1

if（bool）{
    a =1;
}else{
  a=2
}
// 简写
a= bool ? 1 : 2

2

if(bool){
 a=fn()
}

//简写
bool && (a=fn())

if(!bool) {
 a=fn()
}

//简写
bool || (a=fn())

3

if(b===1){
  a='one'
} else if (b===2) {
 a='two'
} else {
  return ''
}
//简写
let res ={1：'one',2:'two'}
a= res[key] ? res[key] : '';

4

function fn(a){
          if(a==='x'){
            doX();
          }else if(a==='y'){
                    doY();
              }else{
                  doZ();
        }
  }
//简写
function fn(a){
       let lookup={x:doX:y:doY},def=doZ;
           lookup[a] ? lookup[a]() : def();
}

5.if多条件

if (x === 'abc' || x === 'def' || x === 'ghi' || x ==='jkl') {}
// 简写
if (['abc', 'def', 'ghi', 'jkl'].includes(x)) {}

6.forEach循环

for (var i = 0; i < testData.length; i++)
  
// 简写
for (let i in testData)
// 或
for (let i of testData)

7.隐式返回

function getArea(diameter) {
  return Math.PI * diameter
}

// 简洁
getArea = diameter => (
  Math.PI * diameter;
)