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 exists
Could I do better than:
if (map[key] == null) map[key] = 0;
map[key] = map[key]++;
9 Answers
Here you go minimize your code.
map[key] = (map[key]+1) || 1 ;
13 Comments
JJJ
By "flag hoisting" you mean "short-circuit evaluation". This doesn't have anything to do with hoisting, although "flag hoisting" is an amusing conflation.
ricky
without having flag hoisting property in JavaScript. You can not use so called "short-circuit evaluation". Ultimately, Its utilizing the same concept.
JJJ
There is no kind of hoisting involved in your code at all.
JJJ
You're misreading that question.
var a = a || 1; would involve hoisting because of the var keyword (the local variable is hoisted). Here you're manipulating a property of an existing object, therefore there's nothing that would get hoisted anywhere.
Sergio Tulentsev
@ricky: nope. Getting a value by non-existing key returns
undefined (without any invisible assignment). undefined + 1 gives NaN, which is a falsey value, so the control goes to || 1 part. I assure you nothing is being hoisted here. Go read up on the term. |
Comments
You can check if the object doesn't have the specific key and set it or increase existing key value by one:
function assignKey(obj, key) {
typeof obj[key] === 'undefined' ? obj[key] = 1 : obj[key]++;
}
var map = {};
assignKey(map, 2);
assignKey(map, 2);
assignKey(map, 4);
assignKey(map, 1);
assignKey(map, 2);
assignKey(map, 5);
assignKey(map, 1);
console.log(map);
Comments
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.
1 Comment
Taylor R
The code snippet has an error;
Uncaught SecurityError. I'm not sure if this is a problem with the code itself or with some StackOverflow issue. Just thought I'd point it out.you can use ternory operator like this
map[key] ? map[key]++ : map[key] = 1;
Comments
It would be better if you convert array's value into integer and increment it. It will be robust code. By default array's value is string. so in case you do not convert it to integer then it might not work cross browser.
if (map[key] == null) map[key] = 0;
map[key] = parseInt(map[key])+1;
function addToMap(map, key, value) {
if (map.has(key)) {
map.set(key, parseInt(map.get(key), 10) + parseInt(value, 10));
} else {
map.set(key, parseInt(value, 10));
}
}
Comments
Creating an object:
tagObject = {};
tagObject['contentID'] = []; // adding an entry to the above tagObject
tagObject['contentTypes'] = []; // same explanation as above
tagObject['html'] = [];
Now below is the occurrences entry which I am addding to the above tag Object..
ES 2015 standards:
function () {} is same as () => {}
let found = Object.keys(tagObject).find(
(element) => {
return element === matchWithYourValueHere;
});
tagObject['occurrences'] = found ? tagObject['occurrences'] + 1 : 1;
this will increase the count of a particular object key..
Comments
The shortest code would be:
map[key] = ++map[key] || 1
hasOwnPropertyto check for the existence of the prop you are looking for ?