brotherton-erpnext/docs/dev/custom_script_examples/docs.dev.custom_script.read_only.md

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Make an Item read-only after Saving

Use the method cur_frm.set_df_property to update the field's display.

In this script we also use the __islocal property of the doc to check if the document has been saved atleast once or is never saved. If __islocal is 1, then the document has never been saved.

cur_frm.cscript.custom_refresh = function(doc) {
    // use the __islocal value of doc, to check if the doc is saved or not
    cur_frm.set_df_property("myfield", "read_only", doc.__islocal ? 0 : 1);
}