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The error is very obvious. There is no style attribute in this.contentli[i]

If you are not sure, just print the line aboveconsole.log(this.contentli[i]);

Try console.log(this.contentli[i]). If the returned object is not a DOM object, then the code is wrong. .

The this.contentli[i].style attribute cannot be accessed because the this.contentli[i] attribute is undefined, and the .style of an undefined object cannot of course be accessed.

First check the value of this.contentli[i] when you run it, and then combine the above code that defines this.contentli[i] to troubleshoot why this.contentli[i] is not defined.

this.contentli[i] is undefined, and it should be that the value of i exceeds this.contentli.length - 1, which is the maximum subscript of this.contentli.