type checking in javascript

How can I check if a variable is currently an integer type? I've looked for some sort of resource for this and I think the === operator is important, but I'm not sure how to check if a variable is an Integer (or an Array for that matter)

Solution 1:

A variable will never be an integer type in JavaScript — it doesn't distinguish between different types of Number.

You can test if the variable contains a number, and if that number is an integer.

(typeof foo === "number") && Math.floor(foo) === foo

If the variable might be a string containing an integer and you want to see if that is the case:

foo == parseInt(foo, 10)

Solution 2:

These days, ECMAScript 6 (ECMA-262) is "in the house". Use Number.isInteger(x) to ask the question you want to ask with respect to the type of x:

js> var x = 3
js> Number.isInteger(x)
true
js> var y = 3.1
js> Number.isInteger(y)
false

Solution 3:

A number is an integer if its modulo %1 is 0-

function isInt(n){
    return (typeof n== 'number' && n%1== 0);
}

This is only as good as javascript gets- say +- ten to the 15th.

isInt(Math.pow(2,50)+.1) returns true, as does Math.pow(2,50)+.1 == Math.pow(2,50)

Solution 4:

A clean approach

You can consider using a very small, dependency-free library like Issable. Solves all problems:

// at the basic level it supports primitives
let number = 10
let array = []
is(number).number() // returns true
is(array).number() // throws error

// so you need to define your own:
import { define } from 'issable'
// or require syntax
const { define } = require('issable')

define({
    primitives: 'number',
    nameOfTyping: 'integer',
    toPass: function(candidate) {
        // pre-ECMA6
        return candidate.toFixed(0) === candidate.toString()
        // ECMA6
        return Number.isInteger(candidate)
    }
})
is(4.4).custom('integer') // throws error
is(8).customer('integer') // returns true

If you make it a habit, your code will be much stronger. Typescript solves part of the problem but doesn't work at runtime, which is also important.

function test (string, boolean) {
    // any of these below will throw errors to protect you
    is(string).string()
    is(boolean).boolean()

    // continue with your code.
}