Checking if a String Starts With a Substring

String startsWith: String.startsWith()

The startsWith() method in JavaScript is used to determine whether a string begins with the characters of a specified substring. It returns a boolean value indicating the result of the comparison.

Syntax:

string.startsWith(searchString, position)

The parameters

1. searchString: The substring to search for at the beginning of the string.
2. position (optional): The position in the string to begin the search. It is an integer value, the default is 0. If a negative value is provided, it is treated as 0.

Return Value:

The startsWith() method returns true if the string starts with the specified substring; otherwise, it returns false.

Example

// Checking if a string starts with a specific substring
const str = 'Hello, world!';

const startsWithHello = str.startsWith('Hello');
console.log(startsWithHello);
// Output: true

In the example above, the startsWith() method is used to check if the string ‘Hello, world!’ starts with the substring ‘Hello’. The result, true, is stored in the variable startsWithHello.

// Checking if a string starts with a substring at a specific position
const str = 'Hello, world!';

const startsWithWorld = str.startsWith('world', 7);
console.log(startsWithWorld);
// Output: true

const startsWithHello = str.startsWith('hello', 7)
console.log(startsWithHello);
// Output: false

In this example, the startsWith() method is used to check if the substring ‘world’ starts at position 7 in the string ‘Hello, world!‘. The result, true, is stored in the variable startsWithWorld.

In a second example, the startsWith() method is again used, but this time to check if the substring ‘hello’ starts at position 7 in the string ‘Hello, world!‘. The startsWithHello variable will be assigned the value false because, starting from position 7, the substring ‘hello’ is not found at the beginning of the remaining part of the string.

Note

the comparison is case-sensitive, so ‘Hello’ and ‘hello’ are considered different in this context.

// Checking if a string starts with a substring at a position beyond the string length
const str = 'Hello, world!';

const startsWithBeyondLength = str.startsWith('world', 15);
console.log(startsWithBeyondLength);
// Output: false

In this example, the startsWith() method is used to check if the substring ‘world’ starts at position 15 in the string ‘Hello, world!‘. Since the position is beyond the length of the string, the result is false, which is stored in the variable startsWithBeyondLength.

// Checking if a string starts with a substring using a negative position
const str = 'Hello, world!';

const startsWithUniverseNegative = str.startsWith('universe', -20);
console.log(startsWithUniverseNegative);
// Output: false

const startsWithHelloNegative = str.startsWith('Hello', -20);
console.log(startsWithHelloNegative);
// Output: true

Note

If a negative value is provided, it is treated as 0. so, the search starts from the beginning of the string.

In this example, the startsWith() method is used to check if the string ‘Hello, world!’ starts with the substring ‘universe’ when starting the search from the beginning. The result, false, is stored in the variable startsWithHelloNegative.

In this second example, the startsWith() method is used to check if the string ‘Hello, world!’ starts with the substring ‘Hello’ when starting the search from beginning. Since ‘Hello’ is found at default index 0, the result is true.

By using the String.startsWith() method, you can efficiently check if a string begins with a specified substring, providing a useful tool for string comparison in JavaScript.