Example Codes for Checking if a String Starts With Another String in JavaScript

Using the startsWith() method:

• The startsWith() method is directly built into JavaScript strings.
• It takes one or two arguments:
  • The first argument is the substring you want to check for at the beginning of the main string.
  • The second argument (optional) is an integer specifying the index from which to start searching within the main string.
• It returns a boolean value:
  • true if the main string starts with the substring.
  • false otherwise.

Example:

let mainString = "Hello, world!";
let substring = "Hello";

if (mainString.startsWith(substring)) {
  console.log("The main string starts with the substring.");
} else {
  console.log("The main string does not start with the substring.");
}

Output:

The main string starts with the substring.

Additional notes:

• The startsWith() method is case-sensitive.
• If the second argument is provided, the search starts at the specified index. For example, mainString.startsWith(substring, 3) would check if the main string starts with "llo" at index 3.

Example Codes for Checking if a String Starts With Another String in JavaScript

This is the most straightforward way to check if a string starts with another string in JavaScript.

let mainString = "Hello, world!";
let substring = "Hello";

if (mainString.startsWith(substring)) {
  console.log("The main string starts with the substring.");
} else {
  console.log("The main string does not start with the substring.");
}

In this example:

• mainString is the string we're checking.
• substring is the substring we want to find at the beginning.
• The startsWith() method returns true if mainString starts with substring, and false otherwise.

Using a regular expression:

You can also use a regular expression to check for a substring at the beginning of a string.

let mainString = "Hello, world!";
let substring = "Hello";

if (new RegExp("^" + substring).test(mainString)) {
  console.log("The main string starts with the substring.");
} else {
  console.log("The main string does not start with the substring.");
}

• ^ is a special character in regular expressions that matches the beginning of a string.
• new RegExp("^" + substring) creates a regular expression that matches the substring at the beginning of the string.
• The test() method returns true if the regular expression matches the string, and false otherwise.

Using a custom function:

You can also create a custom function to check if a string starts with another string.

function startsWith(mainString, substring) {
  return mainString.indexOf(substring) === 0;
}

let mainString = "Hello, world!";
let substring = "Hello";

if (startsWith(mainString, substring)) {
  console.log("The main string starts with the substring.");
} else {
  console.log("The main string does not start with the substring.");
}

• The indexOf() method returns the index of the first occurrence of the substring in the main string.
• If the index is 0, it means the substring is at the beginning of the main string.

Using the slice() method:

• How it works: Extracts a portion of the string from the beginning and compares it with the substring.

let mainString = "Hello, world!";
let substring = "Hello";

if (mainString.slice(0, substring.length) === substring) {
  console.log("The main string starts with the substring.");
} else {
  console.log("The main string does not start with the substring.");
}

• How it works: Finds the index of the substring within the main string. If the index is 0, it means the substring is at the beginning.

let mainString = "Hello, world!";
let substring = "Hello";

if (mainString.indexOf(substring) === 0) {
  console.log("The main string starts with the substring.");
} else {
  console.log("The main string does not start with the substring.");
}

Using a regular expression with the ^ anchor:

• How it works: The ^ anchor matches the beginning of the string.

let mainString = "Hello, world!";
let substring = "Hello";

if (new RegExp("^" + substring).test(mainString)) {
  console.log("The main string starts with the substring.");
} else {
  console.log("The main string does not start with the substring.");
}