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Useful APIs that you probably don't notice

崔晓明 / 3077人阅读

摘要：结果

Date Get the number of days in a month

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

</>复制代码 
  1. function daysInMonth(year, month) {
  2.     let date = new Date(year, month + 1, 0);
  3.     return date.getDate();
  4. }
  5. /**
  6.  * Note that JS Date month starts with 0
  7.  * The following computes how many days in March 2017
  8.  */
  9. console.log(daysInMonth(2017, 2)); // 31 
  10. // how many days in Feb 2017
  11. console.log(daysInMonth(2017, 1)); // 28
  12. // how many days in Feb 2016
  13. console.log(daysInMonth(2016, 1)); // 29
getTimezoneOffset - get the time zone difference, in minutes, from current locale (host system settings) to UTC. 
</>复制代码 
  1. let now = new Date(); 
  2. console.log(now.toISOString()); //2018-03-12T01:12:29.566Z
  3. // China is UTC+08:00
  4. console.log(now.getTimezoneOffset()); // -480
  5. // convert to UTC 
  6. let UTCDate = new Date(now.getTime() + now.getTimezoneOffset() * 60 * 1000);
  7. console.log(UTCDate.toISOString()); //2018-03-11T17:12:29.566Z
  8. //convert to UTC+03:00
  9. let eastZone3Date = new Date(UTCDate.getTime() + 3 * 60 * 60 * 1000);
  10. console.log(eastZone3Date.toISOString()); //2018-03-11T20:12:29.566Z
JSON [JSON.stringify(value[, replacer[, space]])](https://developer.mozilla.org... When replacer is a function - apply replacer before stringify the value. 
</>复制代码 
  1. JSON.stringify({
  2.     a: 4,
  3.     b: [3, 5, "hello"],
  4. }, (key, val) => {
  5.     if(typeof val === "number") {
  6.         return val * 2;
  7.     }
  8.     return val;
  9. }); //{"a":8,"b":[6,10,"hello"]}
when replacer is an array - use replacer as a white list to filter the keys 
</>复制代码 
  1. JSON.stringify({
  2.     a: 4,
  3.     b: {
  4.         a: 5,
  5.         d: 6
  6.     },
  7.     c: 8
  8. }, ["a", "b"]); //{"a":4,"b":{"a":5}}
space can be used to beautify the output 
</>复制代码 
  1. JSON.stringify({
  2.     a: [3,4,5],
  3.     b: "hello"
  4. }, null, "|--
  5. ");
  6. /**结果:
  7. {
  8. |--    "a": [
  9. |--    |--    3,
  10. |--    |--    4,
  11. |--    |--    5
  12. |--    ],
  13. |--    "b": "hello"
  14. }
  15. */
String [String.prototype.split([separator[, limit]])](https://developer.mozilla.org... 
</>复制代码 
  1. "".split("") // []
separator can be a regular expression! 
</>复制代码 
  1. "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.

</>复制代码 
  1. "abc1def2ghi".split(/(d)/);  // ["abc", "1", "def", "2", "ghi"]
Tagged string literals 
</>复制代码 
  1. let person = "Mike";
  2. let age = 28;
  3. function myTag(strings, personExp, ageExp) {
  4.   let str0 = strings[0]; // "that "
  5.   let str1 = strings[1]; // " is a "
  6.   // There is technically a string after
  7.   // the final expression (in our example),
  8.   // but it is empty (""), so disregard.
  9.   // var str2 = strings[2];
  10.   let ageStr;
  11.   if (ageExp > 99){
  12.     ageStr = "centenarian";
  13.   } else {
  14.     ageStr = "youngster";
  15.   }
  16.   return str0 + personExp + str1 + ageStr;
  17. }
  18. let output = myTag`that ${ person } is a ${ age }`;
  19. console.log(output);
  20. // that Mike is a youngster
null vs undefined

If we don"t want to distinguish null and undefined, we can use ==

</>复制代码 
  1. undefined == undefined //true
  2. null == undefined // true
  3. 0 == undefined // false
  4. "" == undefined // false
  5. false == undefined // false

Don"t simply use == to check for the existence of a global variable as it will throw ReferenceError. Use typeof instead.

</>复制代码 
  1. // a is not defiend under global scope
  2. a == null // ReferenceError
  3. typeof a // "undefined"
Spread Operator(...)

spread operator works for objects!

</>复制代码 
  1. const point2D = {x: 1, y: 2};
  2. const point3D = {...point2D, z: 3};
  3. let obj = {a: "b", c: "d", e: "f"};
  4. let {a, ...other} = obj;
  5. console.log(a); //b
  6. console.log(other); //{c: "d", e: "f"}
Reference

Speaking Javascript

MDN

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