Object（obj）中的“'length'”在js中如何工作？

Question

'length' in Object(obj)

How does the above statement works in JavaScript?

I have found the below JavaScript statement in angular.js lib

var length = 'length' in Object(obj) && obj.length;

Answer 1

Object(...) converts primitives to object structure. Examples:

Object(null) // {}
Object(undefined) // {}
Object(10) // Number {[[PrimitiveValue]]: 10}

If you type 'length' in 10 it will throw an error, but 'length' in Object(10) will not (returns false and blocks next statement, which would throw an error if obj has a primitive value).

Your code checks if obj variable is an array or array-like object (has length property).

More about Object and in operator .

Answer 2

That will check whether the property length exists in Object(obj) or not

I guess that is to differentiate between Array and Object.

Because Object doesn't have the property length so it will false for Object.

 var a = { name: "abc", age: "34" }; var l = 'length' in Object(a) && a.length; console.log(l); 

But array have the property length and it will return the length of the array.

 var b = [34, 54, 65]; var ll = 'length' in Object(b) && b.length; console.log(ll); 

When we type checking them, both are same.

 var a = []; var b = {}; console.log(typeof a, typeof b); 

There are other methods to differentiate Array and Object too.

Using Array.isArray

 var a = []; var b = {}; console.log(Array.isArray(a)); console.log(Array.isArray(b)); 

Answer 3

   var length = Object.keys(myArray).length;

Or

Universal function:-

 var objSize = function(obj) { var count = 0; if (typeof obj == "object") { if (Object.keys) { count = Object.keys(obj).length; } else if (window._) { count = _.keys(obj).length; } else if (window.$) { count = $.map(obj, function() { return 1; }).length; } else { for (var key in obj) if (obj.hasOwnProperty(key)) count++; } } return count; }; document.write(objSize({ a: 1, b: 2, c: 3 }));