Generate a series of whole numbers using the spread operator

Question

Generate a series of whole numbers using the spread operator

Although there is no direct equivalent to Python 3's range in ES6, there are several workarounds available. I am curious about the effectiveness of one specific workaround that actually works. Take this example:

[...Array(10).keys()];

The reason why this workaround functions as intended is somewhat mysterious because Array(10).keys() seems empty at first glance.

I acknowledge that many common workarounds can be inefficient as they create unnecessary arrays. By utilizing a generator function, it is possible to avoid this issue without creating extra arrays. For instance:

[...(function*(){let i = 0; while(i<10) yield i++;})()];

The main focus of my question lies in understanding why the initial workaround yields the desired result.

Edit:

Based on responses, some suggest that evaluating Array(10) is the same as evaluating Array.apply(null,Array(10)), but they differ. For example, .hasOwnProperty(0) returns

false</code for the former and <code>true

for the latter. I am willing to consider arguments that they are somehow equivalent in ways relevant to this context since my understanding is clearly lacking. It appears that iterating over keys is influenced by the shared length property rather than defined array indexes. If this behavior is standard, I would like clarification on that aspect.

javascript arrays ecmascript-6 iterator

Answer 1

Answer №1

Array#keys retrieves an Array iterator. The spread syntax then exhausts the iterator by accessing each value until there are none left, collecting all values and spreading them into a new array.

Array(10) creates an array exotic object without integer-indexed keys, only a length property. Despite appearing 'empty', Array(10).keys() is not. The use of Array#keys doesn't rely on actual elements but rather on the length property. The function CreateArrayIterator generates an iterator using the intrinsic object %ArrayIteratorPrototype%. By continuously incrementing until reaching the array's length, Array#keys constructs an iterator for all keys in the array, even if no integer keys were present initially.

To delve into the abstract steps, refer to Section 12.2.5.2 ArrayAcculumation of the ECMAScript Language Specification, focusing on the

SpreadElement : ... AssignmentExpression

production, illustrating the process of iterating through an iterator alongside spread syntax.

For details on gathering values into a new array, check out Section 12.2.5.3 Evaluation. Specifically, the ArrayLiteral : [ ElementList ] production encompasses the process that [...Array.keys()] undergoes. Through ArrayAcculumation, values from the iterator are collected and placed into the new array.

Answer 2

Array#keys retrieves an Array iterator. The spread syntax then exhausts the iterator by accessing each value until there are none left, collecting all values and spreading them into a new array.

Array(10) creates an array exotic object without integer-indexed keys, only a length property. Despite appearing 'empty', Array(10).keys() is not. The use of Array#keys doesn't rely on actual elements but rather on the length property. The function CreateArrayIterator generates an iterator using the intrinsic object %ArrayIteratorPrototype%. By continuously incrementing until reaching the array's length, Array#keys constructs an iterator for all keys in the array, even if no integer keys were present initially.

To delve into the abstract steps, refer to Section 12.2.5.2 ArrayAcculumation of the ECMAScript Language Specification, focusing on the

SpreadElement : ... AssignmentExpression

production, illustrating the process of iterating through an iterator alongside spread syntax.

For details on gathering values into a new array, check out Section 12.2.5.3 Evaluation. Specifically, the ArrayLiteral : [ ElementList ] production encompasses the process that [...Array.keys()] undergoes. Through ArrayAcculumation, values from the iterator are collected and placed into the new array.

Answer 3

Answer №2

Array.prototype.keys function returns a new Array Iterator.
Array(10) represents an array with 10 empty slots.

When you spread the keys of the array, you are iterating over it and creating a new array where each item corresponds to the slots in the original array.

This can be observed using a for of loop:

const arr = Array(10).keys();
for (let key of arr) {
  console.log(key);
}

Alternatively, you can achieve the same result by using Array.from along with a mapping function:

const arr = Array.from(Array(10), (_, idx) => idx);
console.log(arr);

Edit
Following your question edit regarding the result of Array(10):
According to the documentation:

If only an integer between 0 and 2^32-1 is passed as argument to the Array constructor, it creates a new JavaScript array with its length property set to that number (Note: this results in an array of arrayLength empty slots, not undefined values). Passing any other number will throw a RangeError exception.

In summary:

A new array is created
The length property is set to the provided number
You get empty slots equal to the provided number

Using .keys on this array generates an Iterator where each slot index is returned as a value.
For example:

let it = Array(10).keys();

Calling:

it.next();

Will return:

{value: 0, done: false}

Subsequent calls will produce

{value: n, done: false}

until reaching the last slot:

{value: 9, done: false}

after which, the next .next call results in:

{value: undefined, done: true}

indicating the end of iterations.

Example execution:

const it = Array(10).keys();
console.log(it.next());
console.log(it.next());
console.log(it.next());
console.log(it.next());
console.log(it.next());
console.log(it.next());
console.log(it.next());
console.log(it.next());
console.log(it.next());
console.log(it.next());
console.log(it.next()); // done: true

Answer 4

Array.prototype.keys function returns a new Array Iterator.
Array(10) represents an array with 10 empty slots.

When you spread the keys of the array, you are iterating over it and creating a new array where each item corresponds to the slots in the original array.

This can be observed using a for of loop:

const arr = Array(10).keys();
for (let key of arr) {
  console.log(key);
}

Alternatively, you can achieve the same result by using Array.from along with a mapping function:

const arr = Array.from(Array(10), (_, idx) => idx);
console.log(arr);

Edit
Following your question edit regarding the result of Array(10):
According to the documentation:

If only an integer between 0 and 2^32-1 is passed as argument to the Array constructor, it creates a new JavaScript array with its length property set to that number (Note: this results in an array of arrayLength empty slots, not undefined values). Passing any other number will throw a RangeError exception.

In summary:

A new array is created
The length property is set to the provided number
You get empty slots equal to the provided number

Using .keys on this array generates an Iterator where each slot index is returned as a value.
For example:

let it = Array(10).keys();

Calling:

it.next();

Will return:

{value: 0, done: false}

Subsequent calls will produce

{value: n, done: false}

until reaching the last slot:

{value: 9, done: false}

after which, the next .next call results in:

{value: undefined, done: true}

indicating the end of iterations.

Example execution:

const it = Array(10).keys();
console.log(it.next());
console.log(it.next());
console.log(it.next());
console.log(it.next());
console.log(it.next());
console.log(it.next());
console.log(it.next());
console.log(it.next());
console.log(it.next());
console.log(it.next());
console.log(it.next()); // done: true

Answer 5

Answer №3

Array(10) will generate a new array with a length of 10, where each element is initially set to undefined.

The method Array(10).keys() will produce an iterator that loops through the keys of the array, which are the numbers from 0 to 9.

Using the spread syntax in JavaScript for defining array literals (https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Operators/Spread_operator) allows for full iteration of any given iterator and adds the results from the iterator to the newly created array in sequential order.

Answer 6

Array(10) will generate a new array with a length of 10, where each element is initially set to undefined.

The method Array(10).keys() will produce an iterator that loops through the keys of the array, which are the numbers from 0 to 9.

Using the spread syntax in JavaScript for defining array literals (https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Operators/Spread_operator) allows for full iteration of any given iterator and adds the results from the iterator to the newly created array in sequential order.

Generate a series of whole numbers using the spread operator

Answer №1

Answer №2

Answer №3

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