ecma6 yield

function * generator(k){
   console.log(‘begin‘);
   var x = yield k;
   console.log(‘x:‘,x);
   var y = yield x+k;
   console.log(‘y:‘,y);
   return x+y+k;
}

var o = generator(1);
var r = o.next();
console.log(‘1:‘+r.value, ‘done:‘+r.done);
r = o.next(3);
console.log(‘2:‘+r.value,‘done:‘+r.done);
r = o.next(5);
console.log(‘3:‘+r.value,‘done:‘+r.done);

//log
begin
1:1 done:false
x: 3
2:4 done:false
y: 5
3:9 done:true

===========================================================================
function * generator(k){
   console.log(‘begin‘);
   var x = yield k;
   console.log(‘x:‘,x);
   return ‘end‘;
   var y = yield x+k;
   console.log(‘y:‘,y);
   return x+y+k;
}

var o = generator(1);
var r = o.next();
console.log(‘1:‘+r.value,‘done:‘+r.done);
r = o.next(3);
console.log(‘2:‘+r.value,‘done:‘+r.done);
r = o.next(5);
console.log(‘3:‘+r.value,‘done:‘+r.done);

//log
begin
1:1 done:false
x: 3
2:end done:true
3:undefined done:true

迭代构造器可以接受初值，在构造时传入而非首次调用 next 时，
yield 后面的值被返回，后继代码暂时中断，直到调用 next 方法，

yield 赋值表达式：在下一次调用 next 时将传入的值赋給变量，

如果遇到 return 则迭代执行结束