Introduce VUE to report error

problem description

teacher Liang Hao"s vue.js practice chapter 5 last example, after I finished writing, I found that the browser error
imported vue is up-to-date by default.
ReferenceError- index is not defined (.)
He @ vue.min.js-6

the environmental background of the problems and what methods you have tried

I wrote a simple vue example that can be implemented, but not the whole shopping cart instance

related codes

/ / Please paste the code text below (do not replace the code with pictures)

what result do you expect? What is the error message actually seen?

Apr.07,2021

has been resolved, the intermediate code has made a mistake

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