How to detect whether a variable exists in javascript

In JavaScript, you can detect whether a variable exists by judging whether the value of the variable is "null" or whether the data type is "undefined". The syntax is "if(typeof(a)=="undefined"||a ==null){//Does not exist}else{//Exists}".

The operating environment of this tutorial: windows7 system, javascript version 1.8.5, Dell G3 computer.

In the actual development process, there will be scenarios to determine whether a variable exists.

You only need to use it to determine whether the value of the variable is null or whether the data type is undefined; if so, it does not exist.

The first thing that comes to mind is

if(a==undefined){
        console.log("a is undefined")
    }else{
        console.log("a is defiend")
    }

An error will be reported here, it may cause blocking, and it is not elegant enough

Solution:

<script type="text/javascript">
 
//	var a=&#39;xixi&#39;; 
 
	if(typeof(a) == "undefined" || a == null)
		alert("a is undefined");
	else
		alert("a is defined");
</script>

typeof is an operator used to view data types. There are two ways to use it:

typeof(表达式)
typeof 变量名

The first is to operate on expressions, and the second is to operate on variables.

The return type of the typeof operator is a string, and the values ​​include the following:

• 'undefined' --Undefined variable or value

• 'boolean' --A variable or value of Boolean type

• 'string' --A variable or value of string type

• 'number' -- a variable or value of numeric type

• 'object' -- a variable or value of object type, or null (this is a legacy issue from js history, will null is treated as object type)

• 'function' -- a variable or value of function type

Use if (typeof(a )=="undefined") can determine whether variable a is undefined.

[Recommended learning: javascript advanced tutorial]

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