EN
JavaScript - check type of object
17 points
In this article, we are going to show how to check object type in JavaScript.
Quick solution:
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var variable = null;2
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console.log( typeof variable ); // object <--- with typeof4
console.log( variable instanceof Object ); // false (for Object) <--- with instanceof5
console.log( getType(variable) ); // Null <--- with toString6
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// Helper util:9
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function getType(object) {11
var text = Object.prototype.toString.call(object);12
return text.substring(8, text.length - 1);13
}
There are three different ways how to do it.
The most common way is to use typeof or instanceof operators, rarely toString():
typeof variable |
| variable.toString() |
|
Result type: e.g.
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Result type: |
Result type: e.g.
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Check the below table to see an example results summary of different approaches.
Note: in the first impression the results of the operator could be strange.
Summary:
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variable typeof toString instanceof2
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null object Null false (for Object)4
undefined undefined Undefined false (for Object)5
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{ } object Object true (for Object)7
new Object() object Object true (for Object)8
new Student() object Object true (for Student) // custom class used9
new function() { } object Object true (for Object)10
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[ ] object Array true (for Array)12
new Array() object Array true (for Array)13
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true boolean Boolean false (for Boolean) 15
Boolean(true) boolean Boolean false (for Boolean)16
new Boolean(true) object Boolean true (for Boolean)17
18
5 number Number false (for Number)19
Number(5) number Number false (for Number)20
new Number(5) object Number true (for Number)21
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5n bigint BigInt false (for BigInt)23
BigInt(5) bigint BigInt false (for BigInt)24
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'a' string String false (for String)26
String('a') string String false (for String)27
new String('a') object String true (for String)28
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function() { } function Function true (for Function)30
Date function Function true (for Function)31
RegExp function Function true (for Function)32
Function function Function true (for Function)33
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new Function('') function Function true (for Function)35
new Date() object Date true (for Date)36
/[a-z]/g object RegExp true (for RegExp)37
new RegExp('[a-z]', 'g') object RegExp true (for RegExp) In this section, we use typeof operator to show what kind of type is some object.
Simple types and undefined were marked in comments with ----->.
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function Student() {2
// class logic...3
}4
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console.log(typeof( null )); // object6
console.log(typeof( undefined )); // -----> undefined7
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console.log(typeof( { } )); // object9
console.log(typeof( new Object() )); // object10
console.log(typeof( new Student() )); // object11
console.log(typeof( new function() { } )); // object12
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console.log(typeof( [ ] )); // object14
console.log(typeof( new Array() )); // object15
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console.log(typeof( true )); // -----> boolean17
console.log(typeof( Boolean(true) )); // -----> boolean18
console.log(typeof( new Boolean(true) )); // object19
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console.log(typeof( 5 )); // -----> number21
console.log(typeof( Number(5) )); // -----> number22
console.log(typeof( new Number(5) )); // object23
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console.log(typeof( 5n )); // -----> bigint25
console.log(typeof( BigInt(5) )); // -----> bigint26
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console.log(typeof( 'a' )); // -----> string28
console.log(typeof( String('a') )); // -----> string29
console.log(typeof( new String('a') )); // object30
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console.log(typeof( function() { } )); // function32
console.log(typeof( Date )); // function33
console.log(typeof( RegExp )); // function34
console.log(typeof( Function )); // function35
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console.log(typeof( new Function('') )); // function37
console.log(typeof( new Date() )); // object38
console.log(typeof( /[a-z]/g )); // object39
console.log(typeof( new RegExp('[a-z]', 'g') )); // objectIn this section, we used toString method to show how to get a type name. The reason why we used Object.prototype.toString.call() approach to call it is the prevention of local method overloading.
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function getType(object) {2
var text = Object.prototype.toString.call(object);3
return text.substring(8, text.length - 1);4
}5
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// Example:7
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function Student() {9
// class logic...10
}11
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console.log(getType( null )); // Null13
console.log(getType( undefined )); // Undefined14
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console.log(getType( { } )); // Object16
console.log(getType( new Object() )); // Object17
console.log(getType( new Student() )); // Object18
console.log(getType( new function() { } )); // Object19
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console.log(getType( [ ] )); // Array21
console.log(getType( new Array() )); // Array22
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console.log(getType( true )); // Boolean24
console.log(getType( Boolean(true) )); // Boolean25
console.log(getType( new Boolean(true) )); // Boolean26
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console.log(getType( 5 )); // Number28
console.log(getType( Number(5) )); // Number29
console.log(getType( new Number(5) )); // Number30
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console.log(getType( 5n )); // BigInt32
console.log(getType( BigInt(5) )); // BigInt33
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console.log(getType( 'a' )); // String35
console.log(getType( String('a') )); // String36
console.log(getType( new String('a') )); // String37
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console.log(getType( function() { } )); // Function39
console.log(getType( Date )); // Function40
console.log(getType( RegExp )); // Function41
console.log(getType( Function )); // Function42
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console.log(getType( new Function('') )); // Function44
console.log(getType( new Date() )); // Date45
console.log(getType( /[a-z]/g )); // RegExp46
console.log(getType( new RegExp('[a-z]', 'g') )); // RegExpAnother way to check the type of object is to use instanceof operator that returns true or false.
false results were marked in comments with ----->.
Note: main disadvantage of this approach is we are able just to test objest with some type.
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function Student() {2
// class logic...3
}4
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console.log( null instanceof Object ); // -----> false6
console.log( undefined instanceof Object ); // -----> false7
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console.log( { } instanceof Object ); // true9
console.log( new Object() instanceof Object ); // true10
console.log( new Student() instanceof Student ); // true11
console.log( new function() { } instanceof Object ); // true12
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console.log( [ ] instanceof Array ); // true14
console.log( new Array() instanceof Array ); // true15
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console.log( true instanceof Boolean ); // -----> false17
console.log( Boolean(true) instanceof Boolean ); // -----> false18
console.log( new Boolean(true) instanceof Boolean ); // true19
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console.log( 5 instanceof Number ); // -----> false21
console.log( Number(5) instanceof Number ); // -----> false22
console.log( new Number(5) instanceof Number ); // true23
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console.log( 5n instanceof BigInt ); // -----> false25
console.log( BigInt(5) instanceof BigInt ); // -----> false26
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console.log( 'a' instanceof String ); // -----> false28
console.log( String('a') instanceof String ); // -----> false29
console.log( new String('a') instanceof String ); // true30
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console.log( function() { } instanceof Function ); // true32
console.log( Date instanceof Function ); // true33
console.log( RegExp instanceof Function ); // true34
console.log( Function instanceof Function ); // true35
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console.log( new Function('') instanceof Function ); // true37
console.log( new Date() instanceof Date ); // true38
console.log( /[a-z]/g instanceof RegExp ); // true39
console.log( new RegExp('[a-z]', 'g') instanceof RegExp ); // true
