String Methods

                str.trim() / str.slice(start, end) / str.includes(sub) / str.replace(a, b) / str.split(sep)
              

Purpose

String.prototype provides methods for searching, extracting, and transforming text. None of them mutate the string they’re called on — strings are primitives, and primitives can’t be mutated — every method returns a new string (or a different type, like an array or boolean).

Common methods

'  Hello, World!  '.trim(); // "Hello, World!"
'hello'.slice(1, 3); // "el"
'Hello'.toLowerCase(); // "hello"
'Hello, World'.includes('World'); // true
'Hello, World'.replace('World', 'JS'); // "Hello, JS"
'a,b,c'.split(','); // ["a", "b", "c"]
'5'.padStart(2, '0'); // "05"
'hello'.at(-1); // "o" — negative index from the end

Behavior notes

• slice(start, end) accepts negative indexes, counting from the end of the string.
• replace(a, b) only replaces the first match unless the first argument is a regular expression with the global flag (/pattern/g).
• split() with no argument returns an array containing the whole string as one element; split('') splits into individual characters (though this can split multi-code-point characters incorrectly — see the Unicode note below).
• Comparison and length behave per UTF-16 code unit, not per visible character — some emoji and accented characters occupy more than one code unit, so .length can report a larger number than the visible character count suggests.

Common mistakes

• Calling a string method and expecting it to mutate the original (str.trim(); console.log(str);) — the return value must be captured (str = str.trim();) or used directly.
• Assuming .length always equals the number of “characters” a person would count — true for plain ASCII text, not guaranteed for all Unicode text.

Runtime notes

Standard String.prototype methods, identical across every current browser and Node version.