ES6 call parent function from inherited class

Question

I am trying to find an easy way to call parent function from child one. However, couldn't really found a way to directly call it from child instance as like in other programming languages.

class ExchangeHandler {
    constructor(getMarketsUrl, getMarketDataBaseUrl, interval) {
        this.interval = interval;
        this.getMarketsUrl = getMarketDataBaseUrl;
        this.getMarketDataBaseUrl = getMarketDataBaseUrl;
    }

    getMarketsUrl() {
        return this.getMarketsUrl;
    }

    getMarketDataBaseUrl() {
        return this.getMarketDataBaseUrl;
    }

    buildQueryUrl(params) {
        return querystring.stringify(params);
    }

    getIntervalParam() {
        return config.intervalToExchangeInput[[config.exchange]][[this.interval]];
    }
}

class BittrexHandler extends ExchangeHandler {
    constructor(interval) {
        super("https://bittrex.com/api/v1.1/public/getmarkets", 
                "https://bittrex.com/Api/v2.0/pub/market/GetTicks", 
                interval);
    }

    buildGetMarketTickerUrl(symbol) {
        return super.getMarketDataBaseUrl() + "?"
                + super.buildQueryUrl({marketName: this.buildSymbolParam(symbol),
                                    tickInterval: super.getIntervalParam()})
    }

    buildSymbolParam(symbol) {
        return "BTC-" + symbol;
    }
}

var bittrexHandler = ExchangeHandlerFactory.getExchangeHandler("bittrex", "hour");
console.log("getMarketsUrl: " + bittrexHandler.getMarketsUrl());

And I get

TypeError: bittrexHandler.getMarketsUrl is not a function

Isn't this possible in javascript? Thanks a lot!

You’re using super.methodName. Do you understand what the prototype is? — evolutionxbox
– evolutionxbox, Commented Apr 15, 2018 at 23:42
@evolutionxbox I am trying to learn JS can you elaborate your question? — quartaela
– quartaela, Commented Apr 16, 2018 at 0:04

jfriend00 · Accepted Answer · 2018-04-15 23:57:21Z

7

You can't have a data property and a method with the same name as you do with getMarketsUrl. They occupy the same property slot on the object. Change the name of one of them.

When you're trying to execute the method, the interpreter is finding the data property first and thus you can't call the method in the normal way.

In addition, you should not be using super to just call a non-overriden method on the object. For example, change this:

buildGetMarketTickerUrl(symbol) {
    return super.getMarketDataBaseUrl() + "?"
            + super.buildQueryUrl({marketName: this.buildSymbolParam(symbol),
                                tickInterval: super.getIntervalParam()})
}

to this:

buildGetMarketTickerUrl(symbol) {
    return this.getMarketDataBaseUrl() + "?"
            + this.buildQueryUrl({marketName: this.buildSymbolParam(symbol),
                                tickInterval: this.getIntervalParam()})
}

edited Apr 15, 2018 at 23:57

answered Apr 15, 2018 at 23:40

jfriend00

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7 Comments

evolutionxbox Over a year ago

Maybe the OP should use a getter method?

jfriend00 Over a year ago

@evolutionxbox - I think the data property getMarketsUrl just needs a different name. Even if they used a getter, the storage location for that value would still need a different name.

evolutionxbox Over a year ago

Indeed. Common practice is to prepend the property name with an underscore to denote a “private” variable.

jfriend00 Over a year ago

@evolutionxbox - Yes, that would be one way to do it.

jfriend00 Over a year ago

@quartaela - Javascript doesn't directly support private member variables as a keyword feature. Adding an underscore does not actually make it private. That's just a convention. Private properties can be achieved using the constructor as a closure such as shown here How to set javascript private variables in constructor. But, this requires defining any methods that want to access this property inside your constructor (not using ES6 class syntax).

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