如何转换此对象数组以使用reduce javascript

Question

I'm trying to convert this function to use reduce. I am halfway through. When the selected value is true I want to know the index of that element.

let options = [
    {label: 'foo1', selected: false},
    {label: 'foo2', selected: true},
    {label: 'foo2', selected: false},
    {label: 'foo2', selected: false},
    {label: 'foo2', selected: false},
];
const obj = options
              .map((option, index) => (option.selected ? index : -1))
              .filter((val) => val !== -1)[0];

Result: 1

My attempt is this:

const obj = options.reduce((acc, currentValue, index) => {
    const i = currentValue["selected"] ? index : -1;
    return acc.concat(i)
}, []); // [-1,1,-1,-1,-1]

How do I change the entire thing to use reduce?

Answer 1

Add the index to the result only if option.selected is true otherwise do nothing.

 let options = [ {label: 'foo1', selected: false}, {label: 'foo2', selected: true}, {label: 'foo2', selected: false}, {label: 'foo2', selected: false}, {label: 'foo2', selected: false}, ]; const selected = options.reduce((arr, option, index) => { return option.selected ? arr.concat(index) : arr; }, []); console.log(selected); 

Answer 2

Use a ternary in concat() to set the value to be inserted. Array#concat() returns the updated array so that is the return for reduce() .

Or use spread to return new array [...accumulator, selsected ? i :-1] [...accumulator, selsected ? i :-1]

 const res = options.reduce((a, {selected:s}, i) => a.concat(s ? i : -1) , []); // OR // const res = options.reduce((a, {selected:s}, i) => [....a,s ? i : -1] , []); console.log(res) 

 <script> let options = [ {label: 'foo1', selected: false}, {label: 'foo2', selected: true}, {label: 'foo2', selected: false}, {label: 'foo2', selected: false}, {label: 'foo2', selected: false}, ]; </script> 

Answer 3

If you want indexes with selected = true, then you could do something like this using reduce . If the current item in context has selected === true, add the index to the accumulator, else return the accumulator. Currently, you're adding -1 if the condition is false

 let options = [ {label: 'foo1', selected: false}, {label: 'foo2', selected: true}, {label: 'foo2', selected: false}, {label: 'foo2', selected: false}, {label: 'foo2', selected: false}, ]; const selectedIndexes = options.reduce((r, o, i) => o.selected ? r.concat(i) : r, []) console.log(selectedIndexes) 

If you want the whole object, then you can simply use filter :

 let options = [{label:'foo1',selected:false},{label:'foo2',selected:true},{label:'foo2',selected:false},{label:'foo2',selected:false},{label:'foo2',selected:false},]; const filtered = options.filter(o => o.selected) console.log(filtered) 

Answer 4

Using Array#reduce and the ... spread operator keep the existing indexes and add the new index if selected is true .

 let options = [ {label: 'foo1', selected: false}, {label: 'foo2', selected: true}, {label: 'foo2', selected: false}, {label: 'foo2', selected: false}, {label: 'foo2', selected: false}, {label: 'foo2', selected: true}, {label: 'foo2', selected: 1} ]; const obj = options.reduce((acc, currentValue, index) => { // checking only for boolean true not for truthy values return acc = currentValue["selected"] === true ? [...acc, index] : acc; }, []); console.log(obj); 

Answer 5

Another way is use of push instead of concat :

 let options = [ {label: 'foo1', selected: false}, {label: 'foo2', selected: true}, {label: 'foo2', selected: false}, {label: 'foo2', selected: false}, {label: 'foo2', selected: false}, ]; var obj = options.reduce( (acc, currentValue, index) => { currentValue.selected ? acc.push(index) : ''; return acc} ,[] ) ; console.log(obj)