javascript - lodash - chaining filter and forEach - Stack Overflow

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I have the following working code written with lodash, latest version

            _.forEach(_.filter($ctrl.data, ['x', 'y']), function (elem) {
                ...
            });

If I try to transform this into a chain, it will never execute the block code inside forEach.

            _.chain($ctrl.data)
                .filter(['x', 'y'])
                .forEach(function (elem) {
                   ...
                });

Why?

I have the following working code written with lodash, latest version

            _.forEach(_.filter($ctrl.data, ['x', 'y']), function (elem) {
                ...
            });

If I try to transform this into a chain, it will never execute the block code inside forEach.

            _.chain($ctrl.data)
                .filter(['x', 'y'])
                .forEach(function (elem) {
                   ...
                });

Why?

• javascript
• lodash

Share Improve this question Follow asked Jan 25, 2017 at 10:32 tzortziktzortzik 5,1631010 gold badges6060 silver badges9595 bronze badges 6

• if you console.log _.chain($ctrl.data).filter(['x', 'y']), what does it return? – Dani Commented Jan 25, 2017 at 10:39
• I debugged using _.chain($ctrl.data).filter(['x', 'y']).value() and it returns the filtered array, as expected. – tzortzik Commented Jan 25, 2017 at 10:40
• then you should try _.chain($ctrl.data).filter(['x', 'y']).value().forEach(function(elem) { .. }); – Dani Commented Jan 25, 2017 at 10:43
• This is not ok. It will call forEach on javascript Array which is not right. My code should be backwards patile and there are some older browsers which does not support forEach. – tzortzik Commented Jan 25, 2017 at 10:45
• 1 _.chain([1,2,3]).forEach(console.log) works as expected, your code should too. – Stefan Dragnev Commented Jan 25, 2017 at 10:47

 |  Show 1 more ment

3 Answers 3

Sorted by: Reset to default 4

I fixed the problem by using _($ctrl.data) instead of _.chain($ctrl.data). I read that the main difference here is that the global function from lodash calls _.value() when it has a "terminator function" at the end.

An equivalent using _.chain would be to explicitly call _.value() at before _.forEach(). But still can't figure out what is the problem because, from my point of view, a forEach is a "terminator function".

This happens because execution of chained methods in lodash is lazy.

From lodash documentation on sequences:

The execution of chained methods is lazy, that is, it's deferred until _#value is implicitly or explicitly called.

That means that following code:

_.chain(['a','b','c'])
    .filter(_.stubTrue)
    .forEach(i => console.log(i))

will not print anything to console unless the sequence will be ended with value():

_.chain(['a','b','c'])
    .filter(_.stubTrue)
    .forEach(i => console.log(i))
    .value()
a
b
c

I just ran into a similar issue. I was using the _(blah) syntax and I was still running into issues.

In my case, the code looked like this:

_(items).forEach((x) => doSomething(x));

It turns out the doSomething was returning a boolean. I guess lodash sees this return value and treats forEach the same as map, not evaluating it. However, I tried tacking on a value() onto the end and I get a value is not a function error at runtime.

In my case, the problem was solved by wrapping the doSomething call:

_(items).forEach((x) => { doSomething(x); });

Seems more like a bug than anything.

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