To check if a variable is a number in JavaScript:

1. Use the `Number.isFinite()` method to determine whether the passed value is a number.
2. The `Number.isFinite()` method returns `true` if the passed value is not positive `Infinity`, negative `Infinity`, or `NaN`.
``````Number.isFinite(34)         // true
Number.isFinite(2e64)       // true

Number.isFinite('0')        // false
Number.isFinite(null)       // false
Number.isFinite({})         // false

Number.isFinite(Infinity)   // false
Number.isFinite(NaN)        // false
Number.isFinite(-Infinity)  // false
``````

The `Number.isFinite()` method was introduced in ES6 to check if the given value is a finite number. It returns `true` if the passed value is a number, otherwise `false`.

JavaScript also provides a global `isFinite()` function that converts the parameter to a `Number` and returns `true` if the resulting value is not positive or negative `Infinity`, `NaN`, or `undefined`.

This means, with `Number.isFinite()`, only values of the type number and are finite return `true`, and non-numbers always return `false`:

``````isFinite('2')             // true
Number.isFinite('2')      // false

isFinite(null)            // true
Number.isFinite(null)     // false
``````

Alternatively, you can use the `Number.isNaN()` method to check if the passed value is a `Number` and `NaN` (Not a Number). This method is a more robust replacement of the global `isNaN()` function in JavaScript.

``````Number.isNaN(34)         // false <-- 34 is a valid number
Number.isNaN(2e64)       // false

Number.isNaN('0')        // false <-- 0 is a valid number
Number.isNaN(null)       // false (false positive?)
Number.isNaN({})         // false

Number.isNaN(Infinity)   // false
Number.isNaN(NaN)        // true <-- NaN is "Not a Number"
Number.isNaN(-Infinity)  // false
``````

Unlike the global `isNaN()` function, `Number.isNaN()` does not convert the argument to a `Number`; it returns `true` when the argument is a `Number` and is `NaN`.

Finally, the `typeof` operator can also be used to check if the variable type is a `number` in JavaScript:

``````typeof 3 === 'number'       // true
typeof '0' === 'number'     // false ("string" type)

typeof null === 'number'          // false
typeof {} === 'number'            // false
typeof undefined === 'number'     // false

typeof NaN === 'number'         // true (false positive?)
typeof Infinity === 'number'    // true
``````

The `typeof` operator is more robust than `Number.isNaN()` and it correctly identifies `null` and `undefined` are not numbers.

However, it returns `true` for `Infinity` and `NaN` because these are special number values in JavaScript.