访问回调array.protoype中的参数

Question

I am implementing the map function. To access the array I am mapping over I am using this based on a question I had earlier. Now I am wondering how to access different arguments passed to the callback . So in the official map method you can pass the index. I am trying to do this, but unsure how to access this in my custom method.

 Array.prototype.mapz = function(callback) { const arr = []; for (let i = 0; i < this.length; i++) { arr.push(callback(this[i])) } return arr; }; let x = [1, 12, 3].mapz((item, index) => { return item * 2; }) console.log(x); 

Answer 1

You need to hand over the index as second parameter for the callback

 Array.prototype.mapz = function(callback) { const arr = []; for (let i = 0; i < this.length; i++) { arr.push(callback(this[i], i)); } return arr; }; let x = [1, 12, 3].mapz((item, index) => { console.log(index, item); return item * 2; }) console.log(x); 

Answer 2

You just need to add another argument to your callback function which is your index.

 Array.prototype.mapz = function(callback) { const arr = []; for (let i = 0; i < this.length; i++) { arr.push(callback(this[i], i)) } return arr; }; let x = [1, 12, 3].mapz((item, index) => { console.log(index); return item * 2; }) console.log(x);