Question: I have some arrays as below and I want to know how to get the index of the first number by js?

```
Arr1=[null,null,null,null,null,...,343,959,543,..,252,null,null,...,null]

```

Answer: 
You can use `findIndex`:

```
Arr1.findIndex(value => value !== null)

```
You can loop through the array and check it's value:

```js
const Arr1=[null,null,null,null,null,343,959,543,252,null,null,null];

let index = -1;
for(let i = 0; i < Arr1.length; i++)
{
  if (Arr1[i] !== null)
  {
    index = i;
    break;
  }
}

console.log(index);
```