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Object.keys Function (JavaScript)

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

Object.keys(object)

Parameter

Definition

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.

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

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

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 (JavaScript).

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

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

Supported in the following document modes: Internet Explorer 9 standards, Internet Explorer 10 standards, and Internet Explorer 11 standards. Also supported in Store apps (Windows 8 and Windows Phone 8.1). See Version Information. Not supported in the following document modes: Quirks, Internet Explorer 6 standards, Internet Explorer 7 standards, Internet Explorer 8 standards.

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