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
JSON.stringify(value[, replacer[, space]])
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');
/**結(jié)果:
{
|-- "a": [
|-- |-- 3,
|-- |-- 4,
|-- |-- 5
|-- ],
|-- "b": "hello"
}
*/
String
String.prototype.split([separator[, limit]])
''.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"]
Tagged string literals
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
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