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只能绑定一次 this,并返回一个新函数
const fun = function () {}; const fun2 = fun.bind(someThis);
可以一次绑定多个参数,并返回一个新函数
const fun = function () {}; const fun2 = fun.bind(null, arg1, arg2); const fun3 = fun2.bind(null, arg3, arg4); console.log(fun3.length === 4);