title
Compile time

Typically, you won't interact with the Svelte compiler directly, but will instead integrate it into your build system using a bundler plugin. The bundler plugin that the Svelte team most recommends and invests in is vite-plugin-svelte. The SvelteKit framework provides a setup leveraging vite-plugin-svelte to build applications as well as a tool for packaging Svelte component libraries. Svelte Society maintains a list of other bundler plugins for additional tools like Rollup and Webpack.

Nonetheless, it's useful to understand how to use the compiler, since bundler plugins generally expose compiler options to you.

`svelte.compile`

result: {
	js,
	css,
	ast,
	warnings,
	vars,
	stats
} = svelte.compile(source: string, options?: {...})

This is where the magic happens. svelte.compile takes your component source code, and turns it into a JavaScript module that exports a class.

const svelte = require('svelte/compiler');

const result = svelte.compile(source, {
	// options
});

The following options can be passed to the compiler. None are required:

option	default	description
`filename`	`null`	`string` used for debugging hints and sourcemaps. Your bundler plugin will set it automatically.
`name`	`"Component"`	`string` that sets the name of the resulting JavaScript class (though the compiler will rename it if it would otherwise conflict with other variables in scope). It will normally be inferred from `filename`.
`format`	`"esm"`	If `"esm"`, creates a JavaScript module (with `import` and `export`). If `"cjs"`, creates a CommonJS module (with `require` and `module.exports`), which is useful in some server-side rendering situations or for testing.
`generate`	`"dom"`	If `"dom"`, Svelte emits a JavaScript class for mounting to the DOM. If `"ssr"`, Svelte emits an object with a `render` method suitable for server-side rendering. If `false`, no JavaScript or CSS is returned; just metadata.
`errorMode`	`"throw"`	If `"throw"`, Svelte throws when a compilation error occurred. If `"warn"`, Svelte will treat errors as warnings and add them to the warning report.
`varsReport`	`"strict"`	If `"strict"`, Svelte returns a variables report with only variables that are not globals nor internals. If `"full"`, Svelte returns a variables report with all detected variables. If `false`, no variables report is returned.
`dev`	`false`	If `true`, causes extra code to be added to components that will perform runtime checks and provide debugging information during development.
`immutable`	`false`	If `true`, tells the compiler that you promise not to mutate any objects. This allows it to be less conservative about checking whether values have changed.
`hydratable`	`false`	If `true` when generating DOM code, enables the `hydrate: true` runtime option, which allows a component to upgrade existing DOM rather than creating new DOM from scratch. When generating SSR code, this adds markers to `<head>` elements so that hydration knows which to replace.
`legacy`	`false`	If `true`, generates code that will work in IE9 and IE10, which don't support things like `element.dataset`.
`accessors`	`false`	If `true`, getters and setters will be created for the component's props. If `false`, they will only be created for readonly exported values (i.e. those declared with `const`, `class` and `function`). If compiling with `customElement: true` this option defaults to `true`.
`customElement`	`false`	If `true`, tells the compiler to generate a custom element constructor instead of a regular Svelte component.
`tag`	`null`	A `string` that tells Svelte what tag name to register the custom element with. It must be a lowercase alphanumeric string with at least one hyphen, e.g. `"my-element"`.
`css`	`"injected"`	If `"injected"` (formerly `true`), styles will be included in the JavaScript class and injected at runtime for the components actually rendered. If `"external"` (formerly `false`), the CSS will be returned in the `css` field of the compilation result. Most Svelte bundler plugins will set this to `"external"` and use the CSS that is statically generated for better performance, as it will result in smaller JavaScript bundles and the output can be served as cacheable `.css` files. If `"none"`, styles are completely avoided and no CSS output is generated.
`cssHash`	See right	A function that takes a `{ hash, css, name, filename }` argument and returns the string that is used as a classname for scoped CSS. It defaults to returning `svelte-${hash(css)}`
`loopGuardTimeout`	0	A `number` that tells Svelte to break the loop if it blocks the thread for more than `loopGuardTimeout` ms. This is useful to prevent infinite loops. Only available when `dev: true`
`preserveComments`	`false`	If `true`, your HTML comments will be preserved during server-side rendering. By default, they are stripped out.
`preserveWhitespace`	`false`	If `true`, whitespace inside and between elements is kept as you typed it, rather than removed or collapsed to a single space where possible.
`sourcemap`	`object \| string`	An initial sourcemap that will be merged into the final output sourcemap. This is usually the preprocessor sourcemap.
`enableSourcemap`	`boolean \| { js: boolean; css: boolean; }`	If `true`, Svelte generate sourcemaps for components. Use an object with `js` or `css` for more granular control of sourcemap generation. By default, this is `true`.
`outputFilename`	`null`	A `string` used for your JavaScript sourcemap.
`cssOutputFilename`	`null`	A `string` used for your CSS sourcemap.
`sveltePath`	`"svelte"`	The location of the `svelte` package. Any imports from `svelte` or `svelte/[module]` will be modified accordingly.
`namespace`	`"html"`	The namespace of the element; e.g., `"mathml"`, `"svg"`, `"foreign"`.

The returned result object contains the code for your component, along with useful bits of metadata.

const {
	js,
	css,
	ast,
	warnings,
	vars,
	stats
} = svelte.compile(source);

js and css are objects with the following properties:
- code is a JavaScript string
- map is a sourcemap with additional toString() and toUrl() convenience methods
ast is an abstract syntax tree representing the structure of your component.
warnings is an array of warning objects that were generated during compilation. Each warning has several properties:
- code is a string identifying the category of warning
- message describes the issue in human-readable terms
- start and end, if the warning relates to a specific location, are objects with line, column and character properties
- frame, if applicable, is a string highlighting the offending code with line numbers
vars is an array of the component's declarations, used by eslint-plugin-svelte3 for example. Each variable has several properties:
- name is self-explanatory
- export_name is the name the value is exported as, if it is exported (will match name unless you do export...as)
- injected is true if the declaration is injected by Svelte, rather than in the code you wrote
- module is true if the value is declared in a context="module" script
- mutated is true if the value's properties are assigned to inside the component
- reassigned is true if the value is reassigned inside the component
- referenced is true if the value is used in the template
- referenced_from_script is true if the value is used in the <script> outside the declaration
- writable is true if the value was declared with let or var (but not const, class or function)
stats is an object used by the Svelte developer team for diagnosing the compiler. Avoid relying on it to stay the same!

`svelte.parse`

ast: object = svelte.parse(
	source: string,
	options?: {
		filename?: string,
		customElement?: boolean
	}
)

The parse function parses a component, returning only its abstract syntax tree. Unlike compiling with the generate: false option, this will not perform any validation or other analysis of the component beyond parsing it. Note that the returned AST is not considered public API, so breaking changes could occur at any point in time.

const svelte = require('svelte/compiler');

const ast = svelte.parse(source, { filename: 'App.svelte' });

`svelte.preprocess`

A number of community-maintained preprocessing plugins are available to allow you to use Svelte with tools like TypeScript, PostCSS, SCSS, and Less.

You can write your own preprocessor using the svelte.preprocess API.

result: {
	code: string,
	dependencies: Array<string>
} = await svelte.preprocess(
	source: string,
	preprocessors: Array<{
		markup?: (input: { content: string, filename: string }) => Promise<{
			code: string,
			dependencies?: Array<string>
		}>,
		script?: (input: { content: string, markup: string, attributes: Record<string, string>, filename: string }) => Promise<{
			code: string,
			dependencies?: Array<string>
		}>,
		style?: (input: { content: string, markup: string, attributes: Record<string, string>, filename: string }) => Promise<{
			code: string,
			dependencies?: Array<string>
		}>
	}>,
	options?: {
		filename?: string
	}
)

The preprocess function provides convenient hooks for arbitrarily transforming component source code. For example, it can be used to convert a <style lang="sass"> block into vanilla CSS.

The first argument is the component source code. The second is an array of preprocessors (or a single preprocessor, if you only have one), where a preprocessor is an object with markup, script and style functions, each of which is optional.

Each markup, script or style function must return an object (or a Promise that resolves to an object) with a code property, representing the transformed source code, and an optional array of dependencies.

The markup function receives the entire component source text, along with the component's filename if it was specified in the third argument.

Preprocessor functions should additionally return a map object alongside code and dependencies, where map is a sourcemap representing the transformation.

const svelte = require('svelte/compiler');
const MagicString = require('magic-string');

const { code } = await svelte.preprocess(source, {
	markup: ({ content, filename }) => {
		const pos = content.indexOf('foo');
		if(pos < 0) {
			return { code: content }
		}
		const s = new MagicString(content, { filename })
		s.overwrite(pos, pos + 3, 'bar', { storeName: true })
		return {
			code: s.toString(),
			map: s.generateMap()
		}
	}
}, {
	filename: 'App.svelte'
});

The script and style functions receive the contents of <script> and <style> elements respectively (content) as well as the entire component source text (markup). In addition to filename, they get an object of the element's attributes.

If a dependencies array is returned, it will be included in the result object. This is used by packages like rollup-plugin-svelte to watch additional files for changes, in the case where your <style> tag has an @import (for example).

const svelte = require('svelte/compiler');
const sass = require('node-sass');
const { dirname } = require('path');

const { code, dependencies } = await svelte.preprocess(source, {
	style: async ({ content, attributes, filename }) => {
		// only process <style lang="sass">
		if (attributes.lang !== 'sass') return;

		const { css, stats } = await new Promise((resolve, reject) => sass.render({
			file: filename,
			data: content,
			includePaths: [
				dirname(filename),
			],
		}, (err, result) => {
			if (err) reject(err);
			else resolve(result);
		}));

		return {
			code: css.toString(),
			dependencies: stats.includedFiles
		};
	}
}, {
	filename: 'App.svelte'
});

Multiple preprocessors can be used together. The output of the first becomes the input to the second. markup functions run first, then script and style.

const svelte = require('svelte/compiler');

const { code } = await svelte.preprocess(source, [
	{
		markup: () => {
			console.log('this runs first');
		},
		script: () => {
			console.log('this runs third');
		},
		style: () => {
			console.log('this runs fifth');
		}
	},
	{
		markup: () => {
			console.log('this runs second');
		},
		script: () => {
			console.log('this runs fourth');
		},
		style: () => {
			console.log('this runs sixth');
		}
	}
], {
	filename: 'App.svelte'
});

`svelte.walk`

walk(ast: Node, {
	enter(node: Node, parent: Node, prop: string, index: number)?: void,
	leave(node: Node, parent: Node, prop: string, index: number)?: void
})

The walk function provides a way to walk the abstract syntax trees generated by the parser, using the compiler's own built-in instance of estree-walker.

The walker takes an abstract syntax tree to walk and an object with two optional methods: enter and leave. For each node, enter is called (if present). Then, unless this.skip() is called during enter, each of the children are traversed, and then leave is called on the node.

const svelte = require('svelte/compiler');
svelte.walk(ast, {
	enter(node, parent, prop, index) {
		do_something(node);
		if (should_skip_children(node)) {
			this.skip();
		}
	},
	leave(node, parent, prop, index) {
		do_something_else(node);
	}
});

`svelte.VERSION`

The current version, as set in package.json.

const svelte = require('svelte/compiler');
console.log(`running svelte version ${svelte.VERSION}`);

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