I want to go through an array of strings, and depending on what the string is, make an array of objects.
For example, if the array is:
[a,a,a,b,b,c,d]
I want to map through the array and make an object with key and value pairs that add up the strings consecutively:
[{a:1},{a:2},{a:3},{b:1},{b:2},{c:1},{d:1}]
How do I do this?
I've tried mapping through, but I can't get how to add on to the previous object's value (a:1 -> a:2)
While mapping, you need to store a separate count of how many times each item has appeared, and increment the appropriate key each iteration. You might use a Map for this:
const input = ['a','a','a','b','b','c','d'];
const map = new Map();
console.log(
input.map(char => {
const count = (map.get(char) || 0) + 1;
map.set(char, count);
return { [char]: count };
})
)
map.get(char) || 0, which means get either the defined value for that char, or if the value was undefined (or othereise "falsey") replace it with a real zero, so that the subsequent + 1 will work. undefined + 1 isn't legal.
map starts out empty. The first time a key is encountered (like the a on the first iteration), map.get(char) will return undefined, which isn't a number that can be added to. So, map.get(char) || 0 is a quick way of getting an expression that evaluates to 0 if the key does not exist in the map yet. If the key does exist in the map already, then the expression evalutes to that (numeric) count.