JavaScript Advanced Series—Object Usage and Properties

• Object as data type

• Access properties

• Delete properties

• Syntax of property names

Object usage and properties

All variables in JavaScript can be used as objects, with two exceptions null and undefined .

false.toString(); // 'false'
[1, 2, 3].toString(); // '1,2,3'

function Foo(){}
Foo.bar = 1;
Foo.bar; // 1

A common misunderstanding is that numerical literals cannot be used as objects. This is due to a bug in the JavaScript parser, which attempts to parse dot operators as part of a floating-point literal value.

2.toString(); // 出错：SyntaxError

There are many workarounds to make number literals look like objects.

2..toString(); // 第二个点号可以正常解析
2 .toString(); // 注意点号前面的空格
(2).toString(); // 2先被计

Objects as data types

JavaScript objects can be used as hash tables, mainly used to save the correspondence between named keys and values.

Using the object literal syntax - {} - you can create a simple object. This newly created object inherits from Object.prototype and does not have any custom properties.

var foo = {}; // 一个空对象

// 一个新对象，拥有一个值为12的自定义属性'test'
var bar = {test: 12};

Access properties

There are two ways to access the properties of an object, the dot operator or the square bracket operator.

var foo = {name: 'kitten'}
foo.name; // kitten
foo['name']; // kitten

var get = 'name';
foo[get]; // kitten

foo.1234; // SyntaxError
foo['1234']; // works

The two syntaxes are equivalent, but the bracket operator is still valid in the following two situations

• Dynamic setting of attributes

• The attribute name is not a valid variable name (Translator's Note: For example, the attribute name contains spaces, or the attribute name is a JS keyword)

Delete attribute

The only way to delete an attribute is to use the delete operator; setting the attribute to undefined or null does not actually delete the attribute, but only removes the association between the attribute and the value.

var obj = {
    bar: 1,
    foo: 2,
    baz: 3
};
obj.bar = undefined;
obj.foo = null;
delete obj.baz;

for(var i in obj) {
    if (obj.hasOwnProperty(i)) {
        console.log(i, '' + obj[i]);
    }
}

The above output has bar undefined and foo null - only baz is actually deleted, so it disappears from the output.

Syntax of attribute names

var test = {
    'case': 'I am a keyword so I must be notated as a string',
    delete: 'I am a keyword too so me' // 出错：SyntaxError
};

The attribute names of objects can be declared using strings or ordinary characters. However, due to another incorrect design of the JavaScript parser, the second declaration method above will throw a SyntaxError before ECMAScript 5.

The reason for this error is that delete is a keyword in the JavaScript language; therefore, in order to run normally under lower versions of JavaScript engines, string literal declaration must be used.

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