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| -rw-r--r-- | typescript.html.markdown | 16 |
1 files changed, 16 insertions, 0 deletions
diff --git a/typescript.html.markdown b/typescript.html.markdown index cf2111d5..6c6da2c4 100644 --- a/typescript.html.markdown +++ b/typescript.html.markdown @@ -257,8 +257,24 @@ for (const i in list) { console.log(i); // 0, 1, 2 } +// Type Assertion +let foo = {} // Creating foo as an empty object +foo.bar = 123 // Error: property 'bar' does not exist on `{}` +foo.baz = 'hello world' // Error: property 'baz' does not exist on `{}` +// Because the inferred type of foo is `{}` (an object with 0 properties), you +// are not allowed to add bar and baz to it. However with type assertion, +// the following will pass: + +interface Foo { + bar: number; + baz: string; +} + +let foo = {} as Foo; // Type assertion here +foo.bar = 123; +foo.baz = 'hello world' ``` |