• JavaScript

Type Checking in JS

added on October 17, 2022 (2y ago)

Primitives:

  1. String:
if (typeof str === 'string')
if (str instanceof String || Object.prototype.toString.call(str) === '[object String]')
if (str.constructor === String)
  1. Number:
if (typeof num === 'number')
if (num instanceof Number || Object.prototype.toString.call(num) === '[object Number]')
if (num.constructor === Number)
  • Integer:
if (Number.isInteger(num))
if (num % 1 === 0)
if (Number.isSafeInteger(num)) // The **`Number.isSafeInteger()`** method determines whether a value is a safe integer (within the range of **`-2^53`** to **`2^53 - 1`**).
  • Float:
if (!Number.isInteger(num))
if (num % 1 !== 0)
  • NaN:
if (Number.isNaN(value))
if (isNaN(value))
if (value !== value) // As mentioned earlier, **`NaN`** is the only value that is not equal to itself. You can take advantage of this property for detection.
  1. BigInt:
if (typeof bigIntValue === 'bigint')
if (bigIntValue.constructor === BigInt)
if (bigIntValue instanceof BigInt || Object.prototype.toString.call(bigIntValue) === '[object BigInt]')
if (BigInt.asIntN(64, bigIntValue) === bigIntValue) // This method can check if the value is within the range of a certain bit length
  1. Symbol:
if (typeof symbolValue === 'symbol')
if (symbolValue === Symbol.iterator)
if (symbolValue instanceof Symbol || Object.prototype.toString.call(symbolValue) === '[object Symbol]')
if (symbolValue.constructor === Symbol)
  1. Boolean:
if (typeof boolValue === 'boolean')
if (boolValue instanceof Boolean || Object.prototype.toString.call(boolValue) === '[object Boolean]')
if (boolValue.constructor === Boolean)
  1. Null:
if (nullValue === null)
if (typeof nullValue === 'object' && nullValue === null) // The **`typeof`** operator returns **`'object'`** for **`null`**, which is a historical quirk and not a true indication of its type. Therefore, using strict equality (**`===`**) is the preferred way to check for **`null`** values.
  1. Undefined:
if (undefinedValue === undefined)
if (typeof undefinedValue === 'undefined')
if (undefinedValue === undefinedValue) // **Checking if a variable is declared but not assigned a value**
if (undefinedValue === void 0)

Reference:

  1. Object:
if (typeof obj === 'object' && obj !== null)
if (obj instanceof Object && obj !== null)
if (Object.prototype.toString.call(obj) === '[object Object]')
  • Array:
if (Array.isArray(arr))
if (arr instanceof Array)
if (arr.constructor === Array)
if (Object.getPrototypeOf(arr) === Array.prototype)
  • Function:
if (typeof myFunction === 'function')
if (myFunction instanceof Function)
  • Date:
if (myDate instanceof Date)
if (Object.prototype.toString.call(myDate) === '[object Date]')
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