JavaScript Array Methods
All the array methods you need to crack a JavaScript interview — with one-liner usage examples.
Transformation
Methods that return a new array based on a transformation function.
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// map — transform each element
[1,2,3].map(x => x * 2); // [2, 4, 6]
// flatMap — map then flatten one level
[1,2].flatMap(x => [x, x*2]); // [1,2,2,4]
// filter — keep elements that pass the test
[1,2,3,4].filter(x => x % 2 === 0); // [2, 4]Aggregation
Methods that reduce an array to a single value.
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// reduce — accumulate into a single value
[1,2,3,4].reduce((acc, x) => acc + x, 0); // 10
// reduceRight — same but right-to-left
[[1],[2],[3]].reduceRight((acc, x) => acc.concat(x)); // [3,2,1]Search & Test
Methods that find elements or test conditions.
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const arr = [5, 12, 8, 130, 44];
arr.find(x => x > 10); // 12 (first match)
arr.findIndex(x => x > 10); // 1
arr.indexOf(8); // 2
arr.includes(8); // true
arr.some(x => x > 100); // true (at least one)
arr.every(x => x > 0); // true (all match)Sorting & Ordering
Methods that reorder elements (mutate the original array).
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[3,1,2].sort((a,b) => a - b); // [1,2,3] ascending
[3,1,2].sort((a,b) => b - a); // [3,2,1] descending
[1,2,3].reverse(); // [3,2,1]Add / Remove
Mutating methods to add or remove elements.
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const a = [1,2,3];
a.push(4); // a = [1,2,3,4] adds to end
a.pop(); // a = [1,2,3] removes from end
a.unshift(0); // a = [0,1,2,3] adds to start
a.shift(); // a = [1,2,3] removes from start
a.splice(1,1); // a = [1,3] removes at index 1