To extend an existing JavaScript array with another array, without creating a new array, we call the push method.
For instance, we write
const a = [1, 2];
const b = [3, 4, 5];
a.push(...b);
to call a.push with the items in b as arguments to append the items in b into a in the order they’re listed.
We use the spread operator to call push with the entries in b as arguments.
To create a two dimensional array in JavaScript, we put arrays inside an array.
For instance, we write
const items = [
[1, 2],
[3, 4],
[5, 6],
];
console.log(items[0][0]);
console.log(items[0][1]);
console.log(items[1][0]);
console.log(items[1][1]);
to put arrays inside the items array.
Then we access the first array in item‘s items with
console.log(items[0][0]);
console.log(items[0][1]);
We access the 2nd array in item‘s items with
console.log(items[1][0]);
console.log(items[1][1]);
To find the sum of an array of numbers with JavaScript, we call the reduce method.
For instance we write
const sum = [1, 2, 3].reduce((partialSum, a) => partialSum + a, 0);
console.log(sum);
to call reduce with a function that returns the partialSum plus the number a in the array being looped through.
We set the initial value of partialSum to 0 with the 2nd argument.
The sum is returned and it’s 6.
To compare arrays in JavaScript, we use the JSON.stringify method.
For instance, we write
const isSame = JSON.stringify(a1) === JSON.stringify(a2);
to convert arrays a1 and a2 to JSON strings with JSON.stringify.
And then we compare the strings with ===.
To merge/flatten an array of arrays with JavaScript, we call the flat method.
For instance, we write
const arrays = [["$6"], ["$12"], ["$25"], ["$25"], ["$18"], ["$22"], ["$10"]];
const merge3 = arrays.flat(1);
console.log(merge3);
to call arrays.flat with 1 to flatten the arrays array 1 level deep.
The returned flattened array is assigned to merge3.