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In this article Xiaobian for you to introduce in detail "what are the shortcomings of var in JavaScript", the content is detailed, the steps are clear, the details are handled properly, I hope this article "what are the shortcomings of var in JavaScript" can help you solve your doubts, the following follows the editor's ideas slowly in depth, together to learn new knowledge.

The defects of var in JavaScript: 1, the variables defined by var have no block scope; 2, the global variables defined by var will automatically add the attributes of the global window object; 3, the variables defined by var will be loaded in advance.

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

Using var to define variables may have the following problems:

Variables defined by var have no block scope.

Global variables defined by var automatically add properties to the global window object.

Variables defined by var are loaded in advance.

So how to make up for the shortcomings of var? The let keyword can be used, and the emergence of the let keyword solves the above problems.

Variables defined by 1.let have block scope:

Let's look at the code first:

For (let I = 0 +) {console.log (I);} console.log ("outside the loop:" + I); / / error: letScope.html:5 Uncaught ReferenceError: i is not defined

Running result:

The above program uses let to define the loop counter in the for loop, so the loop counter I will only be valid in the for loop, so the program will report an error if it accesses the I variable outside the loop body. If you use var to define the I variable, the scope is spread to the extracorporeal circulation.

Global variables defined by 2.let are not automatically added to the properties of the window object:

Let's look at another example:

Let name = 'Barala'; console.log (name); / / Export Barala console.log (window.name); / / window.name does not exist

The above code uses let to define a global variable name, but unlike using var to define global variables, global variables defined using let will not become properties of the window object, so when you run the above program to access window.name, you will not see any output.

Variables defined by 3.let are not loaded in advance:

Look at the following example:

Var name = 'Barala'; function func () {console.log (name); let name = 'fkit'; console.log (name);} func ()

Running result:

Variables defined using let are not reproduced until the program flow executes to the line of code that defines the variable.

The above program first defines a global variable name, and then the program defines a name variable with the same name using let in the func () function. At this time, the local variable name will override the global name variable. However, because variables defined with let are not loaded in advance, accessing the name variable before using let to define the local variable name results in an error-changing the keyword defining the name variable in the func () function to var will have a different effect.

The result of the modified operation is as follows:

After reading this, the article "what are the disadvantages of var in JavaScript" has been introduced. If you want to master the knowledge of this article, you still need to practice and use it yourself to understand it. If you want to know more about related articles, welcome to follow the industry information channel.

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