by JavaScript function objects:

 Array.prototype.forEach.call(arguments, function(item) {
   alert(item);
 });

Array generic methods can be used on strings as well, since they provide sequential access to their characters in a similar way to arrays:

Array.forEach("a string", function(chr) {
   alert(chr);
});

Here are some further examples of applying array methods to strings, also taking advantage of JavaScript 1.8 expression closures:

var str = 'abcdef';
var consonantsOnlyStr = Array.filter(str, function (c) !(/[aeiou]/i).test(c)).join(''); // 'bcdf'
var vowelsPresent = Array.some(str, function (c) (/[aeiou]/i).test(c)); // true
var allVowels = Array.every(str, function (c) (/[aeiou]/i).test(c)); // false
var interpolatedZeros = Array.map(str, function (c) c+'0').join(''); // 'a0b0c0d0e0f0'
var numerologicalValue = Array.reduce(str, function (c, c2) c+c2.toLowerCase().charCodeAt()-96, 0);
// 21 (reduce() since JS v1.8)

Note that filter and map do not automatically return the characters back into being members of a string in the return result; an array is returned, so we must use join to return back to a string.

Array comprehensions

Introduced in JavaScript 1.7, array comprehensions provide a useful shortcut for constructing a new array based on the contents of another. Comprehensions can often be used in place of calls to map() and filter(), or as a way of combining the two.

The following comprehension takes an array of numbers and creates a new array of the double of each of those numbers.

var numbers = [1, 2, 3, 4];
var doubled = [i * 2 for (i of numbers)];
alert(doubled); // Alerts 2,4,6,8

This is equivalent to the following map() operation:

var doubled = numbers.map(function(i){return i * 2;});

Comprehensions can also be used to select items that match a particular expression. Here is a comprehension which selects only even numbers:

var numbers = [1, 2, 3, 21, 22, 30];
var evens = [i for (i of numbers) if (i % 2 === 0)];
alert(evens); // Alerts 2,22,30

filter() can be used for the same purpose:

var evens = numbers.filter(function(i){return i % 2 === 0;});

map() and filter() style operations can be combined into a single array comprehension. Here is one that filters just the even numbers, then creates an array containing their doubles:

var numbers = [1, 2, 3, 21, 22, 30];
var doubledEvens = [i * 2 for (i of numbers) if (i % 2 === 0)];
alert(doubledEvens); // Alerts 4,44,60

The square brackets of an array comprehension introduce an implicit block for scoping purposes. New variables (such as i in the example) are treated as if they had been declared using let. This means that they will not be available outside of the comprehension.

The input to an array comprehension does not itself need to be an array; iterators and generators can also be used.

Even strings may be used as input; to achieve the filter and map actions (under Array-like objects) above:

var str = 'abcdef';
var consonantsOnlyStr = [c for (c of str) if (!(/[aeiouAEIOU]/).test(c))  ].join(''); // 'bcdf'
var interpolatedZeros = [c+'0' for (c of str) ].join(''); // 'a0b0c0d0e0f0'

Again, the input form is not preserved, so we have to use join() to revert back to a string.

Boolean Object

The Boolean object is a wrapper around the primitive Boolean data type. Use the following syntax to create a Boolean object:

var booleanObjectName = new Boolean(value);

Do not confuse the primitive Boolean values true and false with the true and false values of the Boolean object. Any object whose value is not undefined , null, 0, NaN, or the empty string, including a Boolean object whose value is false, evaluates to true when passed to a conditional statement. See if...else Statement for more information.

Date Object

JavaScript does not have a date data type. However, you can use the Date object and its methods to work with dates and times in your applications. The Date object has a large number of methods for setting, getting, and manipulating dates. It does not have any properties.

JavaScript handles dates similarly to Java. The two languages have many of the same date methods, and both languages store dates as the number of milliseconds since January 1, 1970, 00:00:00.

The Date object range is -100,000,000 days to 100,000,000 days relative to 01 January, 1970 UTC.

To create a Date object:

var dateObjectName = new Date([parameters]);

where dateObjectName is the name of the Date object being created; it can be a new object or a property of an existing object.

Calling Date without the new keyword simply converts the provided date to a string representation.

The parameters in the preceding syntax can be any of the following:

• Nothing: creates today's date and time. For example, today = new Date();.
• A string representing a date in the following form: "Month day, year hours:minutes:seconds." For example, var Xmas95 = new Date("December 25, 1995 13:30:00"). If you omit hours, minutes, or seconds, the value will be set to zero.
• A set of integer values for year, month, and day. For example, var Xmas95 = new Date(1995, 11, 25).
• A set of integer values for year, month, day, hour, minute, and seconds. For example, var Xmas95 = new Date(1995, 11, 25, 9, 30, 0);.

JavaScript 1.2 and earlier
The Date object behaves as follows:

• Dates prior to 1970 are not allowed.
• JavaScript depends on platform-specific date facilities and behavior; the behavior of the Date object varies from platform to platform.

Methods of the Date Object

The Date object methods for handling dates and times fall into these broad categories:

• "set" methods, for setting date and time values in Date objects.
• "get" methods, for getting date and time values from Date objects.
• "to" methods, for returning string values from Date objects.
• parse and UTC methods, for parsing Date strings.

With the "get" and "set" methods you can get and set seconds, minutes, hours, day of the month, day of the week, months, and years separately. There is a getDay method that returns the day of the week, but no corresponding setDay method, because the day of the week is set automatically. These methods use integers to represent these values as follows:

• Seconds and minutes: 0 to 59
• Hours: 0 to 23
• Day: 0 (Sunday) to 6 (Saturday)
• Date: 1 to 31 (day of the month)
• Months: 0 (January) to 11 (December)
• Year: years since 1900

For example, suppose you define the following date:

var Xmas95 = new Date("December 25, 1995");

Then Xmas95.getMonth() returns 11, and Xmas95.getFullYear() returns 1995.

The getTime and setTime methods are useful for comparing dates. The getTime method returns the number of milliseconds since January 1, 1970, 00:00:00 for a Date object.

For example, the following code displays the number of days left in the current year:

var today = new Date();
var endYear = new Date(1995, 11, 31, 23, 59, 59, 999); // Set day and month
endYear.setFullYear(today.getFullYear()); // Set year to this year
var msPerDay = 24 * 60 * 60 * 1000; // Number of milliseconds per day
var daysLeft = (endYear.getTime() - today.getTime()) / msPerDay;
var daysLeft = Math.round(daysLeft); //returns days left in the year

This example creates a Date object named today that contains today's date. It then creates a Date object named endYear and sets the year to the current year. Then, using the number of milliseconds per day, it computes the number of days between today and endYear, using getTime and rounding to a whole number of days.

The parse method is useful for assigning values from date strings to existing Date objects. For example, the following code uses parse and setTime to assign a date value to the IPOdate object:

var IPOdate = new Date();
IPOdate.setTime(Date.parse("Aug 9, 1995"));

Using the Date Object: an Example

In the following example, the function JSClock() returns the time in the format of a digital clock.

function JSClock() {
  var time = new Date();
  var hour = time.getHours();
  var minute = time.getMinutes();
  var second = time.getSeconds();
  var temp = "" + ((hour > 12) ? hour - 12 : hour);
  if (hour == 0)
    temp = "12";
  temp += ((minute < 10) ? ":0" : ":") + minute;
  temp += ((second < 10) ? ":0" : ":") + second;
  temp += (hour >= 12) ? " P.M." : " A.M.";
  return temp;
}

The JSClock function first creates a new Date object called time; since no arguments are given, time is created with the current date and time. Then calls to the getHours, getMinutes, and getSeconds methods assign the value of the current hour, minute, and second to hour, minute, and second.

The next four statements build a string value based on the time. The first statement creates a variable temp, assigning it a value using a conditional expression; if hour is greater than 12, (hour - 12), otherwise simply hour, unless hour is 0, in which case it becomes 12.

The next statement appends a minute value to temp. If the value of minute is less than 10, the conditional expression adds a string with a preceding zero; otherwise it adds a string with a demarcating colon. Then a statement appends a seconds value to temp in the same way.

Finally, a conditional expression appends "P.M." to temp if hour is 12 or greater; otherwise, it appends "A.M." to temp.

Function Object

The predefined Function object specifies a string of JavaScript code to be compiled as a function.

To create a Function object:

var functionObjectName = new Function ([arg1, arg2, ... argn], functionBody);

functionObjectName is the name of a variable or a property of an existing object. It can also be an object followed by a lowercase event handler name, such as window.onerror.

arg1, arg2, ... argn are arguments to be used by the function as formal argument names. Each must be a string that corresponds to a valid JavaScript identifier; for example "x" or "theForm".

functionBody is a string specifying the JavaScript code to be compiled as the function body.

Function objects are evaluated each time they are used. This is less efficient than declaring a function and calling it within your code, because declared functions are compiled.

In addition to defining functions as described here, you can also use the function statement and the function expression. See the JavaScript Reference for more information.

The following code assigns a function to the variable setBGColor. This function sets the current document's background color.

var setBGColor = new Function("document.bgColor = 'antiquewhite'");

To call the Function object, you can specify the variable name as if it were a function. The following code executes the function specified by the setBGColor variable:

var colorChoice="antiquewhite";
if (colorChoice=="antiquewhite") {setBGColor()}

You can assign the function to an event handler in either of the following ways:

1. document.form1.colorButton.onclick = setBGColor;
   

2. <INPUT NAME="colorButton" TYPE="button"
     VALUE="Change background color"
     onClick="setBGColor()">
   

Creating the variable setBGColor shown above is similar to declaring the following function:

function setBGColor() {
  document.bgColor = 'antiquewhite';
}

Assigning a function to a variable is similar to declaring a function, but there are differences:

• When you assign a function to a variable using var setBGColor = new Function("..."), setBGColor is a variable for which the current value is a reference to the function created with new Function().
• When you create a function using function setBGColor() {...}, setBGColor is not a variable, it is the name of a function.

You can nest a function within a function. The nested (inner) function is private to its containing (outer) function:

• The inner function can be accessed only from statements in the outer function.
• The inner function can use the arguments and variables of the outer function. The outer function cannot use the arguments and variables of the inner function.

Math Object

The predefined Math object has properties and methods for mathematical constants and functions. For example, the Math object's PI property has the value of pi (3.141...), which you would use in an application as

Math.PI

Similarly, standard mathematical functions are methods of Math. These include trigonometric, logarithmic, exponential, and other functions. For example, if you want to use the trigonometric function sine, you would write

Math.sin(1.56)

Note that all trigonometric methods of Math take arguments in radians.

The following table summarizes the Math object's methods.

Unlike many other objects, you never create a Math object of your own. You always use the predefined Math object.

Number Object

The Number object has properties for numerical constants, such as maximum value, not-a-number, and infinity. You cannot change the values of these properties and you use them as follows:

var biggestNum = Number.MAX_VALUE;
var smallestNum = Number.MIN_VALUE;
var infiniteNum = Number.POSITIVE_INFINITY;
var negInfiniteNum = Number.NEGATIVE_INFINITY;
var notANum = Number.NaN;

You always refer to a property of the predefined Number object as shown above, and not as a property of a Number object you create yourself.

The following table summarizes the Number object's properties.

The Number prototype provides methods for retrieving information from Number objects in various formats. The following table summarizes the methods of Number.prototype.

RegExp Object

The RegExp object lets you work with regular expressions. It is described in Regular Expressions.

String Object

The String object is a wrapper around the string primitive data type. Do not confuse a string literal with the String object. For example, the following code creates the string literal s1 and also the String object s2:

var s1 = "foo"; //creates a string literal value
var s2 = new String("foo"); //creates a String object

You can call any of the methods of the String object on a string literal value—JavaScript automatically converts the string literal to a temporary String object, calls the method, then discards the temporary String object. You can also use the String.length property with a string literal.

You should use string literals unless you specifically need to use a String object, because String objects can have counterintuitive behavior. For example:

var s1 = "2 + 2"; //creates a string literal value
var s2 = new String("2 + 2"); //creates a String object
eval(s1); //returns the number 4
eval(s2); //returns the string "2 + 2"

A String object has one property, length, that indicates the number of characters in the string. For example, the following code assigns x the value 13, because "Hello, World!" has 13 characters:

var mystring = "Hello, World!";
var x = mystring.length;

A String object has two types of methods: those that return a variation on the string itself, such as substring and toUpperCase, and those that return an HTML-formatted version of the string, such as bold and link.

For example, using the previous example, both mystring.toUpperCase() and "hello, world!".toUpperCase() return the string "HELLO, WORLD!"

The substring method takes two arguments and returns a subset of the string between the two arguments. Using the previous example, mystring.substring(4, 9) returns the string "o, Wo". See the substring method of the String object in the JavaScript Reference for more information.

The String object also has a number of methods for automatic HTML formatting, such as bold to create boldface text and link to create a hyperlink. For example, you could create a hyperlink to a hypothetical URL with the link method as follows:

mystring.link("https://www.helloworld.com")

The following table summarizes the methods of String objects.