mirror of https://github.com/jkjoy/sunpeiwen.git
88 lines
2.5 KiB
Markdown
88 lines
2.5 KiB
Markdown
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# with
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Compile time `with` for strict mode JavaScript
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[](https://github.com/pugjs/with/actions?query=workflow%3A%22Publish+Canary%22)
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[](https://rollingversions.com/pugjs/with)
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[](https://www.npmjs.com/package/with)
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## Installation
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$ npm install with
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## Usage
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```js
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var addWith = require('with');
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addWith('obj', 'console.log(a)');
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// => ';(function (console, a) {
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// console.log(a)
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// }("console" in obj ? obj.console :
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// typeof console!=="undefined" ? console : undefined,
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// "a" in obj ? obj.a :
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// typeof a !== "undefined" ? a : undefined));'
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addWith('obj', 'console.log(a)', ['console']);
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// => ';(function (console, a) {
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// console.log(a)
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// }("a" in obj ? obj.a :
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// typeof a !== "undefined" ? a : undefined));'
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```
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## API
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### addWith(obj, src[, exclude])
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The idea is that this is roughly equivallent to:
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```js
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with (obj) {
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src;
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}
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```
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There are a few differences though. For starters, assignments to variables will always remain contained within the with block.
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e.g.
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```js
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var foo = 'foo';
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with ({}) {
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foo = 'bar';
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}
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assert(foo === 'bar'); // => This fails for compile time with but passes for native with
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var obj = {foo: 'foo'};
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with ({}) {
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foo = 'bar';
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}
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assert(obj.foo === 'bar'); // => This fails for compile time with but passes for native with
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```
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It also makes everything be declared, so you can always do:
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```js
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if (foo === undefined)
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```
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instead of
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```js
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if (typeof foo === 'undefined')
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```
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This is not the case if foo is in `exclude`. If a variable is excluded, we ignore it entirely. This is useful if you know a variable will be global as it can lead to efficiency improvements.
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It is also safe to use in strict mode (unlike `with`) and it minifies properly (`with` disables virtually all minification).
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#### Parsing Errors
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with internally uses babylon to parse code passed to `addWith`. If babylon throws an error, probably due to a syntax error, `addWith` returns an error wrapping the babylon error, so you can
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retrieve location information. `error.component` is `"src"` if the error is in the body or `"obj"` if it's in the object part of the with expression. `error.babylonError` is
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the error thrown from babylon.
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## License
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MIT
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