What is the reason for `downlevelIteration` not being enabled by default?

Question

What is the reason for `downlevelIteration` not being enabled by default?

When directing towards ES5 and using the spread operator ... to convert an Iterator to an Array, it prompts the need for the -downlevelIteration compiler option. Enabling this option allows the spread operators to function without any errors.

I'm curious as to why this specification is necessary. Are there any drawbacks or limitations when enabling it, other than increasing the emitted generated code from tslib?

Here's another example: Creating an array dynamically of size N (e.g., 3):

[...Array(3).keys()]  // output: [0, 1, 2]

At times, VS Code displays an error:
https://i.sstatic.net/cSrYT.png

Error message from tsc:

Type 'IterableIterator' is not an array type or a string type. Use compiler option '--downlevelIteration' to allow iterating of iterators.

Check out the TypeScript Playground to edit the code and view the error

typescript

Answer 1

Answer №1

Having gone through the release notes and the insightful article Downlevel Iteration for ES3/ES5 in TypeScript, I am inclined to believe that the reason why downlevelIteration is disabled lies in the need to configure how TypeScript deals with compatibility code during compilation (to cater to older versions of Javascript).

The article elaborates further, presenting the dilemma of whether to let TypeScript embed essential helper functions (resulting in a larger production bundle size) or opt for configuring TypeScript to utilize tslib as a dependency and leverage its external methods.

I strongly advise delving into Downlevel Iteration for ES3/ES5 in TypeScript for a more profound comprehension... and a potential alternate resolution to your original query.

Answer 2

Having gone through the release notes and the insightful article Downlevel Iteration for ES3/ES5 in TypeScript, I am inclined to believe that the reason why downlevelIteration is disabled lies in the need to configure how TypeScript deals with compatibility code during compilation (to cater to older versions of Javascript).

The article elaborates further, presenting the dilemma of whether to let TypeScript embed essential helper functions (resulting in a larger production bundle size) or opt for configuring TypeScript to utilize tslib as a dependency and leverage its external methods.

I strongly advise delving into Downlevel Iteration for ES3/ES5 in TypeScript for a more profound comprehension... and a potential alternate resolution to your original query.

Answer 3

Answer №2

To simplify, use Array.from(foo) in your situation:

Array.from(Array(3).keys())

TypeScript defines downleveling as transpiling to an older version of JavaScript to ensure compatibility with older JavaScript runtimes. Enabling the --downlevelIteration flag allows for more accurate implementation of modern JS iteration concepts on ES5 environments.

In ECMAScript 6, new iteration primitives like the for / of loop (for (el of arr)), Array spread ([a, ...b]), argument spread (fn(...args)), and Symbol.iterator were introduced. The --downlevelIteration flag ensures these primitives work correctly in ES5 if a Symbol.iterator implementation is available.

For example, when dealing with array spread, consider this scenario. An array spread operation:

// Create a new array with elements starting from 1 followed by elements of arr2
const arr = [1, ...arr2];

Although downleveling to ES5 seems straightforward:

// Essentially the same?
const arr = [1].concat(arr2);

However, there are subtle differences in rare cases. For instance, spreading an array with a missing element will create that index as a property, unlike using concat:

// Generate an array with a missing '1' element
let missing = [0, , 1];
let spreaded = [...missing];
let concated = [].concat(missing);
// true
"1" in spreaded;
// false
"1" in concated;

downlevelIteration leverages Symbol.iterator to accurately replicate ES6 behavior.

For further details and examples, visit here

In your specific case, it's recommended not to alter this behavior just to generate a basic range. Instead of [...Array(3).keys()], opt for Array.from(Array(3).keys())

Answer 4

To simplify, use Array.from(foo) in your situation:

Array.from(Array(3).keys())

TypeScript defines downleveling as transpiling to an older version of JavaScript to ensure compatibility with older JavaScript runtimes. Enabling the --downlevelIteration flag allows for more accurate implementation of modern JS iteration concepts on ES5 environments.

In ECMAScript 6, new iteration primitives like the for / of loop (for (el of arr)), Array spread ([a, ...b]), argument spread (fn(...args)), and Symbol.iterator were introduced. The --downlevelIteration flag ensures these primitives work correctly in ES5 if a Symbol.iterator implementation is available.

For example, when dealing with array spread, consider this scenario. An array spread operation:

// Create a new array with elements starting from 1 followed by elements of arr2
const arr = [1, ...arr2];

Although downleveling to ES5 seems straightforward:

// Essentially the same?
const arr = [1].concat(arr2);

However, there are subtle differences in rare cases. For instance, spreading an array with a missing element will create that index as a property, unlike using concat:

// Generate an array with a missing '1' element
let missing = [0, , 1];
let spreaded = [...missing];
let concated = [].concat(missing);
// true
"1" in spreaded;
// false
"1" in concated;

downlevelIteration leverages Symbol.iterator to accurately replicate ES6 behavior.

For further details and examples, visit here

In your specific case, it's recommended not to alter this behavior just to generate a basic range. Instead of [...Array(3).keys()], opt for Array.from(Array(3).keys())

Answer 5

Answer №3

To update your code, simply replace es5 with es6 in the tsconfig.json configuration file.

https://i.sstatic.net/XJ6xJ.png

Answer 6

To update your code, simply replace es5 with es6 in the tsconfig.json configuration file.

https://i.sstatic.net/XJ6xJ.png

Answer 7

Answer №4

Tip: Easily access the array index by simply utilizing [...Array(10)].

[...Array(10)].map((element, index) => <div key={index}>...insert your code here</div>

This technique can be used in React.

Answer 8

Tip: Easily access the array index by simply utilizing [...Array(10)].

[...Array(10)].map((element, index) => <div key={index}>...insert your code here</div>

This technique can be used in React.

What is the reason for `downlevelIteration` not being enabled by default?

Answer №1

Answer №2

Answer №3

Answer №4

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