测试用例如图

it('test_lookahead2',()=>{
        const it = new PeekIterator(arrayToGenerator([..."abcdefg"]))
        assert.equal(it.next(),'a')
        assert.equal(it.peek(),'b')
        assert.equal(it.peek(),'b')
        assert.equal(it.next(),'b')
        assert.equal(it.next(),'c')
        it.putBack()
        it.putBack()
        assert.equal(it.peek(),'b')
        assert.equal(it.next(),'b')
        assert.equal(it.next(),'c')
        assert.equal(it.next(),'d')
    })

如果在putback以后进行peek的话，下一个值就不应该是stackputback的头元素了而应该是尾部元素了

peek(){
        if(this.stackPutBacks.length>0){
            return this.stackPutBacks.tail
        }
        const val = this.next()
        this.putBack()
        return val

    }

改造方法如上能通过所有测试用例