The usage of javascript typeof and the introduction of typeof operator [details]_javascript skills

The following is a detailed introduction to the typeof operator and some usage and analysis of typeof. Friends who are learning typeof should be able to gain something after reading this article.

Arrays are often used in js, such as multiple inputs with the same name. If they are dynamically generated, you need to determine whether they are arrays when submitting.
if(document.mylist.length != "undefined" ) {} This usage is incorrect.
The correct one is if( typeof(document.mylist.length) != "undefined" ) {}
or if( !isNaN(document.mylist.length) ) {}
The operand of typeof is undefined, and the returned value is "undefined".
The operand is a number typeof(x) = "number"
String typeof(x) = "string"
Boolean value typeof(x) = "boolean"
Object, array and null typeof(x) = "object"
Function typeof(x) = "function"
The typeof operator returns an expression Data type string.
Possible strings are: "number", "string", "boolean", "object", "function" and "undefined".
For example:
alert(typeof (123));//typeof(123) returns "number"
alert(typeof ("123"));//typeof("123") returns "string" "
typeof operator
returns a string representing the data type of the expression.
typeof[()expression[]] ;
expression parameter is any expression for which type information needs to be found.
Explanation
The typeof operator returns type information as a string. There are six possible return values ​​of typeof: "number," "string," "boolean," "object," "function," and "undefined."
The parentheses in the typeof syntax are optional.

typeof operator introduction:
typeof is a unary operation, placed before an operand, and the operand can be of any type.
The return value is a string describing the type of the operand.
Do you know the result of the following typeof operation?
typeof(1);
typeof(NaN);
typeof(Number.MIN_VALUE);
typeof(Infinity);
typeof("123");
typeof(true );
typeof(window);
typeof(document);
typeof(null);
typeof(eval);
typeof(Date);
typeof(sss);
typeof(undefined);
Let’s see how many you know?

If you don’t understand it after reading it, please read below (those who understand don’t need to read further):
typeof is a unary operator, and the result it returns is always a string. , it returns different results for different operands.
The specific rules are as follows:
1. For numeric type operands, the value returned by typeof is number. For example: typeof(1), the returned value is number.
The above are regular numbers. For unconventional number types, the result returned is also number. For example, typeof(NaN), NaN represents a special non-numeric value in
JavaScript, although it itself is a numeric type.
In JavaScript, there are several special number types:
Infinity represents an infinity special value
NaN　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　cessui hand havendent in the special number types [ 🎜 ] The smallest number that can be represented (closest to zero)
Number.NaN　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　 wealthdie/ 　　　 　　　　　　 　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　ccogenegene in us/// For special types, when using typeof to perform operations, the result will be number.
2. For string types, the value returned by typeof is string. For example, the value returned by typeof("123") is string.
3. For Boolean types, the value returned by typeof is boolean. For example, the value returned by typeof(true) is boolean.
4. For objects, arrays, and null, the returned value is object. For example, the values ​​returned by typeof(window), typeof(document), and typeof(null) are all objects.
5. For function types, the returned value is function. For example: the values ​​returned by typeof(eval) and typeof(Date) are functions.
6. If the operand is not defined (such as a non-existent variable, function or undefined), undefined will be returned. For example: typeof(sss), typeof(undefined) both return undefined.

document.write ("typeof(1): " typeof(1) "
");
document.write ("typeof(NaN): " typeof(NaN) "
");
document.write ("typeof(Number.MIN_VALUE): " typeof(Number.MIN_VALUE) "
")
document.write ("typeof(Infinity): " typeof(Infinity) "
")
document.write ("typeof("123"): " typeof("123") "
")
document.write ("typeof(true): " typeof(true) "
")
document.write ("typeof(window): " typeof(window) "
")
document.write ("typeof(document): " typeof(document) "
")
document.write ("typeof(null): " typeof(null) "
")
document.write ("typeof(eval): " typeof(eval) "
")
document.write ("typeof(Date): " typeof(Date) "
")
document.write ("typeof(sss): " typeof(sss) "
")
document.write ("typeof(undefine): " typeof(undefine) "
")