Sum values of objects which are inside an array with underscore.js and reduce

Question

I'm trying to sum values of objects inside and array with underscore.js and its reduce method. But it looks like I'm doing something wrong. Where's my problem?

let list = [{ title: 'one', time: 75 },
        { title: 'two', time: 200 },
        { title: 'three', time: 500 }]

let sum = _.reduce(list, (f, s) => {
    console.log(f.time); // this logs 75
    f.time + s.time
})

console.log(sum); // Cannot read property 'time' of undefined

actually, I forgot about return. But when I tried to return and display that value on html I got NaN — kirqe
– kirqe, Commented Feb 6, 2017 at 13:55
You forgot to give it an initial value. ..., 0) at the end of reduce. — ibrahim mahrir
– ibrahim mahrir, Commented Feb 6, 2017 at 13:58

ibrahim mahrir · Accepted Answer · 2018-07-02 15:48:14Z

17

Use the native reduce since list is already an array.

reduce callback should return something, and have an initial value.

Try this:

let list = [{ title: 'one', time: 75 },
        { title: 'two', time: 200 },
        { title: 'three', time: 500 }];

let sum = list.reduce((s, f) => {
    return s + f.time;               // return the sum of the accumulator and the current time, as the the new accumulator
}, 0);                               // initial value of 0

console.log(sum);

Note: That reduce call can be shortened even more if we omit the block and use the implicit return of the arrow function:

let sum = list.reduce((s, f) => s + f.time, 0);

edited Jul 2, 2018 at 15:48

answered Feb 6, 2017 at 13:54

ibrahim mahrir

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