Array.prototype.flat()
The flat()
method creates a new array with all sub-array
elements concatenated into it recursively up to the specified depth.
Syntax
flat()
flat(depth)
Parameters
depth
Optional-
The depth level specifying how deep a nested array structure should be flattened. Defaults to 1.
Return value
A new array with the sub-array elements concatenated into it.
Alternatives
reduce and concat
const arr = [1, 2, [3, 4]];
// To flat single level array
arr.flat();
// is equivalent to
arr.reduce((acc, val) => acc.concat(val), []);
// [1, 2, 3, 4]
// or with decomposition syntax
const flattened = arr => [].concat(...arr);
reduce + concat + isArray + recursivity
const arr = [1, 2, [3, 4, [5, 6]]];
// to enable deep level flatten use recursion with reduce and concat
function flatDeep(arr, d = 1) {
return d > 0 ? arr.reduce((acc, val) => acc.concat(Array.isArray(val) ? flatDeep(val, d - 1) : val), [])
: arr.slice();
};
flatDeep(arr, Infinity);
// [1, 2, 3, 4, 5, 6]
Use a stack
// non recursive flatten deep using a stack
// note that depth control is hard/inefficient as we will need to tag EACH value with its own depth
// also possible w/o reversing on shift/unshift, but array OPs on the end tends to be faster
function flatten(input) {
const stack = [...input];
const res = [];
while (stack.length) {
// pop value from stack
const next = stack.pop();
if (Array.isArray(next)) {
// push back array items, won't modify the original input
stack.push(...next);
} else {
res.push(next);
}
}
// reverse to restore input order
return res.reverse();
}
const arr = [1, 2, [3, 4, [5, 6]]];
flatten(arr);
// [1, 2, 3, 4, 5, 6]
Use Generator function
function* flatten(array, depth) {
if (depth === undefined) {
depth = 1;
}
for (const item of array) {
if (Array.isArray(item) && depth > 0) {
yield* flatten(item, depth - 1);
} else {
yield item;
}
}
}
const arr = [1, 2, [3, 4, [5, 6]]];
const flattened = [...flatten(arr, Infinity)];
// [1, 2, 3, 4, 5, 6]
Examples
Flattening nested arrays
const arr1 = [1, 2, [3, 4]];
arr1.flat();
// [1, 2, 3, 4]
const arr2 = [1, 2, [3, 4, [5, 6]]];
arr2.flat();
// [1, 2, 3, 4, [5, 6]]
const arr3 = [1, 2, [3, 4, [5, 6]]];
arr3.flat(2);
// [1, 2, 3, 4, 5, 6]
const arr4 = [1, 2, [3, 4, [5, 6, [7, 8, [9, 10]]]]];
arr4.flat(Infinity);
// [1, 2, 3, 4, 5, 6, 7, 8, 9, 10]
Flattening and array holes
The flat method removes empty slots in arrays:
const arr5 = [1, 2, , 4, 5];
arr5.flat();
// [1, 2, 4, 5]
Specifications
Specification |
---|
ECMAScript Language Specification # sec-array.prototype.flat |
Browser compatibility
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