Podemos verificar que la string dada comience con los caracteres de la string especificada o no mediante varios métodos en javascript que se describen a continuación:
Método 1: este es un enfoque simple en el que uniremos los caracteres uno por uno desde el inicio usando el ciclo y si algún carácter no coincide, podemos decir que la string no comienza con el carácter o la string especificada.
Sintaxis:
for (var i = 0; i < pattern.length; i++) {
if(str.charAt(i) != pattern.charAt(i)){
result = false;
break;
}
}
Ejemplo: El siguiente programa demuestra el enfoque anterior:
<script>
// 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);
</script>
Producción:
String = "GeeksforGeeks" String should begin with = "Geeks" String begins with "Geeks" String = "geeksforgeeks" String should begin with = "Geeks" String doesn't begins with "Geeks"
Método 2: En este método, usaremos la función substring() para obtener la substring de la longitud requerida de la string del patrón y luego uniremos la substring con el patrón usando la función localeCompare() .
Sintaxis:
var = string.substring(Startindex, Endindex) var.localeCompare(compareString)
Ejemplo: El siguiente programa demuestra el enfoque anterior:
<script>
// 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);
</script>
Producción:
String = "GeeksforGeeks" String should begin with = "Geeks" String begins with "Geeks" String = "geeksforgeeks" String should begin with = "Geeks" String doesn't begins with "Geeks"
Método 3: esta es la mejor solución sobre todo, en este método usaremos el método beginWith() para verificar directamente si la string dada comienza con algo o no.
Sintaxis:
str.startsWith( searchString , position )
Ejemplo: El siguiente programa demuestra el enfoque anterior:
<script>
// 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);
</script>
Producción:
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"