Are JavaScript strings mutable?

No. Strings are immutable primitives, so no method changes the string in place. Methods like toUpperCase, replace, and trim all return a brand-new string and leave the original unchanged. You must assign the result to keep it.

Difference between slice, substring, and substr?

slice(start, end) accepts negative indices counting from the end. substring(start, end) clamps negatives to 0 and swaps the arguments if start is greater than end. substr(start, length) is legacy and takes a length instead of an end index. Prefer slice.

What does normalize do?

normalize returns the Unicode Normalization Form of a string. The same visible character can be encoded differently, for example e with an accent as one code point or as e plus a combining accent. normalize('NFC') unifies them so comparisons and searches match reliably.

How does replace differ from replaceAll?

replace only replaces the first match when given a string pattern, or all matches when given a global regular expression. replaceAll replaces every occurrence of a string pattern directly, and throws if given a non-global regex. Use replaceAll for clear all-occurrence replacement.

Why does length count emoji as two?

length counts UTF-16 code units, and characters outside the Basic Multilingual Plane such as many emoji are stored as a surrogate pair of two code units. To count visible characters, iterate with the spread operator or for...of, which yield whole code points.

JavaScript String Methods

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JavaScript strings are immutable, so every method here returns a new string or a derived value rather than changing the original. This reference lists each String.prototype method with its signature, return type, and the unicode gotchas that trip people up.

How it works

The methods group into a few jobs:

Search — indexOf, includes, startsWith, endsWith, match, search locate text.
Slice — slice, substring, substr, at extract portions.
Transform — toUpperCase, toLowerCase, trim, pad*, replace, replaceAll, repeat, normalize produce a new string.
Convert — split, charAt, charCodeAt, codePointAt, localeCompare move between strings, arrays, and numbers.

Because strings are immutable, none of these mutate this; you always keep the return value.

Tips

Prefer slice over substring/substr for predictable negative-index behaviour. Use replaceAll for clear all-occurrence replacement and reserve replace for the first match or regex captures. When comparing user text, normalize('NFC') first so visually identical strings with different encodings compare equal. Remember length counts UTF-16 code units — to count visible characters iterate with for...of or [...str].length, which respect whole code points.