Using Javascript string objects

Using the build-in String object.

  • You can create an object with a member variable that is a primitive datatype string with this Javascript build-in String object.
  • You use the new keyword to create an object that contains a primitive string.
    var obj_num= new String("Hello");
  • This Javascript build-in String object is also a wrapper for all primitive strings.
  • This means you can use the properties and methods contained in the String object, not only on all new-created string objects, but also on all primitive strings.
    <script type="text/javascript">
      // create a primitive string
      var prim_str1 = "You can create primitive strings or String objects";
      document.write(prim_str1+"<br>");
      // The same creating an object with a primitive string
      var obj_str1 = new String("You can create primitive strings or String objects");
      document.write(obj_str1+"<br>");
    </script>
  • Javascript will automatically use the valueOf() method, which exits in the String object, when there is a need to convert a String object value to a primitive value.
    For example:
    <script type="text/javascript">
     // First a primitive number.
     var prim_str1="Javascript uses the valueOf() method" ;
     // Here is the same, using an object.
     var obj_str1= new String("Javascript uses the valueOf() method");
     document.write("Javascript uses the valueOf() method "+
                       " before comparing strings<br>");
     if (prim_str1==obj_str1){
       document.write("Strings are equal<br>");
     }
     </script>
  • As you probably already have seen there is no good reason to create string objects.
  • At last; You can use the String object as a function to create a primitive string.
  • Whether you create an object, or use the String object as a function, you can convert a strings with a number to a number object or a primitive number value.
    For example:
    <script type="text/javascript">
      // create a primitive string with numbers
      var prim_str1 = 6 +" hotdogs and "+ 2 + " pizza please!";
      document.write(prim_str1+"<br>");
      // The same; create an object with a primitive string with numbers
      var obj_str1 = new String(6 +" hotdogs and "+ 2 + " pizza please!");
      document.write(obj_str1+"<br>");
    </script>
  • A short notation for converting a single number to a string is:
    var prim_str1 = 6.461 +""
    // prim_str1 will then contains the string; '6.462' 

Most used string methods.

  • Concatenate strings

    You can concatenate strings in Javascript using the + operator:
    <script type="text/javascript">
    var message = "Hello, " + "mister";   // Produces the string "Hello, mister"
    document.write(message+"<br>");
    </script>
  • Length of a string

    You can find the length of a string (the number of characters it contains) using the length property of the string.
    <script type="text/javascript">
    var str1="This string has a lot of characters";
    document.write("The 'str1' string contains "+str1.length+" characters.<br>");
    </script>
  • Get a char from a string

    All characters in a string has an index starting with 0 as the first character and length-1 as the last character. You can get any character from the string using the charAt() method.
    <script type="text/javascript">
    var str1="This string has a lot of characters";
    document.write("The last character of the string is: "+
                    str1.charAt(str1.length - 1)+"<br>");
    </script>
  • Index of a char in a string

    All characters in a string has an index starting with 0 as the first character and length-1 as the last character. You can get any index of string in a string using the indexOf() method.
    <script type="text/javascript">
      var str1="This string has a lot of characters";
      document.write("The string 'lot' starts at index: "+
        str1.indexOf("lot")+" in the string:<br>");
      document.write(str1+"<br>");
    </script>
  • Get a string from a string

    You can get any string from the string using the substring() method or the substr() method.
    <script type="text/javascript">
      var str1="This string has a lot of characters";
      // using the substring(from, to);
      document.write("String from index 18 up to index 21 is: '"+
        str1.substring(18, 21)+"' in the string:<br>");
      document.write(str1+"<br>");
       // using the substr(from, length);
      document.write("String from index 18 with length 3 is: '"+
        str1.substr(18, 3)+"' in the string:<br>");
      document.write(str1+"<br>");
    </script>
  • Replace a string with a string in a string

    You replase any string with a string in a string using the replace() method.
  • This method has two parameters (regexp and replacement):
    1. If the regexp argument is a string, it is used as a literal text pattern to be searched for in the string. Else you can specify a RegExp pattern to be searched for in the string.
    2. The second argument, replacement, is the string for replacement.
    <script type="text/javascript">
      var str1="This#string#has#a#lot#of#characters";
      document.write(str1+"<br>");
      document.write("Now we replace all # with a space character in this string:<br>");
      document.write(str1.replace(/#/g," ")+"<br>");
    </script>

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