What does undefined mean in javascript?

JavaScript is a literal scripting language. It is a dynamically typed, weakly typed, prototype-based language with built-in support for types. Its interpreter is called the JavaScript engine, which is part of the browser and is widely used in client-side scripting languages. It was first used on HTML web pages to add dynamic functions to HTML web pages. It has many special definitions that you may not be very clear about. Today I will talk to you about a special data type undefined in JavaScript.

undefined represents an undeclared variable, or a variable that has been declared but not assigned a value, or an object property that does not exist. When the function does not return a value, undefined is returned by default. .

These are several typical uses of undefined. To determine whether a variable is undefined, use the typeof function. The typeof function mainly returns a string, mainly as follows: "number", " string", "boolean", "object", "function", "undefined".

Note that this special undefined value is different from null.

Usage example:

if (typeof(obj) == "undefined") { 
   /*逻辑*/ 
}

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