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Updated Type Checking JavaScript Files (markdown)

Daniel Rosenwasser 2017-04-25 12:26:01 -07:00
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Type-Checking-JavaScript-Files.md

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By default the TypeScript compiler does not report any errors in `.js` files including using `--allowJs`. With TypeScript 2.3 type-checking errors can also be reported in `.js` files with `--checkJs`.
TypeScript 2.3 and later support a mode of type-checking and reporting errors in `.js` files with `--checkJs`.
You can skip checking some files by adding `// @ts-nocheck` comment to them; conversely you can choose to check only a few `.js` files by adding `// @ts-check` comment to them without setting `--checkJs`. You can also ignore errors on specific lines by adding `// @ts-ignore` on the preceding line.
You can skip checking some files by adding `// @ts-nocheck` comment to them; conversely you can choose to check only a few `.js` files by adding `// @ts-check` comment to them without setting `--checkJs`.
You can also ignore errors on specific lines by adding `// @ts-ignore` on the preceding line.
Here are some notable differences on how checking work in `.js` file from `.ts` file:
## Using types in JSDoc
In a `.js` files types are inferred from initialization just like in `.ts` files. JSDoc in `.js` files fills in what type annotations does in a `.ts` file.
In a `.js` file, types can often be inferred just like in `.ts` files.
Likewise, when types can't be inferred, they can be specified using JSDoc the same way that type annotations do in a `.ts` file.
JSDoc annotations adorning a declaration will be used to set the type of that declaration. For example:
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## Property declaration inferred from assignments in class bodies
ES2015/ES6 does not specify a mean for declaring properties on classes. Properties are dynamically assigned, just like in the case of object literals.
ES2015/ES6 does not have a means for declaring properties on classes. Properties are dynamically assigned, just like in the case of object literals.
In a `.js` file property declarations are inferred from assignments to the properties inside the class body. The type of properties is the union of the types of all the right-hand values in these assignments. Properties defined in the constructor are always assumed to exist, where as ones defined in methods, getters, or setters are considered optional.