brush.extent() returns error thing in d3.v4.min.js about d3-brush

[mbostock]

Please read the documentation for brush.extent and the major changes to d3-brush as part of the 4.0 release. The functionality you are looking for is called the current brush selection (not to be confused with a D3 selection), and can be retrieved using d3.brushSelection or as event.selection on a brush event.

If you have further questions, please use Stack Overflow tag d3.js to ask for help. Although I make an effort to assist everyone that asks, I am not always available to provide help promptly or directly. Stack Overflow provides a better collaborative forum for self-help: tens of thousands of D3-related questions have already been asked there, and some answered questions may be relevant to you.

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Thank you! 🤗