typescript assert non null

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Karl
29 Aug 2019
1interface Foo { bar(): void }
2declare function getFoo(): Foo | undefined;
3
4function assert(value: unknown): asserts value {
5    if (value === undefined) {
6        throw new Error('value must be defined');
7    }
8}
9
10function test() {
11    const foo = getFoo();
12    // foo is Foo | undefined here
13    assert(foo);
14    // foo narrowed to Foo
15    foo.bar();
16}
17
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