How to use javascript to detect the existence of variables

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In javascript, you can detect the existence of a variable by determining whether the value of the variable is "null" or whether the data type is "undefined". The syntax "if (typeof (a) = =" undefined "| | a = = null)".

The operating environment of this tutorial: windows7 system, javascript1.8.5 version, Dell G3 computer.

In the actual development process, there will be scenarios to determine whether a variable exists or not.

You only need to use 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 ("an is undefined")} else {console.log ("an is defiend")}

There will be errors, there may be blocking, and it is not elegant enough.

Solution:

/ / var axioms; if (typeof (a) = = "undefined" | | a = = null) alert ("an is undefined"); else alert ("an is defined")

Typeof is an operator for viewing data types and can be used in two ways:

Typeof (expression) typeof variable name

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'-- an undefined variable or value

'boolean'-- variable or value of Boolean type

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

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

'object'-- A variable or value of the object type, or null (this is a historical legacy of js, which treats null as an object type)

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

Use if (typeof (a) = = "undefined") to determine whether the variable an is undefined.

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