Learn

Strings are sequences of characters wrapped in quotes. JavaScript accepts three quote styles, all producing the same kind of string:

Pick a style and stick with it. Most projects use single quotes for normal strings, backticks when you need to interpolate (next lesson).

Length and indexing

Indexing is zero-based. Out-of-range returns undefined (not an error — a classic footgun).

Strings are IMMUTABLE

You can't change a character in place. Methods that "modify" actually return a NEW string:

Slicing — extract substrings

slice is the modern, recommended method. (Older code uses substring and substr — they have quirks; prefer slice.)

Common string methods

Iterating characters

for...of correctly handles Unicode (including emojis). Indexing with s[i] works on UTF-16 code units, which can split certain emojis.

Converting between types

Template literals also coerce automatically.

Common mistakes

• Trying to mutate: s[0] = "X" silently fails. Build a new string.
• Forgetting that replace only replaces the FIRST match — use replaceAll.
• Using indexOf's -1 in a truthy context: if (s.indexOf("x")) misfires both ways — a match at index 0 gives 0 (falsy, so the branch is skipped) while no match gives -1 (truthy, so the branch runs). Compare to -1 explicitly.