Date

Get the number of days in a month

The 0th day of next month is the last day of the current month.

function daysInMonth(year, month) {

let date = new Date(year, month + 1, 0);

return date.getDate();

} /**

* Note that JS Date month starts with 0

* The following computes how many days in March 2017

*/

console.log(daysInMonth(2017, 2)); // 31

// how many days in Feb 2017

console.log(daysInMonth(2017, 1)); // 28

// how many days in Feb 2016

console.log(daysInMonth(2016, 1)); // 29

getTimezoneOffset — get the time zone difference, in minutes, from current locale (host system settings) to UTC.

let now = new Date();

console.log(now.toISOString()); //2018-03-12T01:12:29.566Z

// China is UTC+08:00

console.log(now.getTimezoneOffset()); // -480

// convert to UTC

let UTCDate = new Date(now.getTime() + now.getTimezoneOffset() * 60 * 1000);

console.log(UTCDate.toISOString()); //2018-03-11T17:12:29.566Z

//convert to UTC+03:00

let eastZone3Date = new Date(UTCDate.getTime() + 3 * 60 * 60 * 1000);

console.log(eastZone3Date.toISOString()); //2018-03-11T20:12:29.566Z

JSON

When replacer is a function — apply replacer before stringify the value.

JSON.stringify({

a: 4,

b: [3, 5, 'hello'],

}, (key, val) => {

if(typeof val === 'number') {

return val * 2;

}

return val;

}); //{"a":8,"b":[6,10,"hello"]}

when replacer is an array — use replacer as a white list to filter the keys

JSON.stringify({

a: 4,

b: {

a: 5,

d: 6

},

c: 8

}, ['a', 'b']); //{"a":4,"b":{"a":5}}

space can be used to beautify the output

JSON.stringify({

a: [3,4,5],

b: 'hello'

}, null, '|--\t');

/**结果:

{

|-- "a": [

|-- |-- 3,

|-- |-- 4,

|-- |-- 5

|-- ],

|-- "b": "hello"

}

*/

String

###

''.split('') // []

separator can be a regular expression!

'abc1def2ghi'.split(/\d/); //["abc", "def", "ghi"]

If the seperator is a regular expression that contains capturing groups, the capturing groups will appear in the result as well.

'abc1def2ghi'.split(/(\d)/); // ["abc", "1", "def", "2", "ghi"]

let person = 'Mike';

let age = 28; function myTag(strings, personExp, ageExp) {

let str0 = strings[0]; // "that "

let str1 = strings[1]; // " is a " // There is technically a string after

// the final expression (in our example),

// but it is empty (""), so disregard.

// var str2 = strings[2]; let ageStr;

if (ageExp > 99){

ageStr = 'centenarian';

} else {

ageStr = 'youngster';

} return str0 + personExp + str1 + ageStr;

} let output = myTag`that ${ person } is a ${ age }`;

console.log(output);

// that Mike is a youngster

null vs undefined

If we don’t want to distinguish null and undefined, we can use ==

undefined == undefined //true

null == undefined // true

0 == undefined // false

'' == undefined // false

false == undefined // false

Don’t simply use == to check for the existence of a global variable as it will throw ReferenceError. Use typeof instead.

// a is not defiend under global scope

a == null // ReferenceError

typeof a // 'undefined'

Spread Operator(…)

spread operator works for objects!

const point2D = {x: 1, y: 2};

const point3D = {...point2D, z: 3}; let obj = {a: 'b', c: 'd', e: 'f'};

let {a, ...other} = obj;

console.log(a); //b

console.log(other); //{c: "d", e: "f"}

Reference

Notice