Working with Data

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Article Summary

Master data handling in JavaScript. Learn essential string and array methods, map(), filter(), reduce(), the Date and Math objects, and how to work with numbers and booleans.

String Methods

Strings in JavaScript come with many built-in methods for working with text.

Common Methods:

let str = "JavaScript is awesome";

Method	Description	Example	Result
`length`	Returns the number of characters	`str.length`	`22`
`toUpperCase()`	Converts to uppercase	`str.toUpperCase()`	`"JAVASCRIPT IS AWESOME"`
`toLowerCase()`	Converts to lowercase	`str.toLowerCase()`	`"javascript is awesome"`
`includes()`	Checks if substring exists	`str.includes("is")`	`true`
`indexOf()`	Finds index of substring	`str.indexOf("Script")`	`4`
`slice()`	Extracts part of the string	`str.slice(0, 10)`	`"JavaScript"`
`replace()`	Replaces part of a string	`str.replace("awesome", "cool")`	`"JavaScript is cool"`
`split()`	Splits into array	`str.split(" ")`	`["JavaScript", "is", ...]`

Array Methods

Arrays also come with helpful methods for managing and manipulating data.

let numbers = [10, 20, 30, 40];

Method	Description	Example	Result
`push()`	Add to end	`numbers.push(50)`	`[10, 20, 30, 40, 50]`
`pop()`	Remove from end	`numbers.pop()`	Removes `50`
`shift()`	Remove from start	`numbers.shift()`	Removes `10`
`unshift()`	Add to start	`numbers.unshift(5)`	`[5, 20, 30, 40]`
`includes()`	Check for item	`numbers.includes(20)`	`true`
`indexOf()`	Get index of item	`numbers.indexOf(30)`	`2`
`join()`	Convert to string	`numbers.join("-")`	`"20-30-40"`
`slice()`	Extract a portion	`numbers.slice(1, 3)`	`[20, 30]`
`splice()`	Insert/remove at position	`numbers.splice(1, 0, 25)`	`[20, 25, 30, 40]`

map(), filter(), reduce()

These are powerful array transformation methods used frequently in modern JavaScript.

map() — Transform each item

let prices = [100, 200, 300];
let withTax = prices.map(p => p * 1.1);
console.log(withTax); // [110, 220, 330]

filter() — Return items that match a condition

let scores = [85, 42, 77, 95];
let passed = scores.filter(score => score >= 60);
console.log(passed); // [85, 77, 95]

reduce() — Reduce array to a single value

let nums = [10, 20, 30];
let total = nums.reduce((sum, num) => sum + num, 0);
console.log(total); // 60

Date Object

The Date object allows you to work with dates and times.

let now = new Date();
console.log(now); // Current date and time

let birthday = new Date("2000-01-01");

Useful Date Methods:

Method	Description
`getFullYear()`	Gets the 4-digit year
`getMonth()`	Month (0–11)
`getDate()`	Day of the month (1–31)
`getDay()`	Day of the week (0–6)
`getHours()`	Hour of the day
`toDateString()`	Human-readable date

Example:

console.log(birthday.getFullYear()); // 2000
console.log(now.toDateString());     // e.g., "Tue Jul 02 2025"

Math Object

The Math object provides built-in math functions and constants.

let num = 7.5;

Common Math Methods:

Method	Description	Example
`Math.round()`	Round to nearest integer	`Math.round(num)` // 8
`Math.floor()`	Round down	`Math.floor(num)` // 7
`Math.ceil()`	Round up	`Math.ceil(num)` // 8
`Math.max()`	Highest of values	`Math.max(3, 7, 2)` // 7
`Math.min()`	Lowest of values	`Math.min(3, 7, 2)` // 2
`Math.random()`	Random number (0 to <1)	`Math.random()`
`Math.sqrt()`	Square root	`Math.sqrt(16)` // 4
`Math.pow()`	Exponentiation	`Math.pow(2, 3)` // 8

Number and Boolean Handling

Number Conversion:

let str = "42";
let num = Number(str); // 42

parseInt("50.99");  // 50
parseFloat("50.99"); // 50.99

Check for NaN or Finite:

isNaN("abc");      // true
isFinite(100);     // true

Boolean Conversion:

Truthy: non-zero numbers, strings, arrays, objects
Falsy: 0, "", null, undefined, NaN, false

Boolean(0);         // false
Boolean("hello");   // true
Boolean([]);        // true

Summary

Use string methods like slice(), replace(), and toUpperCase() to work with text.
Use array methods like push(), splice(), map(), filter(), and reduce() to manage lists.
The Date object lets you create and manipulate dates.
The Math object gives you tools for calculations and random numbers.
Understand how to convert and validate numbers and booleans for safe, predictable code.