Difference between _.forEach and _.forOwn in lodash

foreach lodash
Novellizator picture Novellizator · Apr 23, 2014 · Viewed 21.4k times · Source

What is the difference between these two methods when iterating over an object?

Answer

Thalis K. picture Thalis K. · Apr 30, 2014

The difference lies in that if the collection over which you are iterating is an object which has a length property, then the _.forEach() will iterate over it as if it were an array, whereas the _.forOwn() will iterate over it like an object.

Assume you have the object:

a = {
  x: 100, 
  y: 200, 
  length: 2
}

If you iterate over it as:

_.forEach(a, function(val, key) {
  console.log('a[' + key + '] = ' + val); 
});

you'll get output:

a[0] = undefined
a[1] = undefined

whereas iterating over it with _.forOwn() you'll get the more reasonable:

a[x] = 100
a[y] = 200
a[length] = 2

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