Initialise an object with array properties

Question

How can I initialise an object in JavaScript, the properties of which would be arrays?

I want to have an object of this format:

foo = { prop1: [0, 1], prop2: [1, 1], prop3: [0] }

My usecase is the following:

- When a property does not exist, create this property which should be an array and add a number.

- When a property exists already, push the number to that array; this way I cannot initialise an array every time.

What I've done so far is this:

  var obj = {};
  arr.forEach(x => { !obj[x] && obj[x].push(1) });

I am getting this error:

Uncaught TypeError: Cannot read property 'push' of undefined

which makes sense, as the property has not been initialised as an empty array.

For example, I am iterating over two arrays, I want to have 0/1 (or true/false) if the arrays include some specific items, e.g. array1 = ["cat", "dog"], array2 = ["dog", "mouse"]; the object would be of this format: { "cat": [1, 0], "dog": [1, 1], "mouse": [0,1] } — luthien
– luthien, Commented Aug 14, 2020 at 19:49

ObsoleteAwareProduce · Accepted Answer · 2020-08-14 19:38:27Z

1

Add this snippet:

arr.forEach((x) => {obj[x] = (obj[x] || []).concat(1);})

If obj[x] is undefined, then it hasn't been initialized. Therefore, undefined || [] resolves to [], an empty array, to which 1, or whatever data you want, can be concatenated.

answered Aug 14, 2020 at 19:38

ObsoleteAwareProduce

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2 Comments

Giovanni Esposito Over a year ago

This is elegant :)

ObsoleteAwareProduce Over a year ago

Thanks! Javascript's lazy || operator can be very useful. It's often used in functions with optional parameters, to replace undefined with a default value.

Giovanni Esposito · Accepted Answer · 2020-08-14 19:44:10Z

1

Ciao you could try this:

let obj = {};
let arr = ["prop1","prop2","prop1","prop3"];
arr.forEach((x) => {
   if(obj.hasOwnProperty(x)) obj[x].push(1);
   else {
      obj[x] = [];
      obj[x].push(1);
   } 
})
console.log(obj);

if obj has already property x (hasOwnProperty) push value 1 else init x as array property and push 1.

answered Aug 14, 2020 at 19:44

Giovanni Esposito

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1 Comment

luthien Over a year ago

Thank you, this is also a nice solution!

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