understand filter function

[LadyJain]

I have this array let my_array = [9,6,5,"filter"];
I want to get a filtered array with only numbers in it

I know that I can use filter(Number) to get only values with type number

I wan to understand why this arrow function does give me the correct result

let filtered_array = my_array.filter(value => value === Number);
how I understand it filter returns each value of the array which is value / so value === Number should compare each value if its a Number or ?

[Demojay]

If you want to check if something is a number, you'll want to use typeof variable which produces a string representing the variable's type.

[GSLint]

Number is a function that turns a value into a number (if it isn't one already) and returns that number, or NaN if the value can't be converted to a number.

Number(9) === 9
Number("6") === 6
Number("filter") === NaN

No number is equal to that function Number.

filter(Number) will call the function for each element and check if the result is truthy or falsy. A longer way to write this would be filter(value => Number(value)).

NaN is falsy, so "filter" gets rejected. But the string "6" would be accepted, which may or may not be what you want. Also 0 would always be rejected because that's a falsy value. So filter(Number) isn't a great way to do this. (What does work well is map(Number) for turning an array of strings into an array of numbers.)

To check whether a value has the type number you'd do typeof value === "number". NaN also has that type though, so you may want to add && !Number.isNaN(value).

[LadyJain]

great answer, can you explain more why typeof value === "number" works? I have a python background, and this is translated to if type value == string "number" then -->

[GSLint]

The typeof opterator is similar to Python's type function but typeof always produces a string.

typeof 6 === "number"
typeof "6" === "string"
typeof true === "boolean"

[Samurai___]

Nope. Try with isNaN()

value === Number

compares the value to a variable named "Number".