减少输出undefined，但可能不应该

Question

Here's the thing, when I ran this code, it works fine:

var arr = [1,2,3,4];
arr.reduce(function(acc,val){
    if(val >= 2){
        acc['the key num ' + val + ' is'] = val;
    } 
    return acc;
},{});

But, when I try this one, it output undefined... I can't figure it out why.

 function ext(arr, zname) { arr.reduce(function(acc, val) { if (val[zname]) { acc = val[zname]; } return acc; }, {}); } console.log(ext([{ name: "Alex", isTeacher: true }, { name: "Bob", isTeacher: true }, { name: "David", isTeacher: true }], "name")) 

It should output an object with the names, but instead I'm getting undefined

What am I missing here? :/

Answer 1

Missing a return statement:

 function ext(arr, zname) { // | // V return arr.reduce(function(acc, val) { if (val[zname]) { acc.push(val[zname]); } return acc; }, []); } console.log(ext([{ name: "Alex", isTeacher: true }, { name: "Bob", isTeacher: true }, { name: "David", isTeacher: true }], "name")) 

---

If you just want the names, that can be done more easily:

 const data = [{ name: "Alex", isTeacher: true }, { name: "Bob", isTeacher: true }, { name: "David", isTeacher: true }]; const ext = (arr, v) => arr.map(x => x[v]); console.log(ext(data, "name")); 

Answer 2

 const ext = (arr, zname) => { // Note: the use of `return` below return arr.reduce((acc, val) => { if (val[zname]) { acc = val[zname]; } return acc; }, {}); } console.log(ext([{ name: "Alex", isTeacher: true }, { name: "Bob", isTeacher: true }, { name: "David", isTeacher: true }], "name")) 

You need to return the reduce function to get your result.

Answer 3

The issues are fixed and commented in below code example.

You have another issue with the function you grabbed the code from - your data structure is not suited to create an object from it because an object can only have a maximum of one same-name property.

 function ext(arr, zname) { // you need to return the result of arr.reduce return arr.reduce(function(acc, val) { if (val[zname]) { // if you want a list of names, just push the names in acc acc.push(val[zname]); } return acc; // in that case initialize acc with [] instead of {} }, []); } console.log(ext([{ name: "Alex", isTeacher: true }, { name: "Bob", isTeacher: true }, { name: "David", isTeacher: true }], "name"))