In ES6, a **safe integer** is an integer in the range $(-2^{53}, 2^{53})$. These integers are *safe* because there is a one-to-one mapping between the mathematical integers and their representations in JavaScript.  

Beyond this range, JavaScript integers are *unsafe*, which means that or more mathematical integers are represented as the same JavaScript integer. For example, starting at $2^{53}$, JavaScript can only represent every second mathematical integer. 

```js
> Math.pow(2, 53)
9007199254740992
> Math.pow(2, 53) + 1
9007199254740992
> Math.pow(2, 53) + 2
9007199254740994
> Math.pow(2, 53) + 3
9007199254740996
```

> Formally, JavaScript documentation defines a safe integer as a number that
> - can be exactly represented as an IEEE-754 double precision number
> - the IEEE-754 representation cannot be the result of rounding any other integer to fit the IEEE-754 representation.



# Code

JavaScript ES6 has the built-in method **`Number.isSafeInteger()`** to check if a number is a safe integer.

function warn(x) {
  if (Number.isSafeInteger(x)) {
    return 'Precision safe.';
  }
  return 'Precision may be lost!';
}

console.log(warn(Math.pow(2, 53)));
// expected output: "Precision may be lost!"

console.log(warn(Math.pow(2, 53) - 1));
// expected output: "Precision safe."

What is a safe integer in ES6?

In ES6, a safe integer lies within the range (−2^53, 2^53) and can be checked with Number.isSafeInteger().