Node.js：_.chain下划线中的异步回调

Question

The arr.length is still undefined after I call _.chain() underscore operation:

var arr = _.chain(msg)
   .groupBy(function(item) {return item.ts_simple; })
   .value();

console.log(arr.length); //undefined
console.log(arr); //correct object array

The first console.log(arr.length) for underscore var array length is undefined while the console.log(arr) outputs the correct returned object array.

How do I callback after underscore operation is complete?

Answer 1

groupBy() returns an object, not an array.

It has no length property.