TypeScript/tests/baselines/reference/arrayConcat2.types at ccbc66214888329eb9fefd13b3c36ab6978e07fa · jablko/TypeScript · GitHub

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=== tests/cases/compiler/arrayConcat2.ts ===
var a: string[] = [];
>a : string[]
>[] : undefined[]

a.concat("hello", 'world');
>a.concat("hello", 'world') : string[]
>a.concat : { (...items: ConcatArray<string>[]): string[]; <U extends (string | ConcatArray<string>)[]>(...items: U): (string | Flatten<U[number]>)[]; }
>a : string[]
>concat : { (...items: ConcatArray<string>[]): string[]; <U extends (string | ConcatArray<string>)[]>(...items: U): (string | Flatten<U[number]>)[]; }
>"hello" : "hello"
>'world' : "world"

a.concat('Hello');
>a.concat('Hello') : string[]
>a.concat : { (...items: ConcatArray<string>[]): string[]; <U extends (string | ConcatArray<string>)[]>(...items: U): (string | Flatten<U[number]>)[]; }
>a : string[]
>concat : { (...items: ConcatArray<string>[]): string[]; <U extends (string | ConcatArray<string>)[]>(...items: U): (string | Flatten<U[number]>)[]; }
>'Hello' : "Hello"

var b = new Array<string>();
>b : string[]
>new Array<string>() : string[]
>Array : ArrayConstructor

b.concat('hello');
>b.concat('hello') : string[]
>b.concat : { (...items: ConcatArray<string>[]): string[]; <U extends (string | ConcatArray<string>)[]>(...items: U): (string | Flatten<U[number]>)[]; }
>b : string[]
>concat : { (...items: ConcatArray<string>[]): string[]; <U extends (string | ConcatArray<string>)[]>(...items: U): (string | Flatten<U[number]>)[]; }
>'hello' : "hello"

// #19535

const [x] = (undefined as unknown as string[][]).concat([""]);
>x : string | string[]
>(undefined as unknown as string[][]).concat([""]) : (string | string[])[]
>(undefined as unknown as string[][]).concat : { (...items: ConcatArray<string[]>[]): string[][]; <U extends (string[] | ConcatArray<string[]>)[]>(...items: U): (string[] | Flatten<U[number]>)[]; }
>(undefined as unknown as string[][]) : string[][]
>undefined as unknown as string[][] : string[][]
>undefined as unknown : unknown
>undefined : undefined
>concat : { (...items: ConcatArray<string[]>[]): string[][]; <U extends (string[] | ConcatArray<string[]>)[]>(...items: U): (string[] | Flatten<U[number]>)[]; }
>[""] : string[]
>"" : ""

x == "";
>x == "" : boolean
>x : string | string[]
>"" : ""