function factor(){
    let result = '';
    if(peek() > '0' && peek() < '9') return number();

calc('9+1') return emtpy string

so, You should write if(peek() > '0' && peek() <= '9').

LOL...

Thank you, you are right.

Hello, after listening to your advice,

I have made some tests as follows, but I have not found that Array.from is slower than [...Arr].

console.time("timer")
obj=document.querySelectorAll('a')
for(let i=1;i<9999999;++i){obj[i]=i}
[...obj]
console.timeEnd("timer");
VM299:5 timer: 948.02685546875ms


console.time("timer")
obj=document.querySelectorAll('a')
for(let i=1;i<9999999;++i){obj[i]=i}
Array.from(obj)
console.timeEnd("timer");
VM300:5 timer: 943.575927734375ms

console.time("timer")
obj=document.querySelectorAll('a')
for(let i=1;i<999;++i){obj[i]=i}
[...obj]
console.timeEnd("timer");
VM340:5 timer: 0.406005859375ms

console.time("timer")
obj=document.querySelectorAll('a')
for(let i=1;i<999;++i){obj[i]=i}
Array.from(obj)
console.timeEnd("timer");
VM341:5 timer: 0.410888671875ms

My Configuration

Google chrome
Version 65.0.3325.181 ( official version ) ( 64 bits )

I guess it must have something to do with V8.