Set "snap to document grid" via javascript

This one works:

app.documents[0].gridPreferences.documentGridSnapto = true;

It's possible other settings like 'Snap to Guides' or 'Smart Guides' are influencing object placement. 

So something like this maybe work for you

var myDocument = app.activeDocument;
myDocument.guidePreferences.guidesLocked = false;
myDocument.guidePreferences.guidesShown = false;
app.activeWindow.transformReferencePoint = AnchorPoint.TOP_LEFT_ANCHOR;

You can invoke Document Grid Snap via the menus which turns it on

// Access the 'Snap to Document Grid' menu action
var snapToGridAction = app.menuActions.itemByName("$ID/Snap to Document Grid");

// Check if the action is valid and enabled
if (snapToGridAction.isValid && snapToGridAction.enabled) {
    // Invoke the action to toggle the snapping
    snapToGridAction.invoke();
} else {
    alert("The 'Snap to Document Grid' action is not available.");
}