This works correctly:

  self.getById = function(id) {
        return ko.utils.arrayFirst(self.PostArray(), function(item) {
            if (item.postId === id) {
                return item;
            }
            else {
                return 'not found';
            }
        });
    };

    console.log(self.PostArray().length);
    console.log(self.getById(170));

But if I put return '' or return null in else block I always get null, why is that?

This works correctly:

  self.getById = function(id) {
        return ko.utils.arrayFirst(self.PostArray(), function(item) {
            if (item.postId === id) {
                return item;
            }
            else {
                return 'not found';
            }
        });
    };

    console.log(self.PostArray().length);
    console.log(self.getById(170));

But if I put return '' or return null in else block I always get null, why is that?

• javascript
• knockout.js

• Knockout provides ko.utils.arrayFirst that will execute a function against each item in our array and returns the first item where the function evaluates to true. otherwise it returns null – Evon Dos Commented Aug 18, 2016 at 6:19

1 Answer 1

You're not using arrayFirst correctly. arrayFirst expects a function that returns true or false, evaluating each item. The first item for which the function returns true is returned. Here's how it should look:

self.getById = function(id) {
    return ko.utils.arrayFirst(self.PostArray(), function(item) {
        return item.postId === id;
    }) || 'not found';
};

Basically return 'not found' if item is falsey (null in this case most likely).

See this article for more information on the various utility functions in KnockoutJS.