Examples of javascript object usage and property manipulation

  • 2020-03-30 02:11:24
  • OfStack

All variables in JavaScript are 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 misconception is that the literal value of a number is not an object. This is because of an error in the JavaScript parser, which tried to resolve the point operator as part of a floating point numeric face value.

2.toString(); //Error: SyntaxError

There are many workarounds for making numeric literals look like objects.

2..toString(); //The second dot can be resolved normally
2 .toString(); //Notice the space before the dot
(2).toString(); //The 2 is calculated first

Object as a data type

JavaScript objects can be used as hash tables to store named keys and values.

Use the literal syntax of the object - {} - to create a simple object. This newly created Object inherits from object.prototype without any custom properties.

var foo = {}; //An empty object

//A new object with a custom property of 'test' with a value of 12
var bar = {test: 12}; 

To access attributes

There are two ways to access an object's properties, the dot operator or the 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 syntax is equivalent, but the bracket operator is still valid in the following two cases - dynamically setting the property - the property name is not a valid variable name.

Note: in the JSLint syntax detection tool, the dot operator is recommended.


Delete the properties

The only way to delete a property is to use the delete operator; Setting a property to undefined or null does not actually delete the property, it simply removes the association between the property 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 output above has bar undefined and foo null - only baz is actually deleted, so it disappears from the output.


Syntax for 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' //Error: SyntaxError

The property name of an object can be declared using either a string or an ordinary character. However, the second declaration above throws the SyntaxError error before ECMAScript 5 due to another error design in the JavaScript parser.

The reason for this error is that delete is a keyword in the JavaScript language; So in order to work with a lower version of the JavaScript engine, you have to use string literal declarations.

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