How Can I Check if a String Starts With Another String in JavaScript?

Checking String Start with Another String in JavaScript

In JavaScript, you can use the String.prototype.startsWith() method to check if a string begins with a specified substring. However, in older browsers, this method may not be supported.

ECMAScript 6 Solution (ES6)

If you are working with a browser that supports ECMAScript 6 (ES6), the simplest solution is to use the String.prototype.startsWith() method. It's supported in all major browsers today.

console.log("Hello World!".startsWith("He")); // true

var haystack = "Hello world";
var prefix = 'orl';
console.log(haystack.startsWith(prefix)); // false

Shim/Polyfill for Unsupported Browsers

If you need to support older browsers, you can use a shim/polyfill to add the startsWith() method. Here are two popular options:

• Matthias Bynens's String.prototype.startsWith shim
• The es6-shim (shims the entire ES6 spec as much as possible, including String.prototype.startsWith)

Once you've shimmed the method, you can use it as described above.

Custom Implementation

If you prefer to create your own implementation, it should follow the algorithm outlined in the ECMAScript 6 specification. This involves checking if the substring's length is greater than the string's length, and then iterating over the first n characters of the string, where n is the length of the substring, and comparing them to the substring characters.

Conclusion

By using the String.prototype.startsWith() method or a suitable polyfill, you can easily check if a string starts with another substring in JavaScript, regardless of browser compatibility.

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