JavaScript — String Methods

Remember the rule

Strings never change — every method returns a new string:

const msg = "  Hello World  ";
msg.toUpperCase();   // "  HELLO WORLD  " (new)
msg;                 // "  Hello World  " (untouched)

Case

"hi".toUpperCase();     // "HI"
"HI".toLowerCase();     // "hi"   ← normalize before comparing!

Case-insensitive check idiom:

input.toLowerCase() === "yes"

Extracting pieces

const email = "ada@lovelace.dev";

email.slice(0, 3);       // "ada"      start index, end (excluded)
email.slice(4);          // "lovelace.dev"  to the end
email.slice(-4);         // ".dev"     negatives count from the right
email.at(-1);            // "v"        modern last-char access

Finding things

"banana".indexOf("na");      // 2   first position, -1 if absent
"banana".includes("ana");    // true
"banana".startsWith("ban");  // true
"file.PDF".endsWith(".pdf"); // false — case matters!

includes/startsWith/endsWith return booleans — perfect for if; indexOf when you need where.

Replacing

"a-b-c".replace("-", "+");        // "a+b-c"  FIRST occurrence only!
"a-b-c".replaceAll("-", "+");     // "a+b+c"
"Hello world".replace(/o/g, "0"); // regex flag g = every match

The first-occurrence-only surprise is a rite of passage.

Trimming whitespace

"  hi \n".trim();       // "hi"
"  hi".trimStart();     // left only
"hi  ".trimEnd();       // right only

Mandatory cleanup for form input and pasted data.

Splitting & joining

"red,green,blue".split(",");   // ["red","green","blue"] → ARRAY
["a","b"].join("-");           // "a-b"                  → STRING
"abc".split("");               // ["a","b","c"] chars

Split/join combo does global replaces without regex:

path.replaceAll("\\", "/");    // or: path.split("\\").join("/")

Padding & repeating

"5".padStart(3, "0");   // "005" — clock/ticket formatting
"7".padEnd(3, ".");     // "7.."
"ab ".repeat(3);        // "ab ab ab "

Chaining — the real style

Methods compose because each returns a string:

const raw = "  Ada Lovelace  ";
const handle = raw.trim().toLowerCase().replaceAll(" ", ".");
// "ada.lovelace"

Read left-to-right like a pipeline: trim → lower → join with dots.

Mini Practice

1. Normalize five user emails: trim + lowercase + verify includes("@")
2. Extract domain from "user@example.com" using slice/indexOf
3. Global-replace every comma in a sentence with a line break (\n)
4. Format seconds 5→"05", 105→"1:45" using padStart + arithmetic
5. Build filename checker: endsWith(".pdf"), case-insensitive

Next: numbers →