Checks if the given argument is a number.

Use typeof to check if a value is classified as a number primitive.
To safeguard against NaN, check if val === val (as NaN has a typeof equal to number and is the only value not equal to itself).


const isNumber = val => typeof val === 'number' && val === val;


isNumber(1); // true
isNumber('1'); // false
isNumber(NaN); // false