JS Foundations #11 — String Methods Reference

JavaScript strings have 30+ methods. This is a practical reference for the ones you'll use in interviews: searching, slicing, transforming, and pattern matching. Every method includes a runnable example.

1. Accessing Characters

charAt(index) / Bracket Notation [index]

Difference: charAt returns '' for out-of-bounds; bracket notation returns undefined.

at(index) — Negative Index Support

2. Searching

indexOf(substring, fromIndex?) / lastIndexOf(substring) — Find Position

includes(substring, fromIndex?) — Contains Check

startsWith(substring, fromIndex?) / endsWith(substring, length?) — Edge Checks

3. Extracting Substrings

slice(start, end?) — Extract by Index

Interview use: Getting all but the last part of a path, extracting segments.

substring(start, end) — Similar to Slice

Doesn't support negative indices. slice is preferred.

split(separator, limit?) — String → Array

Interview use: Path parsing in _.get() (#6), squashing/unsquashing keys (#16), multi-class splitting (#24).

4. Transforming

toLowerCase() / toUpperCase() — Case Conversion

Interview use: Case-insensitive tag comparison in DOM traversal (#24).

trim() / trimStart() / trimEnd() — Whitespace Removal

repeat(count) — Duplicate

padStart(length, padString?) / padEnd(length, padString?) — Padding

5. Replace & Match (Without Regex)

replace(substring, newValue) — Replace First Occurrence

To replace all without regex, use replaceAll.

replaceAll(substring, newValue) — Replace All

Interview use: Escaping special characters for JSON.stringify (#17), bracket-notation to dot-notation conversion (#16).

6. String Concatenation

concat(...strings) — Join Strings

Template literals `${a} ${b}` are preferred for readability.

7. Regex-Based Methods

match(regex) — Find Matches

Without the g flag, returns match details (groups, index). With g, returns all matches as an array.

search(regex) — Find Index of First Match

replace(regex, replacement) — Regex Replace

Interview use: Path parsing with dot/bracket notation (#6, #16), JSON string escaping (#17), bracket index extraction (#16).

Quick Reference

Interview Tips

• Use split for path parsing — 'a.b.c'.split('.') is cleaner than manual iteration.
• Use replaceAll for bulk replacement — no need for regex unless the pattern is complex.
• Use slice with negatives — str.slice(-1) for last char, str.slice(0, -1) for all but last.
• Case-insensitive comparison — .toLowerCase() both strings before comparing.

Related Articles

• #6 — Implement _.get()
• #16 — Implement squash()
• #17 — Implement JSON.stringify()
• Regular Expressions in JavaScript