1.语法
- forEach()与map()差不多，但是前者是没有返回值的
2.手写
Array.prototype.forEach = function(callback,thisValue) {
if(this == undefined) {
throw new TypeError("this is null or not undefined!!")
}
if(Object.prototype.toString.call(callback) != "[object Function]") {
throw new TypeError(callback + "is not a function!")
}
let _this = thisValue? thisValue : this
for(let i = 0;i < this.length;i++) {
callback.call(_this,this[i],i,this)
}
}
