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JavaScript Arrays

Arrays are one of the most fundamental data structures in JavaScript. This guide covers essential array operations, from initialization to advanced manipulation techniques.

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Why Arrays Matter

JavaScript arrays are versatile, dynamic collections that can hold any type of value. Understanding array methods and patterns is crucial for writing efficient, readable code.

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Core Topics

Array Initialization

Learn the many ways to create and initialize arrays

Array Methods

Master essential array methods like map, filter, and reduce

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Quick Examples

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Remove Duplicates

Use the Set object to easily remove duplicate values from an array: 
const uniqueElements = arr => [...new Set(arr)];

uniqueElements([1, 2, 2, 3, 4, 4, 5]); // [1, 2, 3, 4, 5]

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Check if All Values Are Equal

Compare all array elements using Array.prototype.every(): 
const allEqual = arr => arr.every(val => val === arr[0]);

allEqual([1, 1, 1]); // true
allEqual([1, 1, 2]); // false

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Check for Duplicates

Compare the array length with the size of a Set: 
const hasDuplicates = arr => arr.length !== new Set(arr).size;

hasDuplicates([1, 2, 2, 3, 4, 4, 5]); // true
hasDuplicates([1, 2, 3, 4, 5]); // false

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Common Patterns

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Comparing with a Mapping Function

For complex values like objects, use a mapping function: 
const allEqualBy = (arr, fn) =>
  arr.every((val, i) => fn(val, i, arr) === fn(arr[0], 0, arr));

allEqualBy([{ a: 1 }, { a: 1 }, { a: 1 }], obj => obj.a); // true
allEqualBy([{ a: 1 }, { a: 1 }, { a: 2 }], obj => obj.a); // false

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Filter Unique Elements By Property

const uniqueElementsBy = (arr, fn) =>
  arr.reduce((acc, v) => {
    if (!acc.some(x => fn(v, x))) acc.push(v);
    return acc;
  }, []);

const data = [
  { id: 0, value: 'a' },
  { id: 1, value: 'b' },
  { id: 2, value: 'c' },
  { id: 1, value: 'd' },
  { id: 0, value: 'e' }
];

const idComparator = (a, b) => a.id == b.id;
uniqueElementsBy(data, idComparator);
// [ { id: 0, value: 'a' }, { id: 1, value: 'b' }, { id: 2, value: 'c' } ]

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Next Steps

Learn Initialization

Discover different ways to create and initialize arrays

Master Methods

Deep dive into essential array methods

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