javascript - How to `reduce` over an object? - Stack Overflow

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I wonder how to use some reduce logic over an object rather than an array. Something like iterating over tuples represented by

[ object_property, property_value ]).

I tried some code like

var obj = { foo: 'bar', bar: 'baz' };
Array.prototype.reduce.call(obj, function(prev, val) {
    console.log('new iteration');
    // whatever code ...
    return prev;
}, []);

but it doesn't perform any iteration. I can't understand why.

Maybe because the object properties are not enumerable?

Is there any way to run a reduce function on an object?

Note: There is no specific final purpose; I am exploring what's possible or what might be better patterns.

I wonder how to use some reduce logic over an object rather than an array. Something like iterating over tuples represented by

[ object_property, property_value ]).

I tried some code like

var obj = { foo: 'bar', bar: 'baz' };
Array.prototype.reduce.call(obj, function(prev, val) {
    console.log('new iteration');
    // whatever code ...
    return prev;
}, []);

but it doesn't perform any iteration. I can't understand why.

Maybe because the object properties are not enumerable?

Is there any way to run a reduce function on an object?

Note: There is no specific final purpose; I am exploring what's possible or what might be better patterns.

• javascript
• arrays
• reduce
• enumerable

Share Improve this question Follow edited Jan 26, 2021 at 14:21 E_net4 30.1k1313 gold badges114114 silver badges151151 bronze badges asked Mar 18, 2019 at 15:28 KamafeatherKamafeather 9,9251515 gold badges7979 silver badges105105 bronze badges 7

• 2 Array.prototype.reduce() assumes the context has a length property and keys [0..length - 1] – Patrick Roberts Commented Mar 18, 2019 at 15:31
• what is the expected output? – brk Commented Mar 18, 2019 at 15:31
• No expected output; that's not the problem. I want to know if it is possible to iterate with reduce, regardless what's the operation in the callback. – Kamafeather Commented Mar 18, 2019 at 15:32
• Also reduce expects that your callback returns the new value of the accumulator. I think you shouldn't be using reduce here at all. – Bergi Commented Mar 18, 2019 at 15:36
• 1 What do you want to acplish on running reduce in an object? What's the desired final result? Reduce objetive is obtaining an unique value. If you want to iterate .map is more adequate. – Nelson Teixeira Commented Mar 19, 2019 at 1:48

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4 Answers 4

Sorted by: Reset to default 5

Array methods like reduce() can only operate on arrays or array-like objects (with length and numeric properties).

You can call Object.values() to get an array of an object's property values.

Several mistakes.

• obj is not an array. Use .values()
• The result of prev.push is not the array, is the result of the push method which is 1 (integer);
• there is no var to get the final result.

Here is a working example:

var obj = { foo: 'bar', bar: 'baz' };
var res = Array.prototype.reduce.call(Object.values(obj), function(prev, val) {
    prev.push(val);
    return prev;
}, []);
console.log(res);

I don't know why you are using Array.prototype.reduce.call. There's no need to call it this way. Maybe you're experimenting with js or something. Also prev is not a good name. Remember that it accumulates values, it's not just the previous interaction's value. Anyway, this is an easier way to do it:

var obj = { foo: 'bar', bar: 'baz' };
var res = Object.values(obj).reduce((acc, val)=>{
    acc.push(val); 
    return acc;
}, []);
console.log(res);

But if this is the answer you want, reduce is not needed at all. Look:

var obj = { foo: 'bar', bar: 'baz' };
console.log(Object.values(obj));

You can run reduce() method on the array returned by Object.keys() function. It looks like this:

var obj = { foo: 'fooValue', bar: 'barValue' };
var str = Object.keys(obj).reduce((accum, key)=>{
    return accum + obj[key];
}, '');
// str = 'fooValuebarValue';

You can use Object.keys(obj) which will give you array of property names on that array you can apply any array method(predefined) like this

var obj = { foo: 'bar', bar: 'baz' };
var prev=[] //let say prev is array in which you want to store some property value

Object.keys(obj).reduce(function(element) {
    // you can have condition here as well
    prev.push(obj[element]);
});

but be careful if you just want to populate another array then you can use other array method like filter,map,forEach(just for learning purpose because it similar to for loop) it would make sense.

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