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支持某个API,如果不支持的话就加载对应的polyfill.然后新旧浏览器就都可以使用这个API了.   "Shims can be used to support an old API in a newer environment,or a new API in an older environment. Shims can also be used for running programs on different software platforms than they were developed for."---Wikipadie       主要区别： What makes a polyfill different from the techniques we have already,like a shim,is this: if you removed the polyfill script,your code would continue to work,without any changes required in spite of the polyfill being removed.---Wikipadie （也就是说，polyfill和shim的主要区别在于，polyfill的代码如果被移除，代码仍然起作用，polyfill是否被移除都不会有任何的改变）polyfill和shim的区别