It seems like your proposed `const value = map.putIfAbsent(key, getExpensiveValue);` is achievable already with `const value = map.has(key) ? map.get(key) : map.set(getExpensiveValue());` - am I understanding your suggestion correctly?
On Wed, Oct 10, 2018 at 12:46 AM Man Hoang <[email protected]> wrote: > Consider the following function > > ``` js > > /** > > * Parses the locale sensitive string [value] into a number. > > */ > > export function parseNumber( > > value: string, > > locale: string = navigator.language > > ): number { > > let decimalSeparator = decimalSeparators.get(locale); > > if (!decimalSeparator) { > > decimalSeparator = Intl.NumberFormat(locale).format(1.1)[1]; > > decimalSeparators.set(locale, decimalSeparator); > > } > > > > let cleanRegExp = regExps.get(decimalSeparator); > > if (!cleanRegExp) { > > cleanRegExp = new RegExp(`[^-+0-9${decimalSeparator}]`, 'g'); > > regExps.set(decimalSeparator, cleanRegExp); > > } > > > > value = value > > .replace(cleanRegExp, '') > > .replace(decimalSeparator, '.'); > > > > return parseFloat(value); > > } > > > > const decimalSeparators = new Map<string, string>(); > > const regExps = new Map<string, RegExp>(); > > ``` > > > > This function can be simplified quite a bit as follows > > ``` js > > export function parseNumber( > > value: string, > > locale: string = navigator.language > > ): number { > > const decimalSeparator = decimalSeparators.putIfAbsent( > > locale, () => Intl.NumberFormat(locale).format(1.1)[1]); > > > > const cleanRegExp = regExps.putIfAbsent( > > decimalSeparator, () => new RegExp(`[^-+0-9${decimalSeparator}]`, > 'g')); > > > > value = value > > .replace(cleanRegExp, '') > > .replace(decimalSeparator, '.'); > > > > return parseFloat(value); > > } > > ``` > > if `Map` has the following instance method > > ``` js > > export class Map<K, V> { > > /** > > * Look up the value of [key], or add a new value if it isn't there. > > * > > * Returns the value associated to [key], if there is one. > > * Otherwise calls [ifAbsent] to get a new value, associates [key] to > > * that value, and then returns the new value. > > */ > > putIfAbsent(key: K, ifAbsent: () => V): V { > > let v = this.get(key); > > if (v === undefined) { > > v = ifAbsent(); > > this.set(key, v); > > } > > return v; > > } > > } > > ``` > > > > Java's Map has a `putIfAbsent` method, which accepts a value rather than a > function for the second parameter. This is not ideal as computing the value > may be expensive. A function would allow the value to be computed lazily. > > > > References > > - [Dart] [Map.putIfAbsent]( > https://api.dartlang.org/stable/2.0.0/dart-core/Map/putIfAbsent.html) > > - [Java] [Map.putIfAbsent]( > https://docs.oracle.com/javase/8/docs/api/java/util/Map.html#putIfAbsent-K-V- > ) > _______________________________________________ > es-discuss mailing list > [email protected] > https://mail.mozilla.org/listinfo/es-discuss >
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