手写apply
方法一：ES6
Function.prototype.apply2 = function (context, arr) {
context = context || globalThis;
const _symbol = Symbol();
context[_symbol] = this;
let result = context[_symbol](...arr);
delete context[_symbol];
return result;
}方法二：ES5
Function.prototype.apply3 = function (context, arr) {
context = context || globalThis;
context.fn = this;
let result;
if (!arr) {
retult = context.fn();
} else {
let args = [];
for (let i = 0, len = arr.length; i < len; i++) {
args.push('arr[' + i + ']');
}
result = eval('context.fn(' + args + ')');
}
delete context.fn;
return result;
}