How to Check String Prefix with JavaScript string startswith

In this article, you will have to learn how to Check String Prefixes with JavaScript string startswith.

One of the valuable function is startswith(), which enables you to check whether a string starts with a specific substring.

We will discuss the different applications of startswith() function and its related concepts, providing you a detailed understanding of how to manipulate strings in JavaScript like an expert.

What is the startswith() Function in JavaScript?

The startswith() function in JavaScript is a string method that defines if a given string starts with the characters of another stated string.

It returns a Boolean value, if it is true the string begins with the stated substring, and false if not.

Here’s the syntax to use with startswith() function:

string.startswith(searchString);

Where a string is the initial string on which the method is called, and searchString is the substring need be searched for at the starting of the original string.

How to Use startswith() in JavaScript?

Using the startswith() function is relatively easy. You can easily apply it to the string you want to check.

Here’s an example code that uses startswith():

const char = "Welcome, Itsourcecode!";
const message = "Welcome";

if (char.startsWith(message)) {
  console.log("The string begins with 'Welcome'");
} else {
  console.log("The string doesn't start with 'Welcome'");
}

In this code example, we use the startsWith() function to check if the char starts with the message.

If it is work, it will log “The string begins with ‘Welcome’“, If not, it will log “The string doesn’t start with ‘Welcome’“.

Now, let’s dive in to understand the applications of startswith() in JavaScript.

In this section, we will discuss the basic syntax and usage of startswith(), along with a practical example to explain its functionality.

Using Case-Insensitive startswith()

If we want to implement a case-insensitive check, we can convert both the string and searchString to lowercase or uppercase before using startswith().

In this method, the function will forget the case of the characters and offer the proper result.

Let’s see an example code:

const message = "Hi, Itsourcecode!";
const searchString = "Hi";

if (message.toLowerCase().startsWith(searchString.toLowerCase())) {
  console.log("The string begins with 'Hi'");
} else {
  console.log("The string doesn't begin with 'Hi'");
}

In this JavaScript code example, we check if the message begins with the searchString (case-insensitive) and displays a proper message in the console.

Using the trim() method

One method to manage leading whitespace is by using the trim() method to eliminate any whitespace from the starting and end of the string.

This is to make sure that the startswith() function only considers the actual content when defining either the string begins with the defined substring.

For example:

const message = "Hello, Itsourcecode!";
const searchString = "Hello";

if (message.trim().startsWith(searchString)) {
  console.log("The trimmed begins with 'Hello'");
} else {
  console.log("The trimmed doesn't begin with 'Hello'");
}

In this example code, the output will be “The trimmed string starts with ‘Hello’“, as we have used trim() to eliminate the leading whitespace before using startswith().

Using Regular Expressions

Another way to handle leading whitespace is by using regular expressions to match and eliminate any whitespace at the beginning of the string.

Let’s take a look at the example code:

const message = "    Hi, Welcome to Itsourcecode Tutorial!";
const searchString = "Hi";

const trimmedStringFunction = message.replace(/^\s+/, '');

if (trimmedStringFunction.startsWith(searchString)) {
  console.log("The trimmed string begins with 'Hi'");
} else {
  console.log("The trimmed string does not begin with 'Hi'");
}

In this example, we use a regular expression /^\s+/ to match one or more whitespace characters at the beginning of the message and change them with an empty string.

The resulting trimmedStringFunction will have no leading whitespace, and the startswith() function will produce the correct outcome.

Now that we have already understood how to handle whitespace in strings, let’s proceed on checking for multiple prefixes within a string.

Using startswith() with Multiple Prefixes

To check for multiple prefixes, you can use a loop or a combination of conditional statements.

The loop iterates through an array of prefixes, while the conditional statements compare the string with each prefix individually.

Let’s have a look at the example that uses a loop():

const message = "Today is a sunny day!";
const prefixesSample = ["Today", "Tomorrow", "Yesterday"];

for (const prefix of prefixesSample) {
  if (message.startsWith(prefix)) {
    console.log(`The string begins with '${prefix}'`);
    break;
  }
}

In this example, the output will be “The string begins with ‘Today’“, as the message starts with the prefix “Today“.

On the other hand, you can use conditional statements to perform the same check.

Here’s an example code:

const message = "Today is a sunny day!";
const prefixes = ["Today", "Tomorrow", "Yesterday"];

if (message.startsWith(prefixes[0])) {
  console.log(`The string begins with '${prefixes[0]}'`);
} else if (message.startsWith(prefixes[1])) {
  console.log(`The string begins with '${prefixes[1]}'`);
} else if (message.startsWith(prefixes[2])) {
  console.log(`The string begins with '${prefixes[2]}'`);
} else {
  console.log("The string doesn't start with any of the defined prefixes");
}

In this example code, the output will also be “The string begins with ‘Today’“, as message starts with the first prefix “Today“.

Finding the Index Position of Startswith

To find the index position of the starting substring, you can merge the startswith() with the indexOf() method.

The indexOf() function returns the position of the first occurrence of a defined value in a string.

For example:

const message = "Welcome to the tutorial of startof!";
const searchString = "Welcome";

const indexPositionSample = message.indexOf(searchString);

if (indexPositionSample !== -1) {
  console.log(`The string begins with '${searchString}' at index ${indexPositionSample}`);
} else {
  console.log("The string doesn't begins with the defined substring");
}

In this example code, the output will be “The string begins with ‘Welcome’ at index 0“, as the message starts with the substring “Welcome” which is found at index position 0.

Now that we understand how to find the index position of the starting substring, let’s see how startswith() can be used within conditional statements for decision-making.

Combining startswith() with if-else Statements

In JavaScript, if-else statements are significant for decision-making. They enable you to execute various blocks of code based on certain conditions.

By combining startswith() with if-else statements, you can execute specific actions or validations depending on whether a string starts with a particular substring or not.

Here’s an example code that uses if-else statement:

const message = "JavaScript string startswith() tutorial";

if (message.startsWith("JavaScript")) {
  console.log("The string begins with 'JavaScript'");
} else {
  console.log("The string does not begins with 'JavaScript'");
}

In this example code, the output will be “The string begins with ‘JavaScript’“, as the message starts with the substring “JavaScript“

Advanced Techniques with startswith()

JavaScript developers can use startswith() effectively in different cases to solve complex problems and increase code efficiency.

In this section, we will discuss some advanced techniques and different use cases of startswith() in JavaScript.

Prefix Filtering

In certain cases, you might be required to filter an array of strings based on either they start with a particular prefix.

The startswith() function can shorten this process.

Here’s an example code:

const name = ["jude", "glenn", "caren", "eliver", "paul"];
const prefixSample = "c";

const wordsStartingWithPrefixSample = name.filter(word => word.startsWith(prefixSample));

console.log(wordsStartingWithPrefixSample);

In this example code, the wordsStartingWithPrefixSample array consists of only the name “caren” as they are the only ones that start with the prefix “c“.

Dynamic Prefix Validation

You can use startswith() to perform dynamic prefix validation in user input situations.

For example:

const userInputSample = "javascript is";


const validPrefixesSample = ["javascript", "laravel", "C++", "php"];


for (const prefix of validPrefixesSample) {
  if (userInputSample.startsWith(prefix)) {
    console.log(`Suggestions for ${prefix}: ...`);
    break;
  }
}

In this example code, if the userInputSample starts with one of the valid prefixes (“javascript”, “laravel”, “C++”, “php”), the relevant recommended will be displayed.

Extracting Data from CSV

When working with CSV (Comma-Separated Values) data, you can use startswith() to extract exact rows based on a particular column value.

Example code:

const csvDataSample = [
  "Jude,Delacruz,32,Male",
  "Ronito,Miguel,29,Male",
  "Jona,Jerson,38,Female",
  "Caren,Manggana,24,Female"
];

function extractRowsWithPrefix(data, prefix) {
  return data.filter(row => row.startsWith(prefix));
}

const prefix = "Jude";
const johnRows = extractRowsWithPrefix(csvDataSample, prefix);

console.log(johnRows); 

In this example, the jude Rows array consists of the row with the prefix “jude ” from the CSV data.

Best Practices for Efficient String Manipulation

• Use startswith() with Precise Prefixes
• Handle Case Sensitivity Carefully
• Consider Trimming Leading Whitespace
• Combine startswith() with Other String Methods
• Optimize Loops for Multiple Prefixes
• Validate User Input

FAQs

Conclusion

In conclusion, startswith() is a powerful and functional string method in JavaScript that enables you to check whether a string starts with a specific substring.

It can be used for different purposes, such as conditional checks, validation, filtering arrays, and custom URL routing.

Additional Resources

• How to use String.valueOf() JavaScript?
• Advanced Usage of Template Strings JavaScript
• How to square a number in JavaScript?