How can i shear rectangle to JavaScript?

use the Matrix properties b & c, the actual skewing values are in percentages of the objects dimensions, not degrees as in the UI...if you need angle values, that would be your homework.

draw a rectangle, select it and run the script to skew or shear 50%

// skew or shear selection, carlos canto 07/04/13
// http://forums.adobe.com/message/5475881#5475881
var idoc = app.activeDocument;
var sel = idoc.selection[0];
var im = app.getIdentityMatrix();
//im.mValueB = .5; // skew y 50%
im.mValueC = .5; // skew x 50%
sel.transform (im, true, true, true, true, 1, undefined);