CODEWITHSUNDEEP
javascript
Geert-Jan
Geert-Jan

Reputation: 18915

How to idiomatically initialize to zero or increment a property?

I love those short js oneliners. I'd like to know if there's something logical and elegant for:

Shorter than this preferrably ;)

var obj = {} ; 

//some iterative called function
obj.prop = obj.prop===undefined?0:obj.prop++;

Upvotes: 30

Views: 19349

Answers (6)

bormat
bormat

Reputation: 1379

To initialise to 1 obj.prop = -~obj.prop

To initialize to 0 ~~++obj.prop OR ++obj.prop | 0

Upvotes: 3

ruohola
ruohola

Reputation: 24077

This works for both plain JS and TypeScript:

obj.prop = (obj.prop ?? 0) + 1;

Upvotes: 5

Данила Летуновский
Данила Летуновский

Reputation: 51

It is not possible to directly pass a variable to the function inc(obj.prop), because in javascript variables are passed by value, but you can pass the object itself and the name of the variable you want to increment.

Object.prototype.inc = function(n){ this[n]? this[n]++ : this[n] = 1; }

let obj = {};

obj.inc("prop");

// obj.prop == 1

You can also add the required fields to the object before

Object.prototype.has_fields = function(obj2){
    for(let p of Object.keys(obj2)){
        if(obj2[p].constructor == Object){
            if(!this[p]) this[p] = {};
            this[p].has_fields(obj2[p]);
        }else{
            if(!this[p]) this[p] = obj2[p];
        }
    }
    return this;
}

let obj = {};

    obj.has_fields({a:{b:{c:{d:{prop:0}}}}});
    obj.a.b.c.d.prop++;
    // obj.a.b.c.d.prop == 1

    obj.has_fields({a:{b:{c:{d:{prop:0}}}}});
    obj.a.b.c.d.prop++;
    // obj.a.b.c.d.prop == 2

Upvotes: 1

aljgom
aljgom

Reputation: 9139

Similar to I Hate Lazy's answer but avoids the double assignment as Bodgan points out 

++obj.prop || (obj.prop=0)

in the global scope

++window.foo || (window.foo=0)

Upvotes: 2

Bogdan D
Bogdan D

Reputation: 5611

A cleaner way of doing this is simply

obj.prop = obj.prop + 1 || 0;

Using the increment operator is wrong or overkill. The suffixed operator, x++, (the example in the question) should not work at all. The prefixed, ++x, leads to assigning twice (like saying x = x = x+1)

Upvotes: 47

I Hate Lazy
I Hate Lazy

Reputation: 48771

This will result in NaN for the first increment, which will default to 0.

obj.prop = ++obj.prop || 0;

Upvotes: 52

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