Flatten array of arrays in TypeScript

Question

I want to flatten string[][] into string[].

The advice given in dozens of SO answers is: [].concat(...arrays).

But that gives me this error:

Argument of type 'string[]' is not assignable to parameter of type 'ConcatArray'.
Types of property 'slice' are incompatible.
Type '(start?: number | undefined, end?: number | undefined) => string[]' is not assignable to type '(start?: number | undefined, end?: number | undefined) => never[]'.
Type 'string[]' is not assignable to type 'never[]'.
Type 'string' is not assignable to type 'never'.

Another way I tried is this:

let foo: string[][] = [["a", "b", "c"], ["a", "b", "c"], ["a", "b", "c"]]; let bar = [].concat(...foo); 

Which gives a similar error:

Argument of type 'string[]' is not assignable to parameter of type 'ConcatArray'.

Why does it work for everyone but me?

Answer 1

Try this:

const a = [["a", "b", "c"], ["a", "b", "c"], ["a", "b", "c"]]  const result = a.reduce((accumulator, value) => accumulator.concat(value), []);  console.log(result)

Answer 2

You can flatten the array with flat()

let foo: string[][] = [["a", "b", "c"], ["a", "b", "c"], ["a", "b", "c"]]; let bar = foo.flat() 

log

console.log(bar)   // a,b,c,a,b,c,a,b,c  

UPDATE

By correcting the type to string[] you can also use concat

let foo: string[][] = [["a", "b", "c"], ["a", "b", "c"], ["a", "b", "c"]]; let bar : string[] = [] bar = bar.concat(foo[0], foo[1], foo[2])