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Fun With forEach

gokatz profile image Gokul Kathirvel ・4 min read

So, what's a forEach?

forEach is a little guy who iterates/loops through the array and executes the given callback for each element. It's more like the traditional for loop, but with a functional touch. A sample snippet would be like,

let colors = ['black', 'blue', 'red'];

colors.forEach((color) => {
    console.log(`${color} is a cool color`);
});

And the output will be:

black is a cool color
blue is a cool color
red is a cool color

So, as you can see, forEach will invoke the callback for each entry of the callee array. Yes, you might already know that what's fun with this?

What's the Fun in there? 👯‍♂️

We are gonna see some snippets that you might not encounter in real-time products/application and try to guess the result of those snippets (without peeking through the output 👀). That's where the real fun lies 😉

Snippet 1

let colors = ['black', 'blue', 'red'];

colors.forEach((color) => {
    colors.push('cyan');
    console.log(`${color} is a cool color`);
});

console.log(colors);

So, When I thought through this snippet, I predicted this would lead to an infinite loop. That's totally understandable, right? But, our little guy, forEach, will run the callback only for the exact number of times as that of the initial array length.

This is a little Twitter poll stating a snippet like this. Check this out. You are not alone 😛

Here, the initial array length is 3 and the callback will be executed only for 3 times. However, the callback can mutate/change the array. But, the callbacks will not be executed for the later elements that are outside the bound (initial length).

So, the output will be:

black is a cool color
blue is a cool color
red is a cool color
[ 'black', 'blue', 'red', 'cyan', 'cyan', 'cyan' ]

Snippet 2

let colors = ['black', 'blue', 'red'];

colors.forEach((color, index) => {
    colors[index+1] = 'cyan';
    console.log(`${color} is a cool color`);
});

console.log(colors);

As per the rule that callback can mutate the array, the output for this snippet will be somewhat straight-forward. From the first run itself, we are changing the array value of the next index to cyan with this colors[index+1] = 'cyan' statement. So, the output will be:

black is a cool color
cyan is a cool color
cyan is a cool color
[ 'black', 'cyan', 'cyan', 'cyan' ]

As you might be noticed, There is an extra element in the resulting array and that's because, on the last run (index = 2), we are assigning the next index (index = 3) element's value as cyan and as said before, the callback will not be run for that last element we just pushed as it resides outside the initial array length.

Snippet 3

let colors = ['black', 'blue', 'red'];

colors.forEach((color, index) => {
    delete colors[index+1];
    console.log(`${color} is a cool color`);
});

console.log(colors);

Now, we delete items from the array. What do you think the output will be? What will be the placeholder for the deleted items? undefined? NULL? or something else?

On Quick skim, a common prediction for the loop would be,

black is a cool color
undefined is a cool color
undefined is a cool color

this is because, we know that callback will be called for the initial length of the array and here in this array, that's 3.

but, deleting the array element will make that space a hole and this forEach guy is pretty smart and will not run the callback for the holes in the array.

So, when the callback is executed for the first element (index = 0), it will delete the second element and the callback for the same will not be executed and the loop will be skipped to the third element. So the output will be:

black is a cool color
red is a cool color
[ 'black', empty, 'red' ] // empty is just the representation of holes in V8

Snippet 4

So, How an empty array will be treated?

let colors = new Array(3);

colors.forEach((color, index) => {
    colors[index] = 'cyan';
    console.log(`${colors[index]} is a cool color`);
});

console.log(colors);

Nothing changes, the empty array will be having all elements as holes. console.log(colors) will result in something like

[empty × 3]

So the callback will not be executed for any of the holes and the actual output of the entire snippet will also be:

[empty × 3]

Snippet 5

Another less used feature in forEach is that it can accept a second parameter, thisArg and if that's passed, the callback will be executed with the passed context. The following snippet is just for the demo you can find a more relevant example on MDN. I haven't used arrow function here as that will make this to be undefined.

class colorHandler {
  isFavorite(color) {
    return color === 'cyan';
  }
}

let colors = ['black', 'blue', 'red', 'cyan'];

colors.forEach(function(color, index) {
    console.log(this.isFavorite(color))
}, new colorHandler());

the output will be:

false
false
false
true

So...

Hope this was fun. That's all for our little guy. There might be a lot of other fun stuff about forEach. Kindly share it in the comments to surprise us. Let's see in some time with another array method/property in Fun With Arrays series.

And a fun fact: This title was inspired from the awesome (😉) show hosted by Sheldon and Amy in The Big Bang Theory series, named, Fun With Flags.

Discussion (2)

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Thomas Philippot

Was not ready to see 'Fun with flags' at the end, but it was fun tought !

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