在ES5 以前，Function.prototype没有bind这个原生方法
    参考 vue 源码中bind兼容

    1. /**
    2.  * Simple bind polyfill for environments that do not support it,
    3.  * e.g., PhantomJS 1.x. Technically, we don't need this anymore
    4.  * since native bind is now performant enough in most browsers.
    5.  * But removing it would mean breaking code that was able to run in
    6.  * PhantomJS 1.x, so this must be kept for backward compatibility.
    7.  */
    9. /* istanbul ignore next */
    10. // 这是Typescript代码，所以参数后面都会有Function等数据类型
    11. function polyfillBind (fn: Function, ctx: Object): Function {
    12.   function boundFn (a) {
    13.     const l = arguments.length
    14.     return l
    15.       ? l > 1
    16.         ? fn.apply(ctx, arguments)
    17.         : fn.call(ctx, a)
    18.       : fn.call(ctx)
    19.   }
    21.   boundFn._length = fn.length
    22.   return boundFn
    23. }
    25. function nativeBind (fn: Function, ctx: Object): Function {
    26.   return fn.bind(ctx)
    27. }
    29. export const bind = Function.prototype.bind
    30.   ? nativeBind
    31.   : polyfillBind