JavaScript String Methods

Extracting Substrings

Use slice() to extract a portion of a string by start and end index.

const text = "Hello World";
text.slice(0, 5); // "Hello"
text.slice(-5);   // "World"

Use substring() for similar behavior but treating negative numbers as zero.
```
"Hello".substring(1, 4); // "ell"
```
Use substr() with length instead of an end index, but avoid it.
```
"Hello".substr(1, 3); // "ell"
```
Slice is preferred over substring and substr for modern code.
Negative indices in slice count from the end of the string.

Use indexOf() to find the first position of a substring.

"hello world".indexOf("world"); // 6
"hello world".indexOf("xyz");   // -1

Use includes() for a simple boolean check without the index.
```
"hello world".includes("world"); // true
```

Use startsWith() and endsWith() to check string boundaries.

"hello".startsWith("he");   // true
"hello".endsWith("lo");     // true

Use lastIndexOf() to find the last occurrence of a substring.
Use search() with a regex to find the index of the first pattern match.
```
"hello123".search(/\d+/); // 5 — index where digits start
```

Use replace() to replace the first occurrence of a substring.

"hello world".replace("world", "there"); // "hello there"

Use replaceAll() to replace every occurrence without a loop.
```
"hello hello".replaceAll("hello", "hi"); // "hi hi"
```
Use a regex for advanced replacements and pattern matching.
```
"hello123".replace(/[0-9]/g, "X"); // "helloXXX"
```
Use toUpperCase() and toLowerCase() for case conversion.
```
"Hello".toUpperCase(); // "HELLO"
```

Use a function as the replace callback to compute each replacement.

"hello world".replace(/\b\w/g, (ch) => ch.toUpperCase());
// "Hello World"

Use split() to break a string into an array by a delimiter.

"a,b,c".split(","); // ["a", "b", "c"]
"hello".split("");   // ["h", "e", "l", "l", "o"]

Use join() to combine an array back into a string.
```
["a", "b", "c"].join(","); // "a,b,c"
```
Combine split and join for text transformations like removing spaces.
```
"h e l l o".split(" ").join(""); // "hello"
```
Pass a limit as the second argument to split for partial results.
```
"a,b,c,d".split(",", 2); // ["a", "b"]
```

Split by regex to handle multiple delimiters at once.

"one, two; three".split(/[,;]\s*/); // ["one", "two", "three"]

Use trim() to remove whitespace from both ends of a string.
```
"  hello  ".trim(); // "hello"
```
Use trimStart() or trimLeft() to remove only leading whitespace.
```
"  hello".trimStart(); // "hello"
```
Use trimEnd() or trimRight() for trailing whitespace only.
```
"hello  ".trimEnd(); // "hello"
```

Use padStart() and padEnd() to add characters for fixed-width strings.

"5".padStart(3, "0");  // "005"
"5".padEnd(3, "0");    // "500"

Use repeat() to create a string repeated a given number of times.
```
"-".repeat(10); // "----------"
```

Tip: Use includes() instead of indexOf() !== -1 for cleaner boolean checks on string presence.

Warning: substr() is deprecated, so use slice() or substring() instead for extracting portions of strings.