如何检查以某些给定字符/pattern 开头的字符串？

我们可以通过javascript中的各种方法检查给定的字符串是否以指定字符串的字符开头，如下所述：

方法1：这是一种简单的方法，我们将从使用循环开始逐个匹配字符，如果任何字符不匹配，那么我们可以说字符串不以字符或指定字符串开头。

句法：

for (var i = 0; i < pattern.length; i++) {
        if(str.charAt(i) != pattern.charAt(i)){
            result = false;
            break;
        }
      }

示例：下面的程序演示了上述方法：

    // Javascript script to check 
    // whether the String begins
    // with something or not
      
    // Function to check String
    // begins with something or not
    function check(str, pattern){
      // Initially we assume that String
      // begins with something
      // so result is true
      var result = true;
        
      // Loop to match character by character
      for (var i = 0; i < pattern.length; i++) {
          
        // If any character doesn't matches 
        // then result is false
        if(str.charAt(i) != pattern.charAt(i)){
            result = false;
            break;
        }
      }
        
      if(result){
        document.write("String begins with \""
                + pattern + "\"<br><br>");
        }
      else{
        document.write("String  doesn't " + 
            "begins with \"" + pattern + "\"<br><br>");
        }
    }
  
    // Driver code
    // String to check
    var str = "GeeksforGeeks";
      
    // Pattern by which string 
    // begins or not
    var pattern = "Geeks";
      
    document.write("String = \"" + str + "\"<br>");
    document.write("String should begin with  = \""
            + pattern + "\"<br>");
      
    // Call check function
    check(str, pattern);
      
    // Change string
    str = "geeksforgeeks";
      
   document.write("String = \"" + str + "\"<br>");
    document.write("String should begin with  = \""
            + pattern + "\"<br>");
              
    // Calling check function
    check(str, pattern);
                    

输出：

String = "GeeksforGeeks"
String should begin with = "Geeks"
String begins with "Geeks"

String = "geeksforgeeks"
String should begin with = "Geeks"
String doesn't begins with "Geeks"

方法二：在这个方法中，我们将使用 substring()函数获取所需长度的模式字符串的子字符串，然后使用 localeCompare()函数将子字符串与模式匹配。

句法：

var = string.substring(Startindex, Endindex)
var.localeCompare(compareString)

示例：下面的程序演示了上述方法：

    // Javascript script to check 
    // whether the String begins
    // with something or not
      
    // Function to check String
    // begins with something or not
    function check(str, pattern){
      // Extracting substring of
      // pattern length
      var sub_str = 
            str.substring(0, pattern.length);
        
      if(sub_str.localeCompare(pattern) == 0){
        document.write("String begins with \""
                + pattern + "\"<br><br>");
        }
      else{
        document.write("String  doesn't " + 
            "begins with \"" + pattern + "\"<br><br>");
        }
    }
  
    // Driver code
    // String to check
    var str = "GeeksforGeeks";
      
    // Pattern by which string 
    // begins or not
    var pattern = "Geeks";
      
    document.write("String = \"" + str + "\"<br>");
    document.write("String should begin with  = \""
            + pattern + "\"<br>");
      
    // Call check function
    check(str, pattern);
  
    // Change string
    str = "geeksforgeeks";
      
   document.write("String = \"" + str + "\"<br>");
    document.write("String should begin with  = \""
            + pattern + "\"<br>");
              
    // Calling check function
    check(str, pattern);
                    

输出：

String = "GeeksforGeeks"
String should begin with = "Geeks"
String begins with "Geeks"

String = "geeksforgeeks"
String should begin with = "Geeks"
String doesn't begins with "Geeks"

方法3：这是最好的解决方案，在这个方法中我们将使用startsWith()方法直接检查给定的字符串是否以某物开头。

句法：

str.startsWith( searchString , position )

示例：下面的程序演示了上述方法：

    // Javascript script to check 
    // whether the String begins
    // with something or not
      
    // Function to check String
    // begins with something or not
    function check(str, pattern){
        
      if(str.startsWith(pattern)){
        document.write("String begins with \""
                + pattern + "\"<br><br>");
        }
      else{
        document.write("String  doesn't " + 
            "begins with \"" + pattern + "\"<br><br>");
        }
    }
  
    // Driver code
    // String to check
    var str = "Burn to shine";
      
    // Pattern by which string 
    // begins or not
    var pattern = "Burn";
      
    document.write("String = \"" + str + "\"<br>");
    document.write("String should begin with  = \""
            + pattern + "\"<br>");
      
    // Call check function
    check(str, pattern);
      
    // Change string
    str = "Happy coding";
    // Change pattern
    pattern = "happy";
      
   document.write("String = \"" + str + "\"<br>");
    document.write("String should begin with  = \""
            + pattern + "\"<br>");
              
    // Calling check function
    check(str, pattern);
                    

输出：

String = "Burn to shine"
String should begin with = "Burn"
String begins with "Burn"

String = "Happy coding"
String should begin with = "happy"
String doesn't begins with "happy"