Summary of typeof usage in Javascript_javascript skills

Arrays are 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 return 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 a string representing the data type of the expression.
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"

The typeof operator returns a string representing the data type of the expression.

typeof[()expression[]] ;

expression parameter is any expression that needs to find type information.

Description

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 typeof syntax are optional. Introduction to typeof operator:
typeof is a unary operation, placed before an operand, and the operand can be of any type.
Its 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, the result it returns is always a string, and 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 is in
Represents a special non-numeric value in JavaScript, although it is a numeric type itself.

In JavaScript, there are several special number types:

Infinity represents the special value of infinity
NaN　　　　　Special non-numeric value
Number.MAX_VALUE　　Maximum representable number
Number.MIN_VALUE　　The smallest representable number (closest to zero)
Number.NaN　　　Special non-numeric value
Number.POSITIVE_INFINITY represents a special value that represents positive infinity
Number.NEGATIVE_INFINITY represents the special value of negative infinity

For the above 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.

The above is the entire content of this article, I hope you all like it.