手动实现reduce

Array.prototype.my_reduce = function(callback,initialValue){
    if(!Array.isArray(this) || !this.length || typeof callback !== 'function'){
        return []
    }

    let hasInitialValue = initialValue !== undefined
    let value = hasInitialValue ? initialValue:this[0]
    for(let index= hasInitialValue?1:0; index<this.length;index++){
        const element = this[index]
        value = callback(value,element,index,this)
    }

    return value
}

//验证
let arr = [1, 2, 3, 4, 5]
let res = arr.my_reduce((pre, cur, i, arr) => {
    return pre + cur
}, 10)
console.log(res)