How to use the JavaScript typeof operator
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Typeof operator
You can use the typeof operator to determine the data type of the JavaScript variable.
Example
Typeof "Bill" / / returns "string"
Typeof 3.14 / / returns "number"
Typeof NaN / / returns "number"
Typeof false / / returns "boolean"
Typeof [1, 2, 3, 4] / / return "object"
Typeof {name:'Bill', age:62} / / returns "object"
Typeof new Date () / / returns "object"
Typeof function () {} / / returns "function"
Typeof myCar / / returns "undefined" *
Typeof null / / returns "object"
Please note:
The data type of NaN is numeric
The data type of the array is an object
The data type of the date is the object
The data type of null is an object
The data type of an undefined variable is undefined
Variables that have not yet been assigned also have the data type undefined
You cannot use typeof to determine whether the JavaScript object is an array (or date).
Data type of typeof
The typeof operator is not a variable. It belongs to the operator. Operators, such as +-* /, have no data type.
However, typeof always returns a string (the type that contains operands).
Constructor attribute
The constructor property returns the constructor functions for all JavaScript variables.
Example
"Bill" .constructor / / returns "function String () {[native code]}"
Constructor / / returns "function Number () {[native code]}"
False.constructor / / returns "function Boolean () {[native code]}"
] .constructor / / returns "function Array () {[native code]}"
{name:'Bill', age:62} .constructor / / returns "function Object () {[native code]}"
New Date () .constructor / / returns "function Date () {[native code]}"
Function () {} .constructor / / returns "function Function () {[native code]}"
You can determine whether an object is an array (including the word "Array") by checking the constructor property:
Example
Function isArray (myArray) {
Return myArray.constructor.toString () .indexOf ("Array") >-1
}
Or more simply, you can check whether the object is an array function:
Example
Function isArray (myArray) {
Return myArray.constructor = Array
}
You can determine whether an object is a date (including the word "Date") by checking the constructor property:
Example
Function isDate (myDate) {
Return myDate.constructor.toString () .indexOf ("Date") >-1
}
Or more simply, you can check whether the object is a date function:
Example
Function isDate (myDate) {
Return myDate.constructor = Date
}
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