| layout | title | subtitle | bigimg |
|---|---|---|---|
post |
Playing with Arrays - part I |
map function |
../img/analog.jpg |
It might be slower then a classical for loop, but if you are planning on using Observables and playing with async arrays it's the only option.
Let's suppose we've got the following array of objects:
{% highlight javascript %} var symbols = getSymbols([ {symbol:"GG", price:500.65}, {symbol:"AA", price:1085.20}, {symbol:"BB", price:135.00} ]); {% endhighlight %}
You can loop through the code above using the standard for loop and returns the symbol this way:
{% highlight javascript %}
function getSymbols(stocks){
var arr = [];
for (var i = 0; i <= stocks.length; i++) {
arr.push(stocks[i].symbol);
}
return arr;
};
{% endhighlight %}
Or you can use the build in map method this way:
{% highlight javascript %} function getSymbols(stocks){ return stocks.map(function(stock){ return stock.symbol; }) }; {% endhighlight %}
What it does it accepts a closure to which each element of the array is passed. Have a look below:
{% highlight javascript %} /*
- Each element of the array is passed through this closure function.
- 'stock' is that single array element */ function(stock){ return stock.symbol; // The stock.symbol is being added to an 'invisible' array. We don't see it but it gets created behind. } {% endhighlight %}
In this case we don't have to take care of a for loop, we don't need to create new variables. We just pass our array through the map() function.
If you want to check the performance have a look here: https://jsperf.com/map-vs-for-loop-performance/6