Good to know: JavaScript #JavaScript

default value for undefined property in Object

Behavior when there is no property

var object = {
  value: 'hoge',
};
console.log(object.key);
// key is not exist, therefore will be undefined
undefined

in js undefined treat as false.

var object = {
  value: 'hoge',
}
var defaultObject = {
  value: 'key' in object ? object.key : 'defaultValue',
};
// or
var defaultObject2 = {
  value: object.hasOwnProperty('key') ? object.key : 'defaultValue',
};
console.log(defaultObject);
// object.key is not exist, therefore will be `defaultValue`
'defaultValue'
'defaultValue'

var object = {
  value: 'hoge',
};
var defaultObject = {
  key: 'defaultValue',
};
$.extend(defaultObject, object);

delete value by key in obj

var obj = {
  keyA: 'a',
  keyB: 'b',
  keyC: 'c',
};

delete arr['keyB'];
// or
delete arr.keyB;
console.log(obj);
Object {keyA: "a", keyC: "c"}

Object.assign

When merge objects with same properties, right object is higher priority.

var o1 = { a: 1, b: 1, c: 1 };
var o2 = { b: 2, c: 2 };
var o3 = { c: 3 };

var obj = Object.assign({}, o1, o2, o3);
console.log(obj); // { a: 1, b: 2, c: 3 }

`||` and `&&` in Javascript

In Javascript || and && are not exactly same with boolean operator.

case 1. || means left value is true return left value.
case 2. || means left value is false return right value.
case 3. && means left value is ture return right value.
case 4. && means left value is false return left value.

// case 1
var x = 'a' || 'b'; // x -> 'a'

// case 2
var x = '' || 'a'; // x -> 'a'

// case 3
var y = 'a' && 'b'; // y -> 'b'

// case 4
var y = '' && 'a'; // y -> ''

`true` and `false` in Javascript

value	type	results
{}	object	true
'hoge'	string	true
''	string	false
1	number	true
-1	number	true
0	number	false
[]	object	true
true	boolean	true
false	boolean	false
undefined	undefined	false
null	null	false

Good to know: jq

Load jquery on Chrome console

var jq = document.createElement('script');
jq.src = "https://ajax.googleapis.com/ajax/libs/jquery/2.1.4/jquery.min.js";
document.getElementsByTagName('head')[0].appendChild(jq);
// ... give time for script to load, then type.
jQuery.noConflict();

jquery selector

// select element by attr
$('[data-react-id]=""')

Good to know: JavaScript