javascript check undefined or null

How to judge undefined in JSThere are two special data types in JavaScript: undefined and null, the following section describes the judgment of NULL, and the following undefined. The following are incorrect usages:var exp =

Null = = Undefinednull!== Undefinednull This is an object, but it is empty. Because it is an object, typeof null returns ' object '. Null is a JavaScript reserved keyword. When NULL participates in numeric operations, its value is automatically converted to 0, so the followi

So, after thinking about it, write down this article. Please take a look.Total offices:Null = undefinedHowever:Null! = UndefinedSo what is the difference between the two?NullThis is an object, but it is null. Because it is an object, typeof null returns 'object '.Null is the reserved keyword of JavaScript.When a null v

without a value assignment.Let's look at a piece of code:Copy codeThe Code is as follows: var age;Var id = 100;Var div02 = document. getElementById ("div02"); // Note: div02 does not exist.Var div01 = document. getElementById ("div01"); // Note: div01 existsAlibaba (id); // 100Alert (age); // undefinedAlert (div02); // nullAlert (div01); // object Let's take a look at the memory:Solve the first problem: Why does undefine inherit from null?In

value assignment.Let's look at a piece of code:Copy codeThe Code is as follows:Var age;Var id = 100;Var div02 = document. getElementById ("div02"); // Note: div02 does not exist.Var div01 = document. getElementById ("div01"); // Note: div01 existsAlibaba (id); // 100Alert (age); // undefinedAlert (div02); // nullAlert (div01); // object Let's take a look at the memory:Solve the first problem: Why does undefine inherit from null?In

There are two primitive types in javascript: null and undefined. These two types often make JavaScript developers confused, when is null, and when is undefined?The undefined type has on

6.0 version of the implementation of document.all, which led to the result is that the first IE only page can be viewed in Opera normal, this is good news, but the bad news is that the second page instead of error Out,!! Document.all is true, opera is considered ie, opera cannot support all the private features of IE. There were a lot of people who gave Opera feedback bugs and developers said they couldn't fix them.During this time, in order to seize the market share, Mozilla's people are also

values and definitions. null is a special object and Nan is a special number. Comparison VaR A1; // The A1 value is undefined.VaR a2 = NULL;VaR a3 = Nan;Alert (a1 = a2); // display "true"Alert (A1! = A2); // display "false"Alert (a1 = A3); // display "false"Alert (A1! = A3); // display "true"Alert (A2 = A3); // display "false"Alert (A2! = A3); // display "true"Alert (A3 = A3); // display "false"Alert (A3!

There are two primitive types in javascript: null and undefined. These two types often make JavaScript developers confused, when is null, and when is undefined?The undefined type has on

There are two primitive types in javascript: null and undefined. These two types often make JavaScript developers confused, when is null, and when is undefined?The undefined type has on

There are two primitive types in javascript: null and undefined. These two types often make JavaScript developers confused, when is null, and when is undefined?The undefined type has on

There are two primitive types in javascript: null and undefined. These two types often make JavaScript developers confused, when is null, and when is undefined?The undefined type has on

Distinguish JS in the Undefined,null, "", 0 and FalseToday, a colleague asked me a JS question, if (0 = = ") is true very do not understand, a separate test, recorded in this.In the various data types defined in the programming language, we will define a null value or a false value for it, such as a null value for the

Distinguish undefined, null, quot;, 0, and false in JS from various data types defined in programming languages, we will define a "null" or "false value" for it, such as the null value of the object type ,.. NET Framework. There are also many "null" and "false" types in

In a variety of data types defined in the programming language, we will define a "null" or "false" value for it, such as the null value of the object type null,.net the null value of the database field in the framework, false value of the Dbnull,boolean type, and so on. There are also many kinds of "

declare a variable,// Whether it is in the stack is both memory usage (of course, allCodeCode segments exist, but memory is used, but I am not talking about this). I mean whether undefined variables exist in the stack.// After thinking for a while, I think it exists because the undefined type has only one undefined value. When the declared variable is not initia

In various data types defined in the programming language, we define a "null" or "false value" for them, such as null value of the object type ,.. NET Framework. There are also many "null" and "false" types in JavaScript, so what are their similarities and differences?In fact, I have already listed all the "

Differences between undefined, null, and NaN in JS, as well as interview questions about object attribute assignment, undefinednan (1) In the following three cases, the return type of typeof is undefined. -- When the variable is not initialized -- When the variable is undefined -- When the function does not return a sp

JS in the 6 major types: Number,string,boolean,null,undefined,object, where null and undefined accounted for 2 seats, and their role is to represent the empty, In my usual work I have not found the difference between the two, then what is the difference between them?It is mainly embodied in semantics:NULL indicates tha

1.Null is the JavaScript keyword, meaning "non-object", which can represent numbers, strings, and objects as "no value".var NULL ; typeof X; // returns "Object"var x=null,y=null; x===y;//returns True!null;//returns True2.