Diving Into JavaScript Arrays 

Syed Muhammad Taha (@smtaha512)

Software Engineer

Arrays

  • Collection of items
const arr1 = [1, 2, 3];

const arr2 = ['a', 'b', 'v'];

const arr3 = [{ a: 1 }, { b: 2 }];

Arrays

  • Collection of arbitrary items
const arr1 = [
  1,
  'a',
  { a: 1 },
  true,
  () => arr1.length
];

Array of Integers

const arr = [1, 2, 3]; // PACKED_SMI_ELEMENTS

ELEMENTS

const obj = {
    a: 1, 
    b: 2, 
    c: 3
  };
const arr = [
    1, 2, 3
  ];

Objects have properties that map to values.

Arrays have indexes that map to elements.

SMI

SMI == SMall Integers

const arr = [
    1, 2, 3
  ]; // PACKED_SMI_ELEMENTS

PACKED_DOUBLE_ELEMENTS

const arr = [
    1, 2, 3
  ]; // PACKED_SMI_ELEMENTS

arr.push(4.6); //PACKED_DOUBLE_ELEMENTS
// [1, 2, 3, 4.6]

PACKED_ELEMENTS

const arr = [
    1, 2, 3
  ]; // PACKED_SMI_ELEMENTS

arr.push(4.6); //PACKED_DOUBLE_ELEMENTS

arr.push('x'); // PACKED_ELEMENTS
// [1, 2, 3, 4.6, 'x']

PACKED_SMI_ELEMENTS

PACKED_DOUBLE_ELEMENTS

 PACKED_ELEMENTS 

const arr = [
    1, 2, 3, 4.6, 'x'
  ]; // PACKED_ELEMENTS

arr.length
// 5


arr[10] = 120; 
// Assigning outside array's bound

HOLEY_ELEMENTS

const arr = [
    1, 2, 3
  ]; // PACKED_SMI_ELEMENTS

arr.push(4.6); // PACKED_DOUBLE_ELEMENTS
arr.push('x'); // PACKED_ELEMENTS
// [1, 2, 3, 4.6, 'x']

arr[10] = 120; // HOLEY_ELEMENTS

Node JS REPL:

Chrome console:

HOLEY_SMI_ELEMENTS

HOLEY_DOUBLE_ELEMENTS

 HOLEY_ELEMENTS 

Elements Kinds Transition

So What!

Let's discuss some tips based on previous knowledge

Avoid creating holes

How to create holes?

const arr = [1, 2, 3];

arr.push(4.6);
arr.push('x');

// Alert holey array ahead! 
arr[10] = 120;  // Holes from index = 5 to index = 9
const arr = [1, 2, 3,,10]; // Hole at index = 3
const arr = new Array(3); HOLEY_SMI_ELEMENTS

arr[0] = 1;
arr[1] = 2;
arr[2] = 3;

Why avoid holes?

index

value

0

1

1

2

2

3

3

4.6

4

5

6

7

8

9

Bounds check:​

 

 

Try to access value on array:

 

 

Try to access value on prototype chain:

 
array[8];   // ???

How JavaScript engine will evaluate this?

'x'

8 ≥ 0 && 8 < array.length // true 

hasOwnProperty(arr, '8');  // false

hasOwnProperty(Array.prototype, '8');  // false
hasOwnProperty(Object.prototype, '8');  // false

10

120

Use array literal 

const arr = [1, 2, 3, 10]; // PACKED_SMI_ELEMENTS
// If you don't know all the values ahead of time 
// try using array literal and then push.
const arr = [];

arr.push(someValue);
arr.push(otherValue);

Avoid reading beyond the length of the array

// Don’t do this!
for (let i = 0, item; (item = items[i]) != null; i++) {
  doSomething(item);
}

Reading beyond the array length

A famous library uses this

Instead, either use:

  • Old fashioned `for loops`.
  • `for-of` loops.
  • For arrays specifically use `forEach`.

Avoid elements kind transition

// Don’t do this!
const array = [3, 2, 1, +0];
// PACKED_SMI_ELEMENTS
array.push(-0);
// PACKED_DOUBLE_ELEMENTS

Avoid `-0` as much as possible

// Don’t do this!
const array = [3, 2, 1];
// PACKED_SMI_ELEMENTS
array.push(NaN, Infinity);
// PACKED_DOUBLE_ELEMENTS

Avoid NaN and Infinity as much as possible

Prefer arrays over array-like object

const elements = document.getElementsByClassName(className);
// Returns HTMLCollection which is an array like object

// Iterating over HTMLCollection using Array prototype
// Not performant as real array
Array.prototype.forEach.call(elements, (val, idx) => {
    console.log(idx, val);
})

Accessing DOM

Better approach

const elements = document.getElementsByClassName(className);
// Returns HTMLCollection which is an array like object

const elemArr = Array.from(elements); 
// Creating array from array like object

elemArr.forEach((val, idx) => {
    console.log(idx, val);
})

Avoid polymorphism

const each = (array, callback) => {
  for (let idx = 0; idx < array.length; ++idx) {
    const item = array[idx];
    callback(item);
  }
};

each(['a', 'b', 'c'], doSomething);

each([1.1, 2.2, 3.3], doSomething);

each([1, 2, 3], doSomething);

Special Thanks

Mathias Bynens(@mathias)

Resources

Thank you

Diving Into JavaScript Arrays

By Syed M. Taha

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Diving Into JavaScript Arrays

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