How to initialize an array's length in JavaScript?

Array Birth Strategies

Let's delve further to understand different tactics for giving birth to arrays with set lengths and default values, taking advantage of the elasticity of JavaScript.

Array.from — The Master Crafter

Array.from specializes in sculpting arrays to your needs:

let indexArray = Array.from({length: 5}, (_, i) => i); // Hitchhiker's Guide to the Galaxy: Now serving numbers 0-4
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The optional map function offers first-class access to array indices, empowering creation of indexed arrays. Further, it morphs array-like species and iterables into true arrays.

The Spread Operator Unleashed

Unchain the spread ... operator with Array(n) to birth arrays:

let undefinedArray = [...Array(5)]; // Five horsemen of the undefined apocalypse.
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This tact particularly shines when the mission is to make an array with slots sans values; primed for future .map() endeavors or other iteration exploits.

Direct Size Setting

Directly pulling the length lever is the most direct route to array dimensions:

let arr = [];
arr.length = 5; // Poof! arr blooms to have 5 undefined elements.
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Clean, simple and quite the hushed secret for specifying array size.

Smooth Array Initialization Techniques

Going deeper, JavaScript brings certain initialization spells up its sleeves to boost performance and to offer more control.

Careful with fill()

Using .fill() with nested arrangements could land on thin ice:

let matrix = Array(3).fill([]); // Clones gone wrong: Spawned arrays share the same DNA!
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For non-shared 2D arrays, couple .map() with ...Array():

let twoDArray = [...Array(3)].map(x => []); // Army of 3 empty arrays. Each, a unique snowflake.
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This shields against shared references, ensuring each sub-array is fiercely independent.

The Looper — forEach( )

.forEach is blind to uninitialized values. However, .fill() can lead the way, merging .forEach() with potential:

Array(3).fill().forEach((_, i) => console.log(i)); // The count begins: 0, 1, 2
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Mind the Edge Cases

Beware of new Array() casting unexpected shadows:

let array = new Array('5'); // Plot twist: an array holding a single '5', not five spots.
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For TypeScript, keep undefined values at bay with .fill(0) during type checks.

Fancy Array Initialization

For complex or artistic patterns, harness mapping functions and template literals:

let grid = Array.from({length: 3}, (_, row) => Array.from({length: 3}, (_, col) => `${row}:${col}`));
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Behold the matrix, a 3x3 array, with elements mirroring their chess board positions.

Masterclass: The Finer Details

To nail the array management, opt let over var to secure block scoping. It's a modern best-practice to avoid any scope fiascos. Run your array length optimization theories by jsperf.com to see if you're holding the most efficient wand.