Essential String Methods

Strings in JavaScript come with powerful built-in methods to manipulate text.

String methods do not modify the original string (strings are immutable); they always return a new string.

Changing Case

You can easily change the case of a string.

const text = "Hello World";
console.log(text.toUpperCase());
console.log(text.toLowerCase());

HELLO WORLD
hello world

Trimming Whitespace

When accepting user input, you often need to clean up extra spaces. trim() removes whitespace from both ends of a string, but leaves internal spaces intact.

console.log('  hello \n'.trim());

hello

Extracting Substrings

You can extract a specific part of a string using slice().

const str = "CodeLudo";
console.log(str.slice(0, 4));

Code

A powerful feature of slice() is its support for negative indices, which count backwards from the end of the string.

const str = "CodeLudo";
const ending = str.slice(-4);
console.log(ending);

Ludo

substring() is an alternative that behaves similarly but handles arguments differently.

Why Two Similar Methods Exist

You might wonder why JavaScript has both slice() and substring() if they do almost the same thing. The language designers added substring() first in early versions of JavaScript. It is forgiving in a way that turned out to be a mistake: pass a start index greater than the end, say substring(4, 0), and it silently swaps them to 0, 4. Your typo returns a plausible string instead of an obvious wrong answer.

slice() came later and does not swap: start greater than end returns an empty string, which is a visible wrong answer rather than a hidden one. slice() also reads negative indices as counting back from the end, where substring() treats any negative number as 0. Reach for slice() unless you have a reason not to.

Both take the end index as exclusive, which is the source of most off-by-one errors here: 'CodeLudo'.slice(0, 4) gives 'Code', not 'CodeL'. The second number is where to stop, not the last character to keep. Once that clicks, slice(4) with no end — everything from index 4 onward — reads naturally too.

Neither method changes the original. Strings are immutable, so every one of these returns a new string and leaves the old one untouched. text.slice(0, 4) on its own line does nothing at all; you have to assign the result somewhere.

What do you think this outputs based on the rules?

let word = "Java";
let result = word.substring(-6);
console.log(result + "Script");

JavaScript

(Note: Because substring treats -6 as 0, it simply returns the entire string "Java"!)

Searching Strings

Search methods are strictly case-sensitive.

console.log('JavaScript'.includes('Script'));
console.log('javascript'.startsWith('Java'));

true
false