What will be the output? (Array Length)

Easy•

This problem demonstrates how setting the array.length property affects array contents.

Quick Navigation: Array.length Property • Comparison with Other Methods • Reference Behavior

Array.length Property

The length property of an array is writable. Setting it can truncate or extend the array.

Setting length to 0

let arr = [1, 2, 3, 4, 5, -6, 7];
arr.length = 0;
console.log(arr); // []

What happens:

•Setting arr.length = 0 removes all elements from the array

•The array becomes empty but still exists

•This is equivalent to arr = [] but modifies the existing array reference

Other length Manipulations

Truncating Array

let arr = [1, 2, 3, 4, 5];
arr.length = 3;
console.log(arr); // [1, 2, 3]

Extending Array

let arr = [1, 2, 3];
arr.length = 5;
console.log(arr);        // [1, 2, 3, empty × 2]
console.log(arr.length); // 5
console.log(arr[3]);     // undefined

Key Points

1. Setting length < current length: Removes elements from the end

2. Setting length = 0: Clears the entire array

3. Setting length > current length: Creates empty slots (sparse array)

4. Empty slots are not the same as undefined values

Comparison with Other Methods

Different Ways to Clear an Array

Method 1: length = 0

let arr = [1, 2, 3];
arr.length = 0;
// arr is now []

•Pros: Fast, modifies existing array

•Cons: Affects all references to the array

Method 2: Assignment

let arr = [1, 2, 3];
arr = [];
// arr is now []

•Pros: Clear intent

•Cons: Creates new array, old references unchanged

Method 3: splice()

let arr = [1, 2, 3];
arr.splice(0, arr.length);
// arr is now []

•Pros: Explicit

•Cons: Slower, more verbose

Method 4: pop() in loop

let arr = [1, 2, 3];
while (arr.length > 0) {
  arr.pop();
}
// arr is now []

•Pros: Works in all environments

•Cons: Very slow for large arrays

Reference Behavior

Important: Reference vs New Array

let arr1 = [1, 2, 3];
let arr2 = arr1;  // arr2 references the same array

arr1.length = 0;  // Modifies the shared array
console.log(arr2); // [] - arr2 is also empty!

// vs

let arr3 = [1, 2, 3];
let arr4 = arr3;
arr3 = [];        // Creates new array, arr3 points elsewhere
console.log(arr4); // [1, 2, 3] - arr4 still has original values

Key Difference:

•arr.length = 0: Modifies the existing array (affects all references)

•arr = []: Creates a new array (only affects the variable, not other references)