JS String Search

JavaScript String Search: Finding Substrings and Patterns

JavaScript provides built-in search methods to check if text contains a word, locate its index, or match regular expression patterns. Choosing the right search method keeps your conditions readable and avoids fragile index checks.

Choosing the Right String Search Method

Each string search method answers a slightly different question:

• includes(sub): Returns true or false when checking whether a substring exists anywhere inside the text.
• startsWith(sub) / endsWith(sub): Checks prefixes and suffixes, which is ideal for URLs, file extensions, or command parsing.
• indexOf(sub) / lastIndexOf(sub): Returns the zero-based position of the first or last match, or -1 when nothing matches.
• matchAll(regex): Finds every regular expression match along with capture groups and positions.

Practical String Search Example

In real apps, normalize letter case with toLowerCase() before calling includes() so search filters behave predictably for users.

const filePath = "/uploads/invoices/2026-May-Report.pdf";
const query = "may";

const isPdf = filePath.toLowerCase().endsWith(".pdf");
const matchesQuery = filePath.toLowerCase().includes(query);
const firstSlash = filePath.indexOf("/", 1); // Start search after index 0

const prices = "Items: $12, $45, and $99";
const foundNumbers = [...prices.matchAll(/\$(\d+)/g)].map((m) => Number(m[1]));

console.log({ isPdf, matchesQuery, firstSlash, foundNumbers });

Why includes() Beats indexOf() !== -1

Older JavaScript code checked str.indexOf("word") !== -1 or str.indexOf("word") >= 0. Modern ES6 str.includes("word") states your intent directly as a boolean and removes off-by-one confusion with 0 being falsy.

Common Mistakes to Avoid

• Forgetting case sensitivity: "JavaScript".includes("script") returns false. Always lowercase both strings for user search inputs.
• Treating indexOf() result 0 as falsy: Checking if (str.indexOf("js")) fails when "js" starts at index 0, because 0 is falsy.
• Calling matchAll() without the /g flag: Passing a regex without the global g flag to matchAll() throws a TypeError at runtime.

Frequently Asked Questions

What is the difference between search() and indexOf()?

indexOf() searches only for a plain string and accepts a starting index. search() accepts a regular expression, ignores a global flag, and always starts from index 0.

How do I count how many times a word appears in a string?

Use [...text.matchAll(/word/gi)].length, or split carefully on word boundaries when matching whole words.

Does includes() accept a starting position?

Yes, pass a second number argument like text.includes("api", 8) to begin searching from index 8 onward.