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If object does not have key, add it.
此脚本不应直接安装，它是一个供其他脚本使用的外部库。如果您需要使用该库，请在脚本元属性加入：// @require https://update.greasyfork.org/scripts/427473/937466/object_setDefault.js
Added method setDefault to object prototype.
function setDefault (key, value)If object does not have key, add it with value value
key (string, required)key to checkvalue (any, optional, default = undefined)value for key if it's not present