如何区分对象和数组

Question

let a = {}
let b = []

typeof a // returns an object
typeof b // returns an object

 a === {} // false
 b === [] // false
 a === b // false

how then can I know if its an array or object, I'm trying to validate user input, where it can either be an array or object but in either Case I don't want the value to be empty

Answer 1

This is really a few questions wrapped up into one. First off, it is counter-intuitive for many that typeof [] is 'object' . This is simply because an Array is an reference type ( null , Date instances, and any other object references also have typeof of object ).

Thankfully, to know if an object is an instance of Array , you can now use the convenient Array.isArray(...) function. Alternatively, and you can use this for any type of object, you can do something like b instanceof Array .

Knowing if one of these is empty can be done by checking Object.keys(a).length === 0 , though for Arrays it's more natural to do b.length === 0 .

Checking any two objects variables (including Arrays) with === will only tell you if the two variables reference the same object in memory, not if their contents are equal.

Answer 2

Since both Arrays and Objects share the same type, you can check for instance:

if (b instanceof Array) {

}

Answer 3

if (Array.isArray(a) && a.length === 0) {
  // a is an array and an empty one
}

Answer 4

Actually, using typeof for both Objects and Arrays will return Object . There are certain methods in JS to check if a variable is an array or not.

1. Array.isArray(variableName)
2. variableName instanceof Array
3. variableName.constructor === Array

All these Methods will return true if variable is an array else will return false .