How to increment a value in a JavaScript object?

Question

var map = {};
map[key] = value;

How can I

• assign value 1 if key does not yet exist in the object
• increment the value by 1 if it

Answer 1

ES6 provides a dedicated class for maps, Map. You can easily extend it to construct a "map with a default value":

class DefaultMap extends Map {

    constructor(defVal, iterable=[]) {
        super(iterable);
        this.defVal = defVal;
    }

    get(key) {
        if(!this.has(key))
            this.set(key, this.defVal);
        return super.get(key);
    }
}

m = new DefaultMap(9);
console.log(m.get('foo'));
m.set('foo', m.get('foo') + 1);
console.log(m.get('foo'))

(Ab)using Objects as Maps had several disadvantages and requires some caution.