You can use _.every, _.some, and _.isObject to differentiate between arrays of primitives, arrays of objects, and arrays containing both primitives and objects.
Basic Syntax
// Is every array element an object?
_.every(array, _.isObject)
// Is any array element an object?
_.some(array, _.isObject)
// Is the element at index `i` an object?
_.isObject(array[i])
More Examples
var primitives = [1, 2, 3]
var objects = [{name: 1}, {name: 2}]
var mixed = [{name: 1}, 3]
// Is every array element an object?
console.log( _.every(primitives, _.isObject) ) //=> false
console.log( _.every(objects, _.isObject) ) //=> true
console.log( _.every(mixed, _.isObject) ) //=> false
// Is any array element an object?
console.log( _.some(primitives, _.isObject) ) //=> false
console.log( _.some(objects, _.isObject) ) //=> true
console.log( _.some(mixed, _.isObject) ) //=> true
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