9个实用的JavaScript开发技巧，你一定要看下

const array = [1,2,3,4,5]; 
console.log（array）; // 5
array.length--; 
console.log（array）; // 4
array.length + = 15; 
console.log（array）; // 19

[1, 2, 3, 4, 5]
[1, 2, 3, 4]
[1, 2, 3, 4, ..15 empty]
undefined

let arr= ['a', 'b', 'c'];
arr.length = 0;
console.log(arr.length); // Ouput=> 0
console.log(arr); // Output=> []

var x=1;
if(x==1){
    console.log(x)
  }
else{
    console.log("Error")
  }

var x=1;
console.log( x==1? x: "Error"); //Output: 1

import defaultExport from "module-name";
import { export1 as alias1 } from "module-name";
import { export1 , export2 } from "module-name";
import { export1 , export2 as alias2 , [...] } from "module-name";
import defaultExport, { export1 [ , [...] ] } from "module-name";
import defaultExport, * as name from "module-name";

import（'some-module'）
     . 
     then（obj => ...）. catch（err => console.log（err））

const name = null ?? 'Anonymous';
console.log(name); // Output=> 'Anonymous'
const age= undefined?? 18;
console.log(bar); // Output=> 18

let firstName = null; 
let lastName = null; 
let nickName = "Guest";  
// shows the first truthy value: 
alert(firstName || lastName || nickName || "Anonymous"); 
//Guest

const array1 = ['a'，'b'，'c']; 
const array2 = ['a'，'e'，'f']; 
const array3 = array1.concat（array2）;
console.log（array3）; 
//输出=>数组[“ a”，“ b”，“ c”，“ a”，“ e”，“ f”]

let list1 = ['a', 'b', 'c'];
let list2 = ['a', 'e', 'f'];
list1.push.apply(list1, list2);
console.log(list1);
//Output=> Array ["a", "b", "c", "a", "e", "f"]

if (var1 !== null || var1 !== undefined || var1 !=='') {
     var2 = var1;
}

var2 = var1 || 'Some value';

function calculator(principle,rate,time) {
  if (principle === undefined)
    principle = 5000;
  if (rate === undefined)
    rate = 6;
  if (time===undefined)
    time = 3;
  ....
}

function calculator(principle=5000, rate=6, time=3){
   ...
}

const car = { make: 'Honda', model: 'Accord', year: 1998 };
console.log('make' in car);
// expected output: true
delete car.make;
if ('make' in car === false) {
  car.make = 'Suzuki';
}
console.log(car.make);
// expected output: "Suzuki"

var q= "onrender" in  document.createElement("div");
if(q){
  ...
}
else{
  ...
}

function submitName(name) {
  if(name=== undefined) {
    throw new Error('Missing parameter');
  }
  return name;
}

mandatory = () => {
  throw new Error('Missing parameter');
}

submitName= (name= mandatory()) => {
  return name;
}