https://github.com/leebyron/ecmascript-reduced

Proposal for short-circuiting Array.prototype.reduce()
https://github.com/leebyron/ecmascript-reduced

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Proposal for short-circuiting Array.prototype.reduce()

• Host: GitHub
• URL: https://github.com/leebyron/ecmascript-reduced
• Owner: leebyron
• Created: 2015-02-23T05:45:39.000Z (over 9 years ago)
• Default Branch: master
• Last Pushed: 2015-02-23T05:56:33.000Z (over 9 years ago)
• Last Synced: 2024-04-14T05:13:54.209Z (7 months ago)
• Language: JavaScript
• Size: 125 KB
• Stars: 4
• Watchers: 4
• Forks: 0
• Open Issues: 0
• Metadata Files:
  • Readme: README.md

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README

        
# Short-circuiting Array.prototype.reduce()

**Stage:** 0, Strawman

**Author:** Lee Byron

Short-circuiting from a `reduce` operation allows it to be more generically

useful. Much of the clojure core library is built on this premise.

```js

// Reduce a sum or "99+"

values.reduce(function (a, v) {

  if (a + v < 100) {

    return a + v;

  } else {

    return { [Symbol.reduced]: "99+" };

  }

});

```

This proposal suggests adding a new well known symbol *@@reduced* and altering

the defintion of **Array.prototype.reduce** to respect this symbol.

# Additions to Spec

## 6.1.5.1  Well-Known Symbols

> One row is added to Table 1.

| Specification Name | [[Description]] | Value and Purpose |

| ------------------ | --------------- | ----------------- |

| @@reduced  | "Symbol.reduced" | A property detected in the return value of a *reduce* function to indicate early completion. |

## 19.4.2  Properties of the Symbol Constructor

### 19.4.2.X  Symbol.reduced

> This property is new

The initial value of Symbol.reduced is the well known symbol @@reduced (Table 1).

This property has the attributes { [[Writable]]: **false**, [[Enumerable]]: **false**, [[Configurable]]: **false** }

## 22.1.3  Properties of the Array Prototype Object

### 22.1.3.18  Array.prototype.reduce ( *callbackfn [ , initialValue ] )

> This existing method has some additions: 10.d.v - 10.d.vii

  1. Let *O* be ToObject(this value).

  2. ReturnIfAbrupt(*O*).

  3. Let *len* be ToLength(Get(*O*, "length")).

  4. ReturnIfAbrupt(*len*).

  5. If IsCallable(*callbackfn*) is **false**, throw a **TypeError** exception.

  6. If *len* is 0 and *initialValue* is not present, throw a **TypeError** exception.

  7. Let *k* be 0.

  8. If *initialValue* is present, then

      * a. Set *accumulator* to *initialValue*.

  9. Else *initialValue* is not present,

      * a. Let *kPresent* be **false**.

      * b. Repeat, while *kPresent* is **false** and *k* < *len*

          * i. Let *Pk* be ToString(*k*).

          * ii. Let *kPresent* be HasProperty(*O*, *Pk*).

          * iii. ReturnIfAbrupt(*kPresent*).

          * iv. If *kPresent* is **true**, then

              1. Let *accumulator* be Get(*O*, *Pk*).

              2. ReturnIfAbrupt(*accumulator*).

          * v. Increase *k* by 1.

      * c. If *kPresent* is **false**, throw a **TypeError** exception.

  10. Repeat, while *k* < *len*

      * a. Let *Pk* be ToString(*k*).

      * b. Let *kPresent* be HasProperty(*O*, *Pk*).

      * c. ReturnIfAbrupt(*kPresent*).

      * d. If *kPresent* is **true**, then

          * i. Let *kValue* be Get(*O*, *Pk*).

          * ii. ReturnIfAbrupt(*kValue*).

          * iii. Let *accumulator* be Call(*callbackfn*, **undefined**, «*accumulator*, *kValue*, *k*, O»).

          * iv. ReturnIfAbrupt(*accumulator*).

          * v. Let *A* be ToObject(*accumulator*).

          * vi. Let *reducedPresent* be HasProperty(*A*, @@reduced).

          * vii. If *reducedPresent* is **true**, then

              1. Let *reduced* be Get(A, @@reduced).

              2. ReturnIfAbrupt(*reduced*).

              3. Return *reduced*.

      * e. Increase *k* by 1.

  11. Return *accumulator*.