Джава скрипт строка выводит NaN

 
s = value[1] - value[0]; 
t = value[0] - value[1]; 
if (value[1] > value[0]){
document.getElementById('k_s').innerText = "-" + +s.toFixed(2);
	}
	else { 
document.getElementById('k_t').innerText = "+" + +t.toFixed(2);
}
 

document.write('<span id="k_s"><div id="s"></div> </span><span id="k_t"><div id="t"></div> </span>') // -1
let v=[0,1];
let a=v[0];
let b=v[1];
let s = b-a; 
let t = a-b; 
if (b > a){
document.getElementById('k_s').innerText = "-" + +s.toFixed(2);
	}
	else { 
document.getElementById('k_t').innerText = "+" + +t.toFixed(2);
}
console.log(v) //Array [ 0, 1 ]

 
s = value[1] - value[0]; 
t = value[0] - value[1]; 
 

var s = value[1] - value[0]; 
var t = value[0] - value[1];