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Random freezes, missing sound and display glitches usually trace back to one bad driver. Find and replace yours safely.Free scan · under a minuteFor a two-dimensional array, call .flat(): const flat = nested.flat(); It flattens one level into a new array. Pass a number to flatten a known number of levels, or Infinity to flatten all nested array levels.
Flatten one level with flat()
For example, this joins the inner arrays into one array of numbers:
const nested = [[1, 2], [3, 4]];
const flat = nested.flat(); // [1, 2, 3, 4]flat() defaults to a depth of 1. It returns a new array and leaves nested unchanged. MDN describes flat() as creating a new array with sub-array elements concatenated recursively up to the specified depth: MDN: Array.prototype.flat().
Choose how many levels to flatten
| Need | Example | Effect |
|---|---|---|
| Flatten one level | nested.flat() |
Joins each immediate inner array into a new array. |
| Flatten a known number of levels | nested.flat(2) |
Flattens two levels; any deeper nested arrays remain. |
| Flatten all nested array levels | nested.flat(Infinity) |
Recursively flattens nested arrays at every depth. |
Use a finite depth when you want to preserve any nesting beyond that point. Use Infinity only when all nested arrays should be removed, regardless of depth.
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Transform and flatten with flatMap()
When each input item needs to produce zero or more output values, flatMap() combines mapping with one level of flattening:
const words = ["hello world", "TypeScript"];
const parts = words.flatMap(word => word.split(" "));
// ["hello", "world", "TypeScript"]flatMap() is equivalent in effect to mapping and then calling flat(1). It only flattens one level, so it is not a general replacement for flat(Infinity). See MDN: Array.prototype.flatMap().
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Fix TypeScript errors about flat()
If TypeScript reports that flat or flatMap does not exist on the array type, check the project’s tsconfig.json lib setting. TypeScript lists the declarations for these APIs under ES2019; include ES2019 or a suitable newer library in the project’s configuration. The TypeScript TSConfig reference for lib explains that library settings provide type definitions for built-in APIs.
Type declarations do not add JavaScript features to the runtime. A project can type-check successfully while deploying to an environment that lacks flat(). Check the actual target runtime and use a polyfill or fallback if necessary. Updating lib alone does not provide runtime support.
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Use a fallback if the runtime lacks flat()
For one-level flattening, a reducer with concat() is an alternative:
const flat = nested.reduce<number[]>(
(result, part) => result.concat(part),
[]
);Choose the type argument to match the elements in your data. This fallback handles one level, like the default flat() call.
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Account for sparse arrays
flat() omits empty slots at the root and at the levels it flattens. If array positions are intentionally sparse, the resulting array may contain fewer slots than the input structure.
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