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I have a chart with dual y-axis, how can i get the min and max value from both?

load: function () {
 var series = this.series,
 max = series[0].dataMax,
 min = series[0].dataMin;

 series.forEach(function (serie) 
 if (serie.dataMax > max) 
 max = serie.dataMax;
 

 if (serie.dataMin < min) 
 min = serie.dataMin;
 
);

 this.yAxis[0].update(
 min: min,
 max: max
 );

I am trying the method above, but it will only get either one side got max value and update the axis value.

How can i make both y axis using the same max value?

To get min/max values from an axis, use getExtremes which returns an ExtremesObject. The ExtremesObject has the following properties:

dataMax, dataMin, max, min, userMax, userMin

If you want to set a new min/max value you should use setExtremes

setExtremes( [newMin] [, newMax] [, redraw] [, animation] [, eventArguments])

Set the minimum and maximum of the axes after render time. If the startOnTick and endOnTick options are true, the minimum and maximum values are rounded off to the nearest tick. To prevent this, these options can be set to false before calling setExtremes. Also, setExtremes will not allow a range lower than the minRange option, which by default is the range of five points.

So the get the min and max from both axis, you could do something like this for the min value (if you want to tailor your view to the data and not manually set extremes):

let firstAxisExtremes = chart.yAxis[0].getExtremes(),
secondAxisExtremes = chart.yAxis[1].getExtremes(),
min, max;
if(firstAxisExtremes.dataMin <= secondAxisExtremes.dataMin) 
 min = firstAxisExtremes.dataMin
 else 
 min = secondAxisExtremes.dataMin

API references: getExtremes, setExtremes, ExtremesObject

Also, you can use your current code and set yAxis.linkedTo property:

...

When an axis is linked to a master axis, it will take the same
extremes as the master, but as assigned by min or max or by
setExtremes. (...)

yAxis: [, 
 linkedTo: 0,
 opposite: true
]

Live demo: http://jsfiddle.net/BlackLabel/n5bzrqs8/

API: https://api.highcharts.com/highcharts/yAxis.linkedTo