How can I use a simple JavaScript lookup table (which is the mapping itself) inside map function? I.e how can i get rid of this "code" field (borrowed from here), and use only the lookup table inside map method?

const Resistors = {
    "black": 0,    "brown": 1,    "red": 2,
    "orange": 3,   "yellow": 4,   "green": 5,
    "blue": 6,     "violet": 7,   "grey": 8,
    "white": 9,

    // Why not Resistors[color]
    "code" : (color: string) => {
        function valueOf<T, K extends keyof T>(obj: T, key: K) {
            return obj[key];
        }
        return valueOf(Resistors, color as any);
    }
}
class ResistorColor {
    private colors: string[];
    constructor(colors: string[]) { this.colors = colors; }
    value = () => {
        return Number.parseInt(
            this.colors
                  .map(Resistors.code) // How can i get rid of .code here?
                  .join("")
        )
    }
}

How can I use a simple JavaScript lookup table (which is the mapping itself) inside map function? I.e how can i get rid of this "code" field (borrowed from here), and use only the lookup table inside map method?

const Resistors = {
    "black": 0,    "brown": 1,    "red": 2,
    "orange": 3,   "yellow": 4,   "green": 5,
    "blue": 6,     "violet": 7,   "grey": 8,
    "white": 9,

    // Why not Resistors[color]
    "code" : (color: string) => {
        function valueOf<T, K extends keyof T>(obj: T, key: K) {
            return obj[key];
        }
        return valueOf(Resistors, color as any);
    }
}
class ResistorColor {
    private colors: string[];
    constructor(colors: string[]) { this.colors = colors; }
    value = () => {
        return Number.parseInt(
            this.colors
                  .map(Resistors.code) // How can i get rid of .code here?
                  .join("")
        )
    }
}

• javascript
• typescript

• That's what you're doing already? – zerkms Commented Jun 29, 2020 at 1:54
• @zerkms I use an auxilary field as mapping function, I'd rather prefer something like .map(x => Lookup[x]) – andreoss Commented Jun 29, 2020 at 1:58

2 Answers 2

It's a little hard to know exactly what you're looking for, but at a glance ... you can! You should be able to do:

const Resistors = {
  black: 0,
  brown: 1,
}

And then...

const numericColorCode = Resistors['black'];
console.log(numericColorCode) // should be 0

Now sometimes the TypeScript piler can get grumpy about this kind of thing. You may need to do something like this to make the piler happy:

const numericColorCode = (Resistors as {[index: string]: number})['black'];

As for your question below that - use Object.keys and a Array.join!

const allTheColors = Object.keys(Resistors).join(',');
console.log(allTheColors); // should be 'black,brown'

Hope this helps!

Further to - and based on - Justin's answer, here's another example around where to define your types:

const messages: {[index: string]: string} = {
  'required': 'The value is required',
  'minimumValue': 'The value is not large enough'
}

const validationErrorType = 'required';
const messageToUser = messages[validationErrorType];