keys

Summary

Returns the names of the enumerable properties and methods of an object.

Syntax

Object.keys( object )

object: Required. The object that contains the properties and methods. This can be an object that you created or an existing Document Object Model (DOM) object.

Return Value

An array that contains the names of the enumerable properties and methods of the object.

Examples

The following example creates an object that has three properties and a method. It then uses the keys method to get the properties and methods of the object.

// Create a constructor function.
 function Pasta(grain, width, shape) {
     this.grain = grain;
     this.width = width;
     this.shape = shape;

     // Define a method.
     this.toString = function () {
         return (this.grain + ", " + this.width + ", " + this.shape);
     }
 }

 // Create an object.
 var spaghetti = new Pasta("wheat", 0.2, "circle");

 // Put the enumerable properties and methods of the object in an array.
 var arr = Object.keys(spaghetti);
 document.write (arr);

 // Output:
 // grain,width,shape,toString

The following example displays the names of all enumerable properties that start with the letter “g” in the Pasta object.

// Create a constructor function.
 function Pasta(grain, width, shape) {
     this.grain = grain;
     this.width = width;
     this.shape = shape;
 }

 var polenta = new Pasta("corn", 1, "mush");

 var keys = Object.keys(polenta).filter(CheckKey);
 document.write(keys);

 // Check whether the first character of a string is "g".
 function CheckKey(value) {
     var firstChar = value.substr(0, 1);
     if (firstChar.toLowerCase() == "g")
         return true;
     else
         return false;
 }

 // Output:
 // grain

Remarks

The keys method returns only the names of enumerable properties and methods. To return the names of both enumerable and non-enumerable properties and methods, you can use Object.getOwnPropertyNames Function.

For information about the enumerable attribute of a property, see Object.defineProperty Function and Object.getOwnPropertyDescriptor Function.

Exceptions

If the value supplied for the object argument is not the name of an object, a TypeError exception is thrown.

Attributions

Microsoft Developer Network: Article