y=y||'world'与y=y?y:'world'

1.y=y||’world’

function log(x,y){
     y=y||’world’;
     console.log(x,y)
}
log(‘hello’)===>hello world
 log('hello','')==>hello world

 

 

 

2.y=y?y:'world'

function log(x,y){
     y=y?y:'world';
     console.log(x,y)
}
log('hello’)==>hello world
 log('hello','')==>hello world

 

 

 

3.传实参

function log(x,y='world'){
console.log(x,y)
}

log('hello')===> hello world
log('hello','')==>hello