How to Check Data Types in JS?

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I. Check undeclared, undefined or null

  1. undeclared check: try-catch-finally

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    try{
      var;   	// must use it here (in any way)
    }
    catch(e) {
      if(e.name === 'ReferenceError') {
        console.log('var is undeclared')
      }
    }

  2. undefined check: typeof var

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    var undefinedVar

    if (typeof undefinedVar === 'undefined') {
       console.log('var is undefined')
    }

  3. null check: if(var===null)

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    var nullVar = null

    if (nullVar === null) {
       console.log('var is null')
    }

II. String

  1. Three ways to generate String

    • String: let var = '1'
    • String: let var = String(1)
    • [object String]: let var = new String(1)

  2. How to check - [ primitive string & String object ]

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    // primitive string
    // String object
    if (typeof var === 'string' || var instanceof String)

III. Number

  1. Check a Number

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    // primitive string
    // String object
    // Must not `NaN`
    if ( typeof var === 'number' || value instanceof var ) && isFinite(var) )

  2. Check NaN

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    // 1. return true for all non-number
    isNaN(var)

    // 2. only return true for exact 'NaN'
    Number.isNaN(var)

IV. Array

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Array.isArray(var);

V. Function

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// Returns if a value is a function
function isFunction (var) {
	return typeof var === 'function';
}

VI. Boolean

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// Returns if a value is a boolean
function isBoolean (var) {
	return typeof var === 'boolean';
}