String Methods and Properties
Primitive values, like "John Doe", cannot have properties or methods (because they are not objects).
But with JavaScript, methods and properties are also available to primitive values, because JavaScript treats primitive values as objects when executing methods and properties.
String Length
The length property returns the length of a string:
Example
var txt = "ABCDEFGHIJKLMNOPQRSTUVWXYZ";
var sln = txt.length;
Finding a String in a String
The indexOf() method returns the index of (the position of) the first occurrence of a specified text in a string:
Example
var str = "Please locate where 'locate' occurs!";
var pos = str.indexOf("locate");
The lastIndexOf() method returns the index of the last occurrence of a specified text in a string:
Example
var str = "Please locate where 'locate' occurs!";
var pos = str.lastIndexOf("locate");
Both the indexOf(), and the lastIndexOf() methods return -1 if the text is not found.
JavaScript counts positions from zero.
0 is the first position in a string, 1 is the second, 2 is the third ...
0 is the first position in a string, 1 is the second, 2 is the third ...
Both methods accept a second parameter as the starting position for the search:
Example
var str = "Please locate where 'locate' occurs!";
var pos = str.indexOf("locate",15);
Searching for a String in a String
The search() method searches a string for a specified value and returns the position of the match:
Example
var str = "Please locate where 'locate' occurs!";
var pos = str.search("locate");
Did You Notice?
The two methods, indexOf() and search(), are equal?
They accept the same arguments (parameters), and return the same value?
The two methods are quite equal. These are the differences:
- The search() method cannot take a second start position argument.
- The search() method can take much more powerful search values (regular expressions).
You will learn more about regular expressions in a later chapter.
Extracting String Parts
There are 3 methods for extracting a part of a string:
- slice(start, end)
- substring(start, end)
- substr(start, length)
The slice() Method
slice() extracts a part of a string and returns the extracted part in a new string.
The method takes 2 parameters: the starting index (position), and the ending index (position).
This example slices out a portion of a string from position 7 to position 13:
Example
var str = "Apple, Banana, Kiwi";
var res = str.slice(7, 13);
The result of res will be:
Banana
If a parameter is negative, the position is counted from the end of the string.
This example slices out a portion of a string from position -12 to position -6:
Example
var str = "Apple, Banana, Kiwi";
var res = str.slice(-12, -6);
The result of res will be:
Banana
If you omit the second parameter, the method will slice out the rest of the string:
Example
var res = str.slice(7);
or, counting from the end:
Example
var res = str.slice(-12);
Negative positions do not work in Internet Explorer 8 and earlier.
The substring() Method
substring() is similar to slice().
The difference is that substring() cannot accept negative indexes.
Example
var str = "Apple, Banana, Kiwi";
var res = str.substring(7, 13);
The result of res will be:
Banana
If you omit the second parameter, substring() will slice out the rest of the string.
The substr() Method
substr() is similar to slice().
The difference is that the second parameter specifies the length of the extracted part.
Example
var str = "Apple, Banana, Kiwi";
var res = str.substr(7, 6);
The result of res will be:
Banana
If the first parameter is negative, the position counts from the end of the string.
The second parameter can not be negative, because it defines the length.
If you omit the second parameter, substr() will slice out the rest of the string.
Replacing String Content
The replace() method replaces a specified value with another value in a string:
Example
str = "Please visit Microsoft!";
var n = str.replace("Microsoft", "W3Schools");
The replace() method does not change the string it is called on. It returns a new string.
By default, the replace() function replaces only the first match:
Example
str = "Please visit Microsoft and Microsoft!";
var n = str.replace("Microsoft", "W3Schools");
To replace all matches, use a regular expression with a /g flag (global match):
Example
str = "Please visit Microsoft and Microsoft!";
var n = str.replace(/Microsoft/g, "W3Schools");
By default, the replace() function is case sensitive. Writing MICROSOFT (with upper-case) will not work:
Example
str = "Please visit Microsoft!";
var n = str.replace("MICROSOFT", "W3Schools");
To replace case insensitive, use a regular expression with an /i flag (insensitive):
Example
str = "Please visit Microsoft!";
var n = str.replace(/MICROSOFT/i, "W3Schools");
You will learn a lot more about regular expressions in the chapter JavaScript Regular Expressions.
Converting to Upper and Lower Case
A string is converted to upper case with toUpperCase():
Example
var text1 = "Hello World!"; // Stringvar text2 = text1.toUpperCase(); // text2 is text1 converted to upper
A string is converted to lower case with toLowerCase():
Example
var text1 = "Hello World!"; // Stringvar text2 = text1.toLowerCase(); // text2 is text1 converted to lower
The concat() Method
concat() joins two or more strings:
Example
var text1 = "Hello";
var text2 = "World";
var text3 = text1.concat(" ", text2);
The concat() method can be used instead of the plus operator. These two lines do the same:
Example
var text = "Hello" + " " + "World!";
var text = "Hello".concat(" ", "World!");
All string methods return a new string. They don't modify the original string.
Formally said: Strings are immutable: Strings cannot be changed, only replaced.
Formally said: Strings are immutable: Strings cannot be changed, only replaced.
Extracting String Characters
There are 2 safe methods for extracting string characters:
- charAt(position)
- charCodeAt(position)
The charAt() Method
The charAt() method returns the character at a specified index (position) in a string:
Example
var str = "HELLO WORLD";
str.charAt(0); // returns H
The charCodeAt() Method
The charCodeAt() method returns the unicode of the character at a specified index in a string:
Example
var str = "HELLO WORLD";
str.charCodeAt(0); // returns 72
Accessing a String as an Array is Unsafe
You might have seen code like this, accessing a string as an array:
var str = "HELLO WORLD";
str[0]; // returns H
This is unsafe and unpredictable:
- It does not work in all browsers (not in IE5, IE6, IE7)
- It makes strings look like arrays (but they are not)
- str[0] = "H" does not give an error (but does not work)
If you want to read a string as an array, convert it to an array first.
Converting a String to an Array
A string can be converted to an array with the split() method:
Example
var txt = "a,b,c,d,e"; // Stringtxt.split(","); // Split on commastxt.split(" "); // Split on spacestxt.split("|"); // Split on pipe
If the separator is omitted, the returned array will contain the whole string in index [0].
If the separator is "", the returned array will be an array of single characters:
Example
var txt = "Hello"; // Stringtxt.split(""); // Split in characters
No comments:
Post a Comment